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A Pascal compiler is a much larger programming +project than most of the ones we've explored so far. You might well ask, +"where do we <EM>begin</EM> in writing a compiler?" My goal in this chapter +is to show some of the parts that go into a compiler design. + +<P>A compiler translates programs from a language like Pascal into the +machine language of some particular computer model. My compiler +translates into a simplified, simulated machine language; the compiled +programs are actually carried out by another Logo program, the simulator, +rather than directly by the computer hardware. The advantage of using this +simulated machine language is that this compiler will work no matter what +kind of computer you have; also, the simplifications in this simulated +machine allow me to leave out many confusing details of a practical compiler. +Our machine language is, however, realistic enough to give you a good sense +of what compiling into a real machine language would be like; it's loosely +based on the MIPS microprocessor design. You'll see in a moment that most +of the structure of the compiler is independent of the target language, +anyway. + +<P> + +<P>Here is a short, uninteresting Pascal program: + +<P><PRE>program test; + +procedure doit(n:integer); + begin + writeln(n,n*n) + end; + +begin + doit(3) +end. +</PRE> + +<P>If you type this program into a disk file and then compile it +using <CODE>compile</CODE> as described in Chapter 4, the compiler will translate +the program into this sequence of instructions, contained in a list in the +variable named <CODE>%test</CODE>: + +<P> + +<P><PRE>[ [add 3 0 0] + [add 4 0 0] + [addi 2 0 36] + [jump "g1] +%doit [store 1 0(4)] + [jump "g2] +g2 [rload 7 36(4)] + [putint 10 7] + [rload 7 36(4)] + [rload 8 36(4)] + [mul 7 7 8] + [putint 10 7] + [newline] + [rload 1 0(4)] + [add 2 4 0] + [rload 4 3(2)] + [jr 1] +g1 [store 5 1(2)] + [add 5 2 0] + [addi 2 2 37] + [store 4 3(5)] + [store 4 2(5)] + [addi 7 0 3] + [store 7 36(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%doit] + [exit] +] +</PRE> + +<P>I've displayed this list of instructions with some extra spacing +thrown in to make it look somewhat like a typical <EM>assembler</EM> listing. +(An assembler is a program that translates a notation like <CODE>add 3 0 0</CODE> +into a binary number, the form in which the machine hardware actually +recognizes these instructions.) A real assembler listing wouldn't have the +square brackets that Logo uses to mark each sublist, but would instead +depend on the convention that each instruction occupies one line. + +<P>The first three instructions carry out initialization that would be the same +for any compiled Pascal program; the fourth is a <CODE>jump</CODE> instruction that +tells the (simulated) computer to skip to the instruction following the +<EM>label</EM> <CODE>g1</CODE> that appears later in the program. (A word that +isn't part of a sublist is a label.) In Pascal, the body of the main +program comes after the declarations of procedures; this <CODE>jump</CODE> +instruction allows the compiler to translate the parts of the program in the +order in which they appear. + +<P>(Two instructions later, you'll notice a <CODE>jump</CODE> to a label that comes +right after the jump instruction! The compiler issues this useless +instruction just in case some internal procedures were declared within +the procedure <CODE>doit</CODE>. A better compiler would include an <EM> +optimizer</EM> that would go through the compiled program looking for +ways to eliminate unnecessary instructions such as this one. The optimizer +is the most important thing that I've left out of my compiler.) + +<P>We're not ready yet to talk in detail about how the compiled instructions +represent the Pascal program, but you might be able to guess certain things. +For example, the variable <CODE>n</CODE> in procedure <CODE>doit</CODE> seems to be +represented as <CODE>36(4)</CODE> in the compiled program; you can see where <CODE> +36(4)</CODE> is printed and then multiplied by itself, although it may not yet be +clear to you what the numbers <CODE>7</CODE> and <CODE>8</CODE> have to do with anything. +Before we get into those details, I want to give a broader overview of +the organization of the compiler. + +<P>The compilation process is divided into three main pieces. First and +simplest is <EM>tokenization.</EM> The compiler initially sees the +source program as a string of characters: <CODE>p</CODE>, then <CODE>r</CODE>, and so on, +including spaces and line separators. The first step in compilation is to +turn these characters into symbols, so that the later stages of compilation +can deal with the word <CODE>program</CODE> as a unit. The second piece of the +compiler is the <EM>parser,</EM> the part that recognizes certain +patterns of symbols as representing meaningful units. "Oh," says the +parser, "I've just seen the word <CODE>procedure</CODE> so what comes next must be +a procedure header and then a <CODE>begin</CODE>-<CODE>end</CODE> block for the body of +the procedure." Finally, there is the process of <EM> +code generation,</EM> in which each unit that was recognized by the +parser is actually translated into the equivalent machine language +instructions. + +<P>(I don't mean that parsing and code generation happen separately, one after +the other, in the compiler's algorithm. In fact each meaningful unit is +translated as it's encountered, and the translation of a large unit like a +procedure includes, recursively, the translation of smaller units like +statements. But parsing and code generation are conceptually two different +tasks, and we'll talk about them separately.) + +<P><H2>Formal Syntax Definition</H2> + +<P>One common starting place is to develop a formal definition for the language +we're trying to compile. The regular expressions of Chapter 1 are an +example of what I mean by a formal definition. A +regular expression tells +us unambiguously that certain strings of characters are accepted as members +of the category defined by the expression, while other strings aren't. A +language like Pascal is too complicated to be described by a regular +expression, but other kinds of formal definition can be used. + +<P>The formal systems of Chapter 1 just gave a yes-or-no decision for any input +string: Is it, or is it not, accepted in the language under discussion? +That's not quite good enough for a compiler. We don't just want to know +whether a Pascal program is syntactically correct; we want a translation of +the program into some executable form. Nevertheless, it turns out to be +worthwhile to begin by designing a formal acceptor for Pascal. That part of +the compiler--the part that determines the syntactic structure of the +source program--is called the <EM>parser.</EM> Later we'll add provisions for +<EM>code generation:</EM> the translation of each syntactic unit of the +source program into a piece of <EM>object</EM> (executable) program that +carries out the meaning (the <EM>semantics</EM>) of that unit. + +<P>One common form in which programming languages are described is the +<EM>production rule</EM> notation mentioned briefly in Chapter 1. For +example, here is part of a specification for Pascal: + +<P><PRE>program: program <EM>identifier</EM> <EM>filenames</EM> ; <EM>block</EM> . + +filenames: | ( <EM>idlist</EM> ) + +idlist: <EM>identifier</EM> | <EM>idlist</EM> , <EM>identifier</EM> + +block: <EM>varpart</EM> <EM>procpart</EM> <EM>compound</EM> + +varpart: | var <EM>varlist</EM> + +procpart: | <EM>procpart</EM> <EM>procedure</EM> | <EM>procpart</EM> <EM>function</EM> + +compound: begin <EM>statements</EM> end + +statements: <EM>statement</EM> | <EM>statements</EM> ; <EM>statement</EM> + +procedure: procedure <EM>identifier</EM> <EM>args</EM> ; <EM>block</EM> ; + +function: function <EM>identifier</EM> <EM>args</EM> : <EM>type</EM> ; <EM>block</EM> ; +</PRE> + +<P>A program consists of six components. Some of these components +are particular words (like <CODE>program</CODE>) or punctuation marks; other +components are defined in terms of even smaller units by other +rules.<SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>The <EM>filenames</EM> component is an optional list of +names of files, part of Pascal's input/output capability; my compiler doesn't +handle file input or output, so it ignores this list if there is one.</SMALL></BLOCKQUOTE></SMALL><P>A vertical bar (<CODE>|</CODE>) in a rule separates alternatives; an idlist +(identifier list) is either a single identifier or a smaller idlist followed +by a comma and another identifier. Sometimes one of the alternatives in a +rule is empty; for example, a varpart can be empty because a block need not +declare any local variables. + +<P>The goal in designing a formal specification is to capture the +syntactic hierarchy +of the language you're describing. For example, you could define +a Pascal type as + +<P><PRE>type: integer | real | char | boolean | array <EM>range</EM> of integer | + packed array <EM>range</EM> of integer | array <EM>range</EM> of real | + ... +</PRE> + +<P>but it's better to say + +<P><PRE>type: <EM>scalar</EM> | array <EM>range</EM> of <EM>scalar</EM> | + packed array <EM>range</EM> of <EM>scalar</EM> + +scalar: integer | real | char | boolean +</PRE> + +<P>Try completing the syntactic description of my subset of Pascal along these +lines. You might also try a similar syntactic description of Logo. Which +is easier? + +<P>Another kind of formal description is + +the <EM>recursive transition network</EM> (RTN). +An RTN is like a finite-state machine except that instead +of each arrow representing a single symbol in the machine's alphabet, an +arrow can be labeled with the name of another RTN; such an arrow represents +any string of symbols accepted by that RTN. + +<P>Below I show two RTNs, one for a program and one for a +sequence of statements (the body of a compound statement). +In the former, the transition from state 5 to state 6 is followed if what +comes next in the Pascal program is a string of symbols accepted by the RTN +named "block." In these diagrams, a word in <CODE>typewriter</CODE> style like +<CODE>program</CODE> represents a single symbol, as in a finite-state machine diagram, +while a word in <EM>italics</EM> like <EM>block</EM> represents any +string accepted by the RTN of that name. The <EM>statements</EM> RTN +is recursive; one path through the network involves a transition that +requires parsing a smaller <EM>statements</EM> unit. + +<P><CENTER><IMG SRC="rtn1.gif" ALT="figure: rtn1"></CENTER> +<P><CENTER><IMG SRC="rtn2.gif" ALT="figure: rtn2"></CENTER> + +<P><H2>Tokenization</H2> + + +<P>In both the production rules and the RTNs I've treated words like <CODE> +program</CODE> as a single symbol of the "alphabet" of the language. It would +be possible, of course, to use single characters as the alphabetic symbols +and describe the language in this form: + +<P><CENTER><IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/tokenrtn.gif" ALT="figure: tokenrtn"></CENTER> + +<P>Extending the formal description down to that level, though, makes +it hard to see the forest for the trees; the important structural patterns +get lost in details about, for instance, where spaces are required between +words (as in <CODE>program tower</CODE>), where they're optional (as in <CODE> +2 + 3</CODE>), and where they're not allowed at all (<CODE>prog +ram</CODE>). A similar complication is that a comment in braces can +be inserted anywhere in the program; it would be enormously complicated if +every state of every RTN had to have a transition for a left brace beginning +a comment. + +<P>Most language processors therefore group the characters of the source +program into <EM>tokens</EM> used as the alphabet for the formal +grammar. A token may be a single character, such as a punctuation mark, or +a group of characters, such as a word or a number. Spaces do not ordinarily +form part of tokens, although in the Pascal compiler one kind of token is a +quoted character string that can include spaces. Comments are also removed +during tokenization. Here's what the <CODE>tower</CODE> program from Chapter 4 +looks like in token form: + +<P><CENTER><IMG SRC="tokenized.gif"></CENTER> + +<P>Tokenization is what the Logo <CODE>readlist</CODE> operation does when +it uses spaces and brackets to turn the string of characters you type into +a sequence of words and lists. + +<P>Tokenization is also called <EM>lexical analysis.</EM> This term has +nothing to do with lexical scope; the word "lexical" is used not to remind +us of a dictionary but because the root "lex" means <EM>word</EM> and +lexical analysis divides the source program into words. + +<P>I've been talking as if the Pascal compiler first went through the entire +source file tokenizing it and then went back and parsed the result. That's +not actually how it works; instead, the parser just calls a procedure named +<CODE>token</CODE> whenever it wants to see the next token in the source file. +I've already mentioned that Pascal was designed to allow the compiler to +read straight through the source program without jumping around and +re-reading parts of it. + +<P><H2>Lookahead</H2> + + +<P>Consider the situation when the parser has recognized the first token +(<CODE>program</CODE>) as the beginning of a program and it invokes <CODE>token</CODE> to +read the second token, the program name. In the <CODE>tower</CODE> program, the +desired token is <CODE>tower</CODE>. <CODE>Token</CODE> reads the letter <CODE>t</CODE>; since +it's a letter, it must be the beginning of an identifier. Any number of +letters or digits following the <CODE>t</CODE> will be part of the identifier, but +the first non-alphanumeric character ends the token. (In this case, the +character that ends the token will be a semicolon.) + +<P>What this means is that <CODE>token</CODE> has to read one character too many in +order to find the end of the word <CODE>tower</CODE>. The semicolon isn't +part of that token; it's part of the <EM>following</EM> token. (In fact it's +the entire following token, but in other situations that need not be true.) +Ordinarily <CODE>token</CODE> begins its work by reading a character from the +source file, but the next time we call <CODE>token</CODE> it has to deal with the +character it's already read. It would simplify things enormously if <CODE> +token</CODE> could "un-read" the semicolon that ends the token <CODE> +tower</CODE>. It's possible to allow something like un-reading by using a +technique called <EM>lookahead.</EM> + +<P><PRE>to getchar +local "char +if namep "peekchar + [make "char :peekchar + ern "peekchar + output :char] +output readchar +end +</PRE> + +<P><CODE>Getchar</CODE> is the procedure that <CODE>token</CODE> calls to read the next +character from the source file. Ordinarily <CODE>getchar</CODE> just invokes the +primitive <CODE>readchar</CODE> to read a character from the file.<SUP>*</SUP> But if there is a variable named <CODE>peekchar</CODE>, then <CODE> +getchar</CODE> just outputs whatever is in that variable without looking at the +file. <CODE>Token</CODE> can now un-read a character by saying + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>I'm lying. +The real <CODE>getchar</CODE> is slightly more complicated because it checks for an +unexpected end of file and because it prints the characters that it reads +onto the screen. The program listing at the end of the chapter tells the +whole story.</SMALL></BLOCKQUOTE></SMALL><P><PRE>make "peekchar :char +</PRE> + +<P>This technique only allows <CODE>token</CODE> to un-read a single +character at a time. It would be possible to replace <CODE>peekchar</CODE> with a +<EM>list</EM> of pre-read characters to be recycled. But in fact one is +enough. When a program "peeks at" characters before they're read "for +real," the technique is called <EM>lookahead.</EM> <CODE>Getchar</CODE> uses +<EM>one-character lookahead</EM> because <CODE>peekchar</CODE> only stores a single +character. + +<P>It turns out that, for similar reasons, the Pascal parser will occasionally +find it convenient to peek at a <EM>token</EM> and re-read it later. <CODE> +Token</CODE> therefore provides for one-<EM>token</EM> lookahead using a similar +mechanism: + +<P><PRE>to token +local [token char] +if namep "peektoken [make "token :peektoken + ern "peektoken output :token] +make "char getchar +if equalp :char "|{| [skipcomment output token] +if equalp :char char 32 [output token] +if equalp :char char 13 [output token] +if equalp :char char 10 [output token] +if equalp :char "' [output string "'] +if memberp :char [+ - * / = ( , ) |[| |]| |;|] [output :char] +if equalp :char "|<| [output twochar "|<| [= >]] +if equalp :char "|>| [output twochar "|>| [=]] +if equalp :char ". [output twochar ". [.]] +if equalp :char ": [output twochar ": [=]] +if numberp :char [output number :char] +if letterp ascii :char [output token1 lowercase :char] +(throw "error sentence [unrecognized character:] :char) +end + +to twochar :old :ok +localmake "char getchar +if memberp :char :ok [output word :old :char] +make "peekchar :char +output :old +end +</PRE> + +<P>As you can see, <CODE>token</CODE> is mainly a selection of special cases. +<CODE>Char 32</CODE> is a space; <CODE>char 13</CODE> or <CODE>char 10</CODE> is the end-of-line +character. <CODE>Skipcomment</CODE> skips over characters until it sees a right +brace. <CODE>String</CODE> accumulates characters up to and including a single +quote (apostrophe), except that two single quotes in a row become one single +quote inside the string and don't end the string. <CODE>Number</CODE> is a little +tricky because of decimal points (the string of characters <CODE>1.10</CODE> is a +single token, but the string <CODE>1..10</CODE> is three tokens!) and exponent +notation. I'm not showing you all the details because the compiler is a +very large program and we'll never get through it if I annotate every +procedure. But I did want to show you <CODE>twochar</CODE> because it's a good, +simple example of character lookahead at work. If the character <CODE><</CODE> is +seen in the source program, it may be a token by itself or it may be part of +the two-character tokens <CODE><=</CODE> or <CODE><></CODE>. <CODE>Twochar</CODE> takes a peek +at the next character in the file to decide. + +<P>If the character that <CODE>token</CODE> reads isn't part of any recognizable +token, the procedure generates an error. (The error is caught by the +toplevel procedure <CODE>compile</CODE> so that it can close the source file.) +This extremely primitive error handling is one of the most serious +deficiencies in my compiler; it would be better if the compilation process +continued, despite the error, so that any other errors in the program could +also be discovered. In a real compiler, more than half of the parsing +effort goes into error handling; it's relatively trivial to parse a correct +source program. + +<P><H2>Parsing</H2> + + +<P>There are general techniques for turning a formal language specification, +such as a set of production rules, into an algorithm for parsing the +language so specified. These techniques are analogous to the program in +Chapter 1 that translates a regular expression into a finite-state machine. +A program that turns a formal specification into a parser is called a +<EM>parser generator.</EM> + +<P>The trouble is that the techniques that work for <EM>any</EM> set of rules +are quite slow. The time required to parse a sequence of length <EM>n</EM> is +O(<EM>n</EM><SUP><SMALL>2</SMALL></SUP>) if the grammar is unambiguous or O(<EM>n</EM><SUP><SMALL>3</SMALL></SUP>) if it's +ambiguous. A grammar is <EM>ambiguous</EM> if the same input sequence +can be parsed correctly in more than one way. For example, if the +production rule + +<P><PRE>idlist: <EM>identifier</EM> | <EM>idlist</EM> , <EM>identifier</EM> +</PRE> + +<P>is applied to the string + +<P><PRE>Beatles,Who,Zombies,Kinks +</PRE> + +<P>then the only possible application of the rule to accept the +string produces this left-to-right grouping: + +<P><CENTER><IMG SRC="unambiguous.gif" ALT="figure: unambiguous"></CENTER> + +<P>However, if the rule were + +<P><PRE>idlist: <EM>identifier</EM> | <EM>idlist</EM> , <EM>idlist</EM> +</PRE> + +<P>this new rule would accept the same strings, but would allow +alternative groupings like + +<P><CENTER><IMG SRC="ambiguous.gif" ALT="figure: ambiguous"></CENTER> + +<P>The former rule could be part of an unambiguous grammar; the new +rule makes the grammar that contains it ambiguous. + +<P> + +<P>It's usually not hard to devise an unambiguous grammar for any practical +programming language, but even a quadratic algorithm is too slow. Luckily, +most programming languages have <EM>deterministic grammars,</EM> +which is a condition even stricter than being unambiguous. It means that a +parser can read a program from left to right, and can figure out what to do +with the next token using only a fixed amount of lookahead. A parser for a +deterministic grammar can run in linear time, which is a lot better than +quadratic. + +<P>When I said "figure out what to do with the next token," I was being +deliberately vague. A deterministic parser doesn't necessarily know exactly +how a token will fit into the complete program--which production rules will +be branch nodes in a parse tree having this token as a leaf node--as soon +as it reads the token. As a somewhat silly example, pretend that the word +<CODE>if</CODE> is not a "reserved word" in Pascal; suppose it could be the +name of a variable. Then, when the parser is expecting the beginning of a +new statement and the next token is the word <CODE>if</CODE>, the parser doesn't +know whether it is seeing the beginning of a conditional statement such as +<CODE>if x > 0 then writeln('positive')</CODE> or the beginning of an assignment +statement such as <CODE>if := 87</CODE>. But the parser could still be deterministic. +Upon seeing the word <CODE>if</CODE>, it would enter a state (as in a finite state +machine) from which there are two exits. If the next token turned out to be +the <CODE>:=</CODE> assignment operator, the parser would follow one transition; if +the next token was a variable or constant value, the parser would choose a +different next state. + +<P>The real Pascal, though, contains no such syntactic cliffhangers. A Pascal +compiler can always tell which production rule the next token requires. +That's why the language includes keywords like <CODE>var</CODE>, <CODE> +procedure</CODE>, and <CODE>function</CODE>. For the most part, you could figure out +which kind of declaration you're reading without those keywords by looking +for clues like whether or not there are parentheses after the identifier +being declared. (If so, it's a procedure or a function.) But the keywords +let you know from the beginning what to expect next. That means we can +write what's called a <EM>predictive grammar</EM> for Pascal, +even simpler to implement than a deterministic one. + +<P>There are general algorithms for parsing deterministic languages, and there +are parser generators using these algorithms. One widely used example is +the YACC (Yet Another Compiler Compiler) program that translates +production rules into a parser in the C programming language.<SUP>*</SUP> But because Pascal's +grammar is so simple I found it just as easy to do the translation by hand. +For each production rule in a formal description of Pascal, the compiler +includes a Logo procedure that parses each component part of the production +rule. A parser written in this way is called a <EM> +recursive descent parser.</EM> Here's a sample: + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>A parser +generator is also called a <EM>compiler compiler</EM> because it treats +the formal specification as a kind of source program and produces a compiler +as the object program. But the name isn't quite accurate because, as you +know, there's more to a compiler than the parser.</SMALL></BLOCKQUOTE></SMALL><P><PRE>to statement +local [token type] +ifbe "begin [compound stop] +ifbe "for [pfor stop] +ifbe "if [pif stop] +ifbe "while [pwhile stop] +ifbe "repeat [prepeat stop] +ifbe "write [pwrite stop] +ifbe "writeln [pwriteln stop] +make "token token +make "peektoken :token +if memberp :token [|;| end until] [stop] +make "type gettype :token +if emptyp :type [(throw "error sentence :token [can't begin statement])] +if equalp :type "procedure [pproccall stop] +if equalp :type "function [pfunset stop] +passign +end + +to pif +local [cond elsetag endtag] +make "cond pboolean pexpr +make "elsetag gensym +make "endtag gensym +mustbe "then +code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +statement +code (list "jump (word "" :endtag)) +code :elsetag +ifbe "else [statement] +code :endtag +end +</PRE> + +<P>Many of the details of <CODE>pif</CODE> have to do with code +generation, but never mind those parts now. For the +moment, my concern is with the parsing aspect of these procedures: how they +decide what to accept. + +<P><CODE>Statement</CODE> is an important part of the parser; it is invoked whenever a +Pascal statement is expected. It begins by checking the next token from the +source file. If that token is <CODE>begin</CODE>, <CODE>for</CODE>, <CODE>if</CODE>, <CODE>while</CODE>, +or <CODE>repeat</CODE> then we're finished with the token and <CODE>statement</CODE> turns +to a subprocedure to handle the syntax of whatever structured statement type +we've found. If the token isn't one of those, then the statement has to be +a simple statement and the token has to be an identifier, i.e., the name of +a procedure, a function, or a variable. (One other trivial possibility is +that this is an <EM>empty</EM> statement, if we're already up to the +semicolon, <CODE>end</CODE>, or <CODE>until</CODE> that marks the end of a statement.) +In any of these cases, the token we've just read is important to the parsing +procedure that will handle the simple statement, so <CODE>statement</CODE> un-reads +it before deciding what to do next. <CODE>Gettype</CODE> outputs the type of the +identifier, either a variable type like <CODE>real</CODE> or else <CODE>procedure</CODE> +or <CODE>function</CODE>. (The compiler data structures that underlie the work of +<CODE>gettype</CODE> will be discussed later.) If the token is a +procedure name, then this is a procedure call statement. If the token is a +function name, then this is the special kind of assignment inside a function +definition that provides the return value for the function. Otherwise, the +token must be a variable name and this is an ordinary assignment statement. + +<P>The procedure <CODE>pif</CODE> parses <CODE>if</CODE> statements. (The letter <CODE>p</CODE> in +its name stands for "Pascal"; many procedures in the compiler have such +names to avoid conflicts with Logo procedures with similar purposes.) The +syntax of Pascal <CODE>if</CODE> is + +<P><PRE>ifstatement: if <EM>boolean</EM> then <EM>statement</EM> | + if <EM>boolean</EM> then <EM>statement</EM> else <EM>statement</EM> +</PRE> + +<P>When <CODE>pif</CODE> begins, the token <CODE>if</CODE> has just been read by +<CODE>statement</CODE>. So the first thing that's required is a boolean expression. +<CODE>Pexpr</CODE> parses an expression; that task is relatively complicated and +will be discussed in more detail later. <CODE>Pboolean</CODE> ensures that the +expression just parsed does indeed produce a value of type <CODE>boolean</CODE>. + +<P>The next token in the source file <EM>must</EM> be the word <CODE>then</CODE>. The +instruction + +<P><PRE>mustbe "then +</PRE> + +<P>in <CODE>pif</CODE> ensures that. Here's <CODE>mustbe</CODE>: + +<P><PRE>to mustbe :wanted +localmake "token token +if equalp :token :wanted [stop] +(throw "error (sentence "expected :wanted "got :token)) +end +</PRE> + +<P>If <CODE>mustbe</CODE> returns successfully, <CODE>pif</CODE> then invokes +<CODE>statement</CODE> recursively to parse the true branch of the conditional. +The production rule tells us that there is then an <EM>optional</EM> false +branch, signaled by the reserved word <CODE>else</CODE>. The instruction + +<P><PRE>ifbe "else [statement] +</PRE> + +<P>handles that possibility. If the next token matches the first +input to <CODE>ifbe</CODE> then the second input, an instruction list, is carried +out. Otherwise the token is un-read. There is also an <CODE>ifbeelse</CODE> that +takes a third input, an instruction list to be carried out if the next token +isn't equal to the first input. (<CODE>Ifbeelse</CODE> still un-reads the token in +that case, before it runs the third input.) These must be macros so that +the instruction list inputs can include <CODE>output</CODE> or <CODE>stop</CODE> +instructions (as discussed in Volume 2), as in the invocations of <CODE>ifbe</CODE> +in <CODE>statement</CODE> seen a moment ago. + +<P><PRE>.macro ifbe :wanted :action +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output [] +end + +.macro ifbeelse :wanted :action :else +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output :else +end +</PRE> + +<P>If there were no code generation involved, <CODE>pif</CODE> would be written this +way: + +<P><PRE>to pif +pboolean pexpr +mustbe "then +statement +ifbe "else [statement] +end +</PRE> + +<P>This simplified procedure is a straightforward translation of the +RTN + +<P><CENTER><IMG SRC="pif.gif" ALT="figure: pif"></CENTER> + +<P>The need to generate object code complicates the parser. But +don't let that distract you; in general you can see the formal structure of +Pascal syntax reflected in the sequence of instructions used to parse that +syntax. + +<P>The procedures that handle other structured statements, such as <CODE>pfor</CODE> +and <CODE>pwhile</CODE>, are a lot like <CODE>pif</CODE>. Procedure and function +declarations (procedures <CODE>procedure</CODE>, <CODE>function</CODE>, and <CODE>proc1</CODE> in +the compiler) also use the same straightforward parsing technique, but are a +little more complicated because of the need to keep track of type +declarations for each procedure's parameters and local variables. +Ironically, the hardest thing to compile is the "simple" assignment +statement, partly because of operator precedence (multiplication before +addition) in expressions (procedure <CODE>pexpr</CODE> in the compiler) and partly +because of the need to deal with the complexity of variables, including +special cases such as assignments to <CODE>var</CODE> parameters and array elements. + +<P>I haven't yet showed you <CODE>pboolean</CODE> because you have to understand how +the compiler handles expressions first. But it's worth noticing that Pascal +can check <EM>at compile time</EM> whether or not an expression is going to +produce a <CODE>boolean</CODE> value even though the program hasn't been run yet +and the variables in the expression don't have values yet. It's the strict +variable typing of Pascal that makes this compile-time checking possible. +If we were writing a Logo compiler, the checking would have to be postponed +until run time because you can't, in general, know what type of datum will +be computed by a Logo expression until it's actually evaluated. + +<P><H2>Expressions and Precedence</H2> + +<P> + +<P>Arithmetic or boolean expressions appear not only on the right side of +assignment statements but also as actual parameters, array index values, and +as "phrases" in structured statements. One of the classic problems in +compiler construction is the translation of these expressions to executable +form. The interesting difficulty concerns <EM> +operator precedence</EM>--the rule that in a string of alternating +operators and operands, multiplications are done before additions, so + +<P><PRE>a + b * c + d +</PRE> + +<P>means + +<P><PRE>a + (b * c) + d +</PRE> + +<P>Pascal has four levels of operator precedence. The highest level, number 4, + +is the <EM>unary</EM> operators <CODE>+</CODE>, <CODE>-</CODE>, and <CODE>not</CODE>. (The first + +two can be used as unary operators (<CODE>-3</CODE>) or <EM>binary</EM> ones (<CODE> +6-3</CODE>); it's only in the unary case that they have this +precedence.)<SUP>*</SUP> Then comes multiplication, division, and +logical <CODE>and</CODE> at level 3. Level 2 has binary addition, subtraction, and +<CODE>or</CODE>. And level 1 includes the relational operators like +<CODE>=</CODE>. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>It's unfortunate that the word "binary" is used in +computer science both for base-2 numbers and for two-input operations. + Kenneth Iverson, in his documentation for the +language APL, used the words <EM>monadic</EM> and <EM> +dyadic</EM> instead of unary and binary to avoid that ambiguity. But +those terms haven't caught on.</SMALL></BLOCKQUOTE></SMALL><P> + +<P>The formalization of precedence could be done using the mechanisms we've +already seen. For example, here is a production rule grammar for +expressions using only the four basic arithmetic operations. + +<P><PRE>expression: <EM>term</EM> | <EM>expression</EM> + <EM>term</EM> | <EM>expression</EM> - <EM>term</EM> +term: <EM>factor</EM> | <EM>term</EM> * <EM>factor</EM> | <EM>term</EM> / <EM>factor</EM> +factor: <EM>variable</EM> | <EM>number</EM> | ( <EM>expression</EM> ) +</PRE> + +<P>This grammar also introduces into the discussion the fact that the +precedence of operations can be changed by using parentheses. + +<P>This grammar, although formally correct, is not so easy to use in a +recursive descent parser. +One subtle but important problem is that it's <EM>left recursive:</EM> Some +of the alternative forms for an <CODE>expression</CODE> start with an <CODE> +expression</CODE>. If we tried to translate this into a Logo procedure it would +naturally start out + +<P><PRE>to expression +local [left op right] +make "left expression +ifbe "+ + [make "op "+ + make "right term] + [ifbe "- + [make "op "- + make "right term] + [make "op []] ] +... +</PRE> + +<P>But this procedure will never get past the first <CODE>make</CODE>; it's +an infinite loop. It will never actually read a token from the source file; +instead it keeps invoking itself recursively. + +<P>Left association is a problem for automatic compiler compilers, too. There +are ways to solve the problem but I don't want to get into that because in +fact arithmetic expressions are generally handled by an entirely different +scheme, which I'll show you in a moment. The problem wouldn't come up if +the order of the operands were reversed, so the rules said + +<P><PRE>expression: <EM>term</EM> | <EM>term</EM> + <EM>expression</EM> | <EM>term</EM> - <EM>expression</EM> +</PRE> + +<P>and so on. Unfortunately this changes the meaning, and the rules +of Pascal say that equal-precedence operations are performed left to right. + +<P>In any case, the formalization of precedence with production rules gets more +complicated as the number of levels of precedence increases. I showed you a +grammar with two levels. Pascal, with four levels, might reasonably be done +in a similar way, but think about the C programming language, which has 15 +levels of precedence! + +<P><H2>The Two-Stack Algorithm for Expressions</H2> + + +<P>What we're after is an algorithm that will allow the compiler to read an +expression once, left to right, and group operators and operands correctly. +The algorithm involves the use of two stacks, one for operations and one for +data. For each operation we need to know whether it's unary or binary and +what its precedence level is. I'll use the notation "<CODE>*</CODE><SUB><SMALL>2,3</SMALL></SUB>" to +represent binary <CODE>*</CODE> at precedence level 3. So the expression + +<P><PRE>a + b * - c - d +</PRE> + +<P>will be represented in this algorithm as + +<P> +<IMG SRC="vdash.gif"><SUB><SMALL>0</SMALL></SUB> a +<SUB><SMALL>2,2</SMALL></SUB> b *<SUB><SMALL>2,3</SMALL></SUB> -<SUB><SMALL>1,4</SMALL></SUB> c -<SUB><SMALL>2,2</SMALL></SUB> d <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"><SUB><SMALL>0</SMALL></SUB> + + +<P>The symbols <IMG SRC="vdash.gif"> and <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"> aren't really part of the source +expression; they're imaginary markers for the beginning and end of the +expression. When we read a token that doesn't make sense as part of an +expression, we can un-read that token and pretend we read a <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"> instead. +These markers are given precedence level zero because they form a boundary +for <EM>any</EM> operators inside them, just as a low-precedence operator like +<CODE>+</CODE> is a boundary for the operands of a higher-precedence operator like +<CODE>*</CODE>. (For the same reason, you'll see that parentheses are considered +precedence zero.) + +<P>The two minus signs in this expression have two different meanings. As you +read the following algorithm description, you'll see how the algorithm knows +whether an operation symbol is unary or binary. + +<P><STRONG>Step 1.</STRONG> We initialize the two stacks this way: + +<P><PRE>operation: [ <IMG SRC="vdash.gif"><SUB><SMALL>0</SMALL></SUB> ] + +data: [ ] +</PRE> + +<P><STRONG>Step 2.</STRONG> We are now expecting a datum, such as a variable. Read a +token. If it's an operation, it must be unary; subscript it accordingly and +go to step 4. If it's a datum, push it onto the data stack. (If it's +neither an operation nor a datum, something's wrong.) + +<P><STRONG>Step 3.</STRONG> We are now expecting a binary operation. Read a token. If +it's an operation, subscript it as binary and go to step 4. If not, we've +reached the end of the expression. Un-read the token, and go to step 4 with +the token <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"><SUB><SMALL>0</SMALL></SUB>. + +<P><STRONG>Step 4.</STRONG> We have an operation in hand at this point and we know its +precedence level and how many arguments it needs. Compare its precedence +level with that of the topmost (most recently pushed) operation on the stack. +If the precedence of the new operation is less than or equal to that of the +one on the stack, go to step 5. If it's greater, go to step 6. + +<P><STRONG>Step 5.</STRONG> The topmost operation on the stack has higher precedence than +the one we just read, so we should do it right away. (For example, we've +just read the <CODE>+</CODE> in <CODE>a*b+c</CODE>; the multiplication operation and both +of its operands are ready on the stacks.) Pop the operation off +the stack, pop either one or two items off the data stack depending on the +first subscript of the popped operation, then compile machine instructions to +perform the indicated computation. Push the result on the data stack +as a single quantity. However, if the operation we popped is <IMG SRC="vdash.gif">, then +we're finished. There should be only one thing on the data stack, and it's +the completely compiled expression. Otherwise, we still have the new +operation waiting to be processed, so return to step 4. + +<P><STRONG>Step 6.</STRONG> The topmost operation on the stack has lower precedence than +the one we just read, so we can't do it yet because we're still reading its +right operand. (For example, we've just read the <CODE>*</CODE> in <CODE>a+b*c</CODE>; +we're not ready to do either operation until we read the <CODE>c</CODE> later.) +Push the new operation onto the operation stack, then return to step 2. + +<P>Here's how this algorithm works out with the sample expression above. In +the data stack, a boxed entry like <IMG SRC="a+b.gif"> means the result from +translating that subexpression into the object language. + +<P><IMG SRC="stack1.gif"> +<BR><IMG SRC="stack2.gif"> + +<P>The final value on the data stack is the translation of the +entire expression. + +<P>The algorithm so far does not deal with parentheses. They're handled +somewhat like operations, but with slightly different rules. A left +parenthesis is stored on the operation stack as <CODE>(</CODE><SUB><SMALL>0</SMALL></SUB>, like the +special marker at the beginning of the expression, but it does not invoke +step 5 of the algorithm before being pushed on the stack. A right +parenthesis <EM>does</EM> invoke step 5, but only as far down the stack as +the first matching left parenthesis; if it were an ordinary operation of +precedence zero it would pop everything off the stack. You might try to +express precisely how to modify the algorithm to allow for parentheses. + +<P>Here are the procedures that embody this algorithm in the compiler. +<CODE>Pgetunary</CODE> and <CODE>pgetbinary</CODE> output a list like + +<P><PRE>[sub 2 2] +</PRE> + +<P>for binary <CODE>-</CODE> or + +<P><PRE>[minus 1 4] +</PRE> + +<P>for unary minus. (I'm leaving out some complications +having to do with type checking.) They work by looking for a <CODE>unary</CODE> or +<CODE>binary</CODE> property on the property list of the operation symbol. +Procedures with names like <CODE>op.prec</CODE> are selectors for the members +of these lists. + +<P>In this algorithm, only step 5 actually generates any instructions in the +object program. This is the step in which an operation is removed from +the operation stack and actually performed. Step 5 is carried out by the +procedure <CODE>ppopop</CODE> (Pascal pop operation); most of that procedure deals +with code generation, but I've omitted that part of the procedure in the +following listing because right now we're concerned with the parsing +algorithm. We'll return to code generation shortly. + +<P><CODE>Pexpr1</CODE> invokes <CODE>pdata</CODE> when it expects to read an operand, which +could be a number, a variable, or a function call. <CODE>Pdata</CODE>, which I'm +not showing here, generates code to make the operand available and +outputs the location of the result in the simulated computer, in a form +that can be used by <CODE>pexpr</CODE>. + +<P><PRE>to pexpr +local [opstack datastack parenlevel] +make "opstack [[popen 1 0]] ; step 1 +make "datastack [] +make "parenlevel 0 +output pexpr1 +end + +to pexpr1 +local [token op] +make "token token ; step 2 +while [equalp :token "|(|] [popen make "token token] +make "op pgetunary :token +if not emptyp :op [output pexprop :op] +push "datastack pdata :token +make "token token ; step 3 +while [and (:parenlevel > 0) (equalp :token "|)| )] + [pclose make "token token] +make "op pgetbinary :token +if not emptyp :op [output pexprop :op] +make "peektoken :token +pclose +if not emptyp :opstack [(throw "error [too many operators])] +if not emptyp butfirst :datastack [(throw "error [too many operands])] +output pop "datastack +end + +to pexprop :op ; step 4 +while [(op.prec :op) < (1 + op.prec first :opstack)] [ppopop] +push "opstack :op ; step 6 +output pexpr1 +end + +to ppopop ; step 5 +local [op function args left right type reg] +make "op pop "opstack +make "function op.instr :op +if equalp :function "plus [stop] +make "args op.nargs :op +make "right pop "datastack +make "left (ifelse equalp :args 2 [pop "datastack] [[[] []]]) +make "type pnewtype :op exp.type :left exp.type :right +... code generation omitted ... +push "datastack (list :type "register :reg) +end + +to popen +push "opstack [popen 1 0] +make "parenlevel :parenlevel+1 +end + +to pclose +while [(op.prec first :opstack) > 0] [ppopop] +ignore pop "opstack +make "parenlevel :parenlevel - 1 +end +</PRE> + +<P><H2>The Simulated Machine</H2> + +<P>We're ready to move from parsing to code generation, but first you must +understand what a computer's native language is like. Most computer models +in use today have a very similar structure, although there are differences +in details. My simulated computer design makes these detail choices in +favor of simplicity rather than efficiency. (It wouldn't be very efficient +no matter what, compared to real computers. This "computer" is actually an +interpreter, written in Logo, which is itself an interpreter. So we have +two levels of interpretation involved in each simulated instruction, whereas +on a real computer, each instruction is carried out directly by the +hardware. Our compiled Pascal programs, as you've probably already noticed, +run very slowly. That's not Pascal's fault, and it's not even primarily my +compiler's fault, even though the compiler doesn't include optimization +techniques. The main slowdown is in the interpretation of the machine +instructions.) + +<P>Every computer includes a <EM>processor,</EM> which decodes +instructions and carries out the indicated arithmetic operations, and a +<EM>memory,</EM> in which information (such as the values of +variables) is stored. In modern computers, the processor is generally +a single <EM>integrated circuit,</EM> nicknamed a <EM>chip,</EM> +which is a rectangular black plastic housing one or two inches on a side +that contains thousands or even millions of tiny components made of silicon. +The memory is usually a <EM>circuit board</EM> containing several memory +chips. Computers also include circuitry to connect with input and output +devices, but we're not going to have to think about those. What makes one +computer model different from another is mainly the processor. If you +have a PC, its processor is probably an Intel design with a name like +80486 or Pentium; if you have a Macintosh, the processor might be a Motorola +68040 or a Power PC chip. + +<P>It turns out that the wiring connecting the processor to the memory is +often the main limiting factor on the speed of a computer. Things happen at +great speed within the processor, and within the memory, but only one value +at a time can travel from one to the other. Computer designers have invented +several ways to get around this problem, but the important one for our +purposes is that every modern processor includes a little bit of memory +within the processor chip itself. By "a little bit" I mean that a typical +processor has enough memory in it to hold 32 values, compared to several +million values that can be stored in the computer's main memory. The 32 +memory slots within the processor are called <EM> +registers.</EM><SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>One current topic in computer architecture research +is the development of <EM>parallel</EM> computers with many processors working +together. In some of these designs, each processor includes its own +medium-size memory within the processor chip.</SMALL></BLOCKQUOTE></SMALL><P>Whenever you want to perform an arithmetic operation, the operands must +already be within the processor, in registers. So, for example, the Pascal +instruction + +<P><PRE>c := a + b +</PRE> + +<P>isn't compiled into a single machine instruction. First we must +<EM>load</EM> the values of <CODE>a</CODE> and <CODE>b</CODE> from memory into registers, +then add the two registers, then <EM>store</EM> the result back into memory: + +<P><PRE>rload 8 a +rload 9 b +add 10 8 9 +store 10 c +</PRE> + +<P>The first <CODE>rload</CODE> instruction loads the value from memory +location <CODE>a</CODE> into register 8.<SUP>*</SUP> The <CODE>add</CODE> instruction adds +the numbers in registers 8 and 9, putting the result into register 10. (In +practice, you'll see that the compiler would be more likely to conserve +registers by reusing one of the operand registers for the result, but for +this first example I wanted to keep things simple.) Finally we store the +result into the variable <CODE>c</CODE> in memory. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>Really I should have called this +instruction <CODE>load</CODE>, but my machine simulator uses Logo procedures to +carry out the machine instructions, and I had to pick a name that wouldn't +conflict with the Logo <CODE>load</CODE> primitive.</SMALL></BLOCKQUOTE></SMALL><P>The instructions above are actually not machine language instructions, but +rather <EM>assembly language</EM> instructions, a kind of shorthand. +A program called an <EM>assembler</EM> translates assembly language into +machine language, in which each instruction is represented as a number. +For example, if the instruction code for <CODE>add</CODE> is <CODE>0023</CODE>, then +the <CODE>add</CODE> instruction above might be translated into <CODE>0023100809</CODE>, +with four digits for the instruction code and two digits for each of the three +register numbers. (In reality the encoding would use binary numbers rather +than the decimal numbers I've shown in this example.) Since a machine +language instruction is just a number, the instructions that make up a +computer program are stored in memory along with the program's data values. +But one of the simplifications I've made in my simulated computer is that +the simulator deals directly with assembly language instructions, and those +instructions are stored in a Logo list, separate from the program's data memory. + +<P>The simulated computer has 32 processor registers plus 3000 locations of +main memory; it's a very small computer, but big enough for my sample +Pascal programs. (You can change these sizes by editing procedure <CODE> +opsetup</CODE> in the compiler.) The registers are numbered from 0 to 31, and +the memory locations are numbered from 0 to 2999. The number of a memory +location is called its <EM>address.</EM> Each memory location can hold +one numeric value.<SUP>*</SUP> A Pascal array will be represented by a contiguous block of +memory locations, one for each member of the array. Each register, too, can +hold one numeric value. In this machine, as in some real computers, register +number 0 is special; it always contains the value zero. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>This, too, is a simplification. In real computers, +different data types require different amounts of memory. A character +value, for example, fits into eight <EM>bits</EM> (binary digits) of memory, +whereas an integer requires 32 bits in most current computers. Instead of a +single <CODE>load</CODE> instruction, a real computer has a separate one for each +datum size.</SMALL></BLOCKQUOTE></SMALL><P>The simulated computer understands 50 instruction codes, fewer than most +real computers. The first group we'll consider are the 14 binary arithmetic +instructions: <CODE>add</CODE>, <CODE>sub</CODE>, <CODE>mul</CODE>, <CODE>div</CODE> (real quotient), +<CODE>quo</CODE> (integer quotient), <CODE>rem</CODE> (remainder), <CODE>land</CODE> (logical +and), <CODE>lor</CODE> (logical or), <CODE>eql</CODE> (compare two operands for equality), +<CODE>neq</CODE> (not equal), <CODE>less</CODE>, <CODE>gtr</CODE> (greater than), <CODE>leq</CODE> (less +than or equal), and <CODE>geq</CODE> (greater than or equal). The result of each +of the six comparison operators is <CODE>0</CODE> for false or <CODE>1</CODE> for true. +The machine also has four unary arithmetic instructions: <CODE>lnot</CODE> +(logical not), <CODE>sint</CODE> (truncate to integer), <CODE>sround</CODE> (round to +integer), and <CODE>srandom</CODE>. Each of these 18 arithmetic instructions takes +its operands from registers and puts its result into a register. + +<P>All but the last three of these are typical instructions of real +computers.<SUP>*</SUP> +(Not every computer has all of them; for example, if a computer has <CODE>eql</CODE> +and <CODE>lnot</CODE>, then it doesn't really need a <CODE>neq</CODE> instruction because +the same value can be computed by a sequence of two instructions.) The +operations <CODE>sint</CODE>, <CODE>sround</CODE>, and <CODE>srandom</CODE> are less likely to be +machine instructions on actual computers. On the other hand, most real +computers have a <EM>system call</EM> mechanism, which is a machine +instruction that switches the computer from the user's program to a part of +the operating system that performs some task on behalf of the user. System +calls are used mainly for input and output, but we can pretend that there are +system calls to compute these Pascal library functions. (The letter <CODE>s</CODE> +in the instruction names stands for "system call" to remind us.) + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>One important simplification is that in the simulated +computer, the same instructions are used for all kinds of numbers. A typical +computer has three <CODE>add</CODE> instructions: one for integers, one for short +reals (32 bits), and one for long reals (64 bits).</SMALL></BLOCKQUOTE></SMALL><P>The simulated computer also has another set of 18 <EM>immediate</EM> +instructions, with the letter <CODE>i</CODE> added to the instruction name: <CODE> +addi</CODE>, <CODE>subi</CODE>, and so on. In these instructions, the rightmost operand +in the instruction is the actual value desired, rather than the number of +a register containing the operand. For example, + +<P><PRE>add 10 8 9 +</PRE> + +<P>means, "add the number in register 8 and the number in register 9, +putting the result into register 10." But + +<P><PRE>addi 10 8 9 +</PRE> + +<P>means, "add the number in register 8 to the value 9, putting +the result in register 10." + +<P>It's only the right operand that can be made immediate. So, for example, +the Pascal assignment + +<P><PRE>y := x - 5 +</PRE> + +<P>can be translated into + +<P><PRE>rload 8 x +subi 8 8 5 +store 8 y +</PRE> + +<P>but the Pascal assignment + +<P><PRE>y := 5 - x +</PRE> + +<P>must be translated as + +<P><PRE>addi 8 0 5 +rload 9 x +sub 8 8 9 +store 8 y +</PRE> + +<P>This example illustrates one situation in which it's useful to +have register 0 guaranteed to contain the value 0. + +<P>Our simulated machine has six more system call instructions having to do +with printing results. One of them, <CODE>newline</CODE>, uses no operands and +simply prints a newline character, moving to the beginning of a new line +on the screen. Four more are for printing the value in a register; the +instruction used depends on the data type of the value in the register. +The instructions are <CODE>putch</CODE> for a character, <CODE>puttf</CODE> for a boolean +(true or false) value, <CODE>putint</CODE> for an integer, and <CODE>putreal</CODE> for a +real number. Each takes two operands; the first, an immediate value, gives +the minimum width in which to print the value, and the second is a register +number. So the instruction + +<P><PRE>putint 10 8 +</PRE> + +<P>means, "print the integer value in register 8, using at least +10 character positions on the line." The sixth printing instruction, +<CODE>putstr</CODE>, is used only for constant character strings in the Pascal +program; its first operand is a width, as for the others, but its second +is a Logo list containing the string to print: + +<P><PRE>putstr 1 [The shuffled deck:] +</PRE> + +<P>This is, of course, unrealistic; in a real computer the second +operand would have to be the memory address of the beginning of the +array of characters to print. But the way I handle printing isn't very +realistic in any case; I wanted to do the simplest possible thing, because +worrying about printing really doesn't add anything to your understanding +of the process of compilation, which is the point of this chapter. + +<P>The next group of instructions has to do with the flow of control in the +computer program. Ordinarily the computer carries out its instructions +in sequence, that is, in the order in which they appear in the program. +But in order to implement conditionals (such as <CODE>if</CODE>), loops (such as +<CODE>while</CODE>), and procedure calls, we must be able to jump out of sequence. +The <CODE>jump</CODE> instruction takes a single operand, a <EM>label</EM> +that appears somewhere in the program. When the computer carries out a jump +instruction, it looks for the specified label and starts reading instructions +just after where that label appears in the program. (We saw an example of +labels at the beginning of this chapter.) + +<P>The <CODE>jump</CODE> instruction is used for unconditional jumps. In order to +implement conditionals and loops, we need a way to jump if some condition +is true. The instruction <CODE>jumpt</CODE> (jump if true) has two operands, +a register number and a label. It jumps to the specified label if and only +if the given register contains a true value. (Since registers hold only +numbers, we use the value 1 to represent true, and 0 to represent false.) +Similarly, <CODE>jumpf</CODE> jumps if the value in the given register is false. + +<P>For procedure and function calls, we need a different mechanism. The jump +is unconditional, but the computer must remember where it came from, so that +it can continue where it left off once the called procedure or function +returns. The instruction <CODE>jal</CODE> (jump and link) takes two operands, a +register and a label. It puts into the register the address of the +instruction following the <CODE>jal</CODE> instruction.<SUP>*</SUP> Then it jumps to the specified label. +To return from the called procedure, we use the <CODE>jr</CODE> (jump register) +instruction. It has one operand, a register number; it jumps to the +instruction whose address is in the register. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>In a real computer, +each instruction is stored in a particular memory location, so the address +of an instruction is the address of the memory location in which it's stored. +In this simulated computer, I keep the program in the form of a Logo list, +and so I cheat and put the sublist starting at the next instruction into +the register. This isn't quite as much of a cheat as it may seem, though, +since you know from Chapter 3 that Logo represents a list with the memory +address of the first pair of the list.</SMALL></BLOCKQUOTE></SMALL><P>One final instruction that affects the flow of control is the <CODE>exit</CODE> +system call. It requires no operands; it terminates the running of the +program. In this simulated computer, it returns to a Logo prompt; in a +real computer, the operating system would start running another user program. + +<P>The only remaining instructions are <CODE>rload</CODE> and <CODE>store</CODE>. You already +know what these do, but I've been showing them in oversimplified form so far. +The second operand can't just be a variable name, because that variable might +not be in the same place in memory every time the procedure is called. Think, +for example, about a recursive procedure. Several invocations may be in +progress at once, all of them carrying out the same compiled instructions, +but each referring to a separate set of local variables. The solution to +this problem is that the compiler arranges to load into a register +the address of a block of memory containing all the local variables for a +given procedure call. If the variable <CODE>c</CODE>, for example, is in the sixth +memory location of that block, an instruction to load or store that variable +must be able to say "the memory location whose address is the contents of +register 4 (let's say) plus five." So each load and store instruction +contains an <EM>index register</EM> in parentheses following an +<EM>offset</EM> to be added to the contents of that register. We'd say + +<P><PRE>store 8 5(4) +</PRE> + +<P>to store the contents of register 8 into the variable <CODE>c</CODE>, +provided that register 4 points to the correct procedure invocation's +local variables and that <CODE>c</CODE> is in the sixth position in the block. +(The first position in the block would have offset 0, and so on.) + +<P><H2>Stack Frames</H2> + +<P>The first step in invoking a procedure or function is to set aside, or +<EM>allocate,</EM> a block of memory locations for use by that invocation. +This block will include the procedure's local variables, its arguments, and +room to save the values of registers as needed. The compiler's data +structures include, for each procedure, how much memory that procedure needs +when it's invoked. That block of memory is called a <EM>frame.</EM> + +<P>In most programming languages, including Pascal and Logo (but not, as it +turns out, Lisp), the frame allocated when a procedure invocation begins can +be released, or <EM>deallocated,</EM> when that invocation returns to its +caller. In other words, the procedure's local variables no longer exist +once the invocation is finished. In these languages, the frames for all the +active procedure invocations can be viewed as a <EM>stack,</EM> a data structure +to which new elements are added by a Push operation, and elements are removed +using a Pop operation that removes the most recently pushed element. (In +this case, the elements are the frames.) That is, suppose that procedure A +invokes B, which invokes C, which invokes D. For each of these invocations +a new frame is pushed onto the stack. Which procedure finishes first? It +has to be D, the last one invoked. When D returns, its frame can be popped +off the stack. Procedure C returns next, and its frame is popped, and so on. +The phrase <EM>stack frame</EM> is used to refer to frames that +behave like elements of a stack. + +<P>My Pascal compiler allocates memory starting at location 0 and working +upward. At the beginning of the program, a <EM>global frame</EM> is allocated +to hold the program's global variables. Register 3, the +<EM>global pointer,</EM> always contains the +address of the beginning of the global frame, so that every procedure can +easily make use of global variables. (Since the global frame is the first +thing in memory, its address is always zero, so the value in register 3 is +always 0. But in a more realistic implementation the program itself would +appear in memory before the global frame, so its address would be greater +than zero.) + +<P>At any point in the program, register 4, the <EM>frame pointer,</EM> +contains the address of the beginning of the current frame, that is, the +frame that was created for the current procedure invocation. Register 2, +the <EM>stack pointer,</EM> contains the address of the first +currently unused location in memory. + +<P>My compiler is a little unusual in that when a procedure is called, the +stack frame for the new invocation is allocated by the caller, not by the +called procedure. This simplifies things because the procedure's arguments +can be stored in its own frame; if each procedure allocates its own frame, +then the caller must store argument values in its (the caller's) frame, +because the callee's frame doesn't exist yet. So, in my compiler, the +first step in a procedure call is to set register 5, the <EM>new frame +pointer,</EM> to point to the first free memory location, and change the +stack pointer to allocate the needed space. If <CODE>N</CODE> memory locations +are needed for the new frame, the calling procedure will contain the +following instructions: + +<P><PRE>add 5 2 0 +addi 2 2 N +</PRE> + +<P>The first instruction copies the value from register 2 (the +first free memory location) into register 5; the second adds <CODE>N</CODE> to +register 2. (I've left out a complication, which is that the old value +in register 5 must be saved somewhere before putting this new value +into it. You can read the code generation instructions at the beginning +of <CODE>pproccall1</CODE>, in the program listing at the end of the chapter, +for all the details.) The current frame pointer is also saved in +location 3 of the new frame: + +<P><PRE>store 4 3(5) +</PRE> + +<P>The compiler uses data abstraction to refer to these register +numbers and frame slots; for example, the procedure <CODE>reg.frameptr</CODE> takes +no arguments and always outputs 4, while <CODE>frame.prevframe</CODE> outputs 3. + +<P>The next step is to put the argument values into the new frame. During +this process, the calling procedure must use register 4 to refer to its own +variables, and register 5 to refer to the callee's variables. The final +step, just before calling the procedure, is to make the +frame pointer (register 4) point to the new frame: + +<P><PRE>add 4 5 0 +</PRE> + +<P>Once the caller has set up the new frame and saved the necessary registers, +it can call the desired procedure, putting the return address in register 1: + +<P><PRE>jal 1 "proclabel +</PRE> + +<P>The first step in the called procedure is to save the return +address in location zero of its frame: + +<P><PRE>store 1 0(4) +</PRE> + +<P>The procedure then carries out the instructions in its body. When it's +ready to return, it must load the saved return address back into register +1, then restore the old stack pointer and frame pointer to deallocate +its frame, and finally return to the caller: + +<P><PRE>rload 1 0(4) +add 2 4 0 +rload 4 3(2) +jr 1 +</PRE> + +<P>(Procedure <CODE>proc1</CODE> in the compiler generates these +instructions for each procedure.) + +<P>One final complication about stack frames comes from Pascal's block +structure. Suppose we have a program with internal procedures arranged +in this structure: + +<P><CENTER><IMG SRC="blocks.gif" ALT="figure: blocks"></CENTER> + +<P>Then suppose that the main program calls procedure A, which +calls B, which calls C, which calls itself recursively. The current (inner) +invocation of C has access to its own variables, those of procedure A, and +the global variables, but not to procedure B's variables. How does +procedure C know where procedure A's stack frame is located? The answer is +that every frame, in addition to saving a pointer to the previous frame, +must include a pointer to the <EM>lexically enclosing</EM> frame. The +calling procedure sets this up; it can do this because it knows its own +lexical depth and that of the called procedure. For example, when procedure +B calls procedure C, C's lexically enclosing frame will be the same as B's +(namely, the frame for the invocation of A), because B and C are at the +same lexical depth. (They are both declared inside A.) But when procedure +A calls procedure B, which is declared within itself, A must store its own +frame pointer as B's lexically enclosing frame. Here is a picture of what's +where in memory: + +<P><CENTER><IMG SRC="memory.gif" ALT="figure: memory"></CENTER> + +<P>If all these pointers between frames confuse you, it might help to keep in +mind that the two kinds of pointers have very different purposes. The +pointer to the previous frame is used only when a procedure returns, to +help in putting everything back the way it was before the procedure was +called (in particular, restoring the old value of register 4). The pointer +to the lexically enclosing frame is used while the procedure is running, +whenever the procedure makes reference to a variable that belongs to some +outer procedure (for example, a reference in procedure B or C to a variable +that belongs to procedure A).<SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>If procedures used the previous-frame +pointers to make variable references, we would be compiling a dynamically +scoped language! In this example, because Pascal is lexically scoped, +procedure C can't refer to procedure B's variables, even though B called C.</SMALL></BLOCKQUOTE></SMALL><P><H2>Data Structures</H2> + +<P>In this section I'll describe the main data structures used during +compilation (abstract data types for identifiers and for expressions) +and during the running of the program (registers and frames). + +<P>The main body of information that the compiler must maintain is the list of +Pascal identifiers (variable, procedure, and function names). Since Pascal +is lexically scoped, some attention is necessary to ensure that each compiled +Pascal procedure has access to precisely the variables that it should. +At any point during the compilation, the value of <CODE>:idlist</CODE> is a list of +just those identifiers that may be used in the part of the program being +compiled. We'll see in a moment how that's accomplished. + +<P>There are two main categories of identifier: procedure names (including the +main program and functions in this category) and variable names. The +information maintained for a procedure name looks like this example: + +<P><PRE>[myproc procedure %myproc [2 46]] +</PRE> + +<P>The first member of this list is the Pascal name of the program, +procedure, or function. The second member is the type indicator, which +will be one of the words <CODE>program</CODE>, <CODE>procedure</CODE>, or <CODE>function</CODE>. +The third member is the procedure's "Logo name," the unique name used within +the compiler to represent this program or procedure. The program's Logo name +is used as the variable name whose value will be the compiled program; the +Logo names for procedures and functions are used as the labels in the +compiled program at which each procedure or function begins. The fourth +member of the list contains the frame information for the procedure; it's +a list of two numbers, the lexical depth and the frame size. The lexical +depth is 0 for the main program, 1 for a procedure declared inside the +main program, 2 for a procedure declared inside a depth-1 procedure, and +so on. The frame size indicates how many memory locations must be allocated +for each invocation of the procedure. (For the main program, the frame size +indicates the size of the global frame.) + +<P>Because of the Pascal scope rules, there can be two procedures with the same +name, each declared within a different region of the program. But there is +no scoping of labels in the compiled program; each label must be unique. +The simplest solution would be to use a distinct program-generated name for +every Pascal procedure; the Pascal <CODE>doit</CODE> would become the +Logo <CODE>g14</CODE>. In fact I chose to modify this approach somewhat. When an +identifier <CODE>symbol</CODE> is declared in the source program, the compiler +looks to see whether another identifier with the same name has appeared +anywhere in the program. If not, the Logo name <CODE>%symbol</CODE> is used; if +so, a generated symbol is used. This rule makes the compiled +program a little easier to read, while preserving the rule that all Logo +names must be unique. The percent sign in <CODE>%symbol</CODE> ensures that this +Logo name doesn't conflict with any names used in the +compiler itself. Procedure <CODE>newlname</CODE> in the compiler takes a Pascal +identifier as input and generates a new Logo name to correspond. + +<P>The selectors <CODE>id.type</CODE>, <CODE>id.lname</CODE>, and <CODE>id.frame</CODE> are used +for the second through fourth members of these lists. There's no selector +for the first member, the Pascal name, because the compiler never +extracts this information explicitly. Instead, the Pascal name is used +by procedure <CODE>getid</CODE>, which takes a Pascal name as its input and +returns the corresponding identifier list. + +<P>For variable names, the identifier information looks a little different: + +<P><PRE>[i integer [1 41] false] +</PRE> + +<P>The first two members of this list are the Pascal name and the +type, the same as for a procedure. The third member is the <EM>pointer</EM> +information for the variable: its lexical depth and the offset within +a frame where it should be kept. The compiler will use this information +to issue instructions to load or store the value of the variable. The +fourth member of the list is <CODE>true</CODE> if this variable is a <CODE>var</CODE> +(call by reference) parameter, <CODE>false</CODE> otherwise. + +<P>The variable <CODE>i</CODE> above has a scalar type, so its type indicator is +a word. Had it been an array, the type indicator would be a list such as + +<P><PRE>[integer [0 6] [5 3]] +</PRE> + +<P>for a variable declared as <CODE>array [0..5, 5..7] of integer</CODE>. + +<P>For each dimension of the array, the first number in the list +is the smallest possible index, while the second number is the number of +possible index values in this dimension. That is, the range <CODE>[3..7]</CODE> +is represented by the list <CODE>[3 5]</CODE> because there are five possible +values starting from 3. Notice that there is no "Logo name" for a +variable; in the compiled program, a variable is represented as an +offset and an index register, such as <CODE>41(4)</CODE>. + +<P>For variables, the selectors used are <CODE>id.type</CODE>, <CODE>id.pointer</CODE>, +and <CODE>id.varp</CODE>. + +<P>The information about currently accessible identifiers is kept in the list +<CODE>idlist</CODE>. This variable holds a list of lists; each Pascal identifier +is represented by a list as indicated above. <CODE>Idlist</CODE> is a local +variable in the compiler procedures <CODE>program</CODE>, <CODE>procedure</CODE>, and <CODE> +function</CODE>. That is, there is a separate version for each block of the +Pascal source program. Each local version starts out with the same value as +the higher-level version; identifiers declared within a block are added to +the local version but not to the outer one. When the compiler finishes a +block, the (Logo) procedure in charge of that block stops and the outer <CODE> +idlist</CODE> becomes current again. + +<P>This arrangement may or may not seem strange to you. Recall that we had to +invent this <CODE>idlist</CODE> mechanism because Pascal's +lexical scope is different from Logo's dynamic scope. The reason we have +these different versions of <CODE>idlist</CODE> is to keep track of which +identifiers are lexically available to which blocks. And yet we are using +Logo's dynamic scope to determine which <CODE>idlist</CODE> is available at any +point in the compilation. The reason this works is that <EM>the +dynamic environment at compile time reflects the +lexical environment at run time.</EM> For example, in the <CODE>tower</CODE> +program, the fact that <CODE>tower</CODE> <EM>contains</EM> <CODE>hanoi</CODE>, which in +turn contains <CODE>movedisk</CODE>, is reflected in the fact that <CODE>program</CODE> +(compiling <CODE>tower</CODE>) <EM>invokes</EM> <CODE>procedure</CODE> (compiling <CODE> +hanoi</CODE>), which in turn <EM>invokes</EM> <CODE>procedure</CODE> recursively +(compiling <CODE>movedisk</CODE>). Earlier I said that lexical scope is easier for +a compiler than dynamic scope; this paragraph may help you see why that's +true. Even dynamically scoped Logo naturally falls into providing lexical +scope for a Pascal compiler. + +<P>Here is how procedure and function declarations are compiled: + +<P><PRE>to procedure +proc1 "procedure framesize.proc +end + +to function +proc1 "function framesize.fun +end + +to proc1 :proctype :framesize +localmake "procname token +localmake "lexical.depth :lexical.depth+1 +localmake "frame (list :lexical.depth 0) +push "idlist (list :procname :proctype (newlname :procname) :frame) +localmake "idlist :idlist + ... +end +</PRE> + +<P>(I'm leaving out the code generation part for now.) What I want +to be sure you understand is that the <CODE>push</CODE> instruction adds the new +procedure name to the <EM>outer</EM> <CODE>idlist</CODE>; after that, it creates a +new <CODE>idlist</CODE> whose initial value is the same as the old one. It's very +important that the instruction + +<P><PRE>localmake "idlist :idlist +</PRE> + +<P>comes where it does and not at the beginning of the procedure. +<CODE>Proc1</CODE> needs access to the outer <CODE>idlist</CODE> when it starts, and +then later it "shadows" that variable with its own local version. This +example shows that Logo's <CODE>local</CODE> command really is an executable +command and not a declaration like Pascal's <CODE>var</CODE> declaration. In +Pascal it would be unthinkable to declare a new local variable in the middle +of a block. + +<P><CODE>Getid</CODE> depends on Logo's dynamic scope to give it access to the right +version of <CODE>idlist</CODE>. Think about writing a Pascal compiler in Pascal. +There would be a large block for <CODE>program</CODE> with many other procedures +inside it. Two of those inner procedures would be the ones for <CODE> +procedure</CODE> and <CODE>function</CODE>. (Of course they couldn't have those names, +because they're Pascal reserved words. They'd be called <CODE> +compileprocedure</CODE> or some such thing. But I think this will be easier to +follow if I stick with the names used in the Logo version of the compiler.) +Those two procedures should be at the same level of block structure; neither +should be lexically within the other. That's because a Pascal procedure +block can include a function definition or vice versa. Now, where in the +lexical structure does <CODE>getid</CODE> belong? It needs access to the local +<CODE>idlist</CODE> of either <CODE>procedure</CODE> or <CODE>function</CODE>, whichever is +currently active. Similarly, things like <CODE>statement</CODE> need to be +lexically within both <CODE>procedure</CODE> and <CODE>function</CODE>, and actually also +within <CODE>program</CODE> because the outermost program block has statements too. +It would theoretically be possible to solve the problem by writing three +identical versions of each of these subprocedures, but that solution is too +horrible to contemplate. Instead a more common technique is to have only +one <CODE>idlist</CODE> variable, a global one, and write the compiler so that it +explicitly maintains a stack of old values of that variable. The Pascal +programmer has to do the work that the programming language should be doing +automatically. This is an example in which dynamic scope, while not +absolutely essential, makes the program much easier to write and more +straightforward to understand. + +<P>For every procedure or function in the Pascal source program, the compiler +creates a global Logo variable with the same name as the corresponding +label--that is, either a percent-prefix name or a generated symbol. The +value of this variable is a list of types, one for each argument to the +procedure or function. (For a function, the first member of the list is the +type of the function itself; the butfirst is the list of types of its +arguments.) The compiler examines this "type signature" variable when a +procedure or function is invoked, to make sure that the types of the actual +arguments match the types of the formal parameters. + +<P>The other important compile-time data structure is the one that represents +a compiled expression. When the compiler calls <CODE>pexpr</CODE>, its job is to +parse an expression from the Pascal source program and generate code to +compute (when the compiled program runs!) the value of the expression. + +The generated code leaves the computed value in some register. What <CODE> +pexpr</CODE> returns to its caller is a data structure indicating which register +and what type the expression has, like this: + +<P> + +<P><PRE>[real register 8] +</PRE> + +<P>The first member of this list is the type of the expression. +Most of the time, the second member is the word <CODE>register</CODE> and the +third member is the register number in which the expression's value +can be found. The only exception is for a constant expression; if +the expression is, for example, <CODE>15</CODE> then <CODE>pexpr</CODE> will output + +<P><PRE>[integer immediate 15] +</PRE> + +<P>For the most part, these immediate expressions are useful +only within recursive calls to <CODE>pexpr</CODE>. In compiling the Pascal +assignment + +<P><PRE>x := 15 +</PRE> + +<P>we're going to have to get the value 15 into a register anyway +in order to be able to store it into <CODE>x</CODE>; the generated code will be +something like + +<P><PRE>addi 7 0 15 +store 7 48(4) +</PRE> + +<P>An immediate expression is most useful in compiling something like + +<P><PRE>x := a+15 +</PRE> + +<P>in which we can avoid loading the value 15 into a register, but +can directly add it to the register containing <CODE>a</CODE>: + +<P><PRE>rload 7 53(4) +addi 7 7 15 +store 7 48(4) +</PRE> + +<P>The members of an expression list are examined using the selectors <CODE> +exp.type</CODE>, <CODE>exp.mode</CODE> (the word <CODE>register</CODE> or <CODE>immediate</CODE>), +and <CODE>exp.value</CODE> (the register number or immediate value). + +<P>In this compiler an "expression" is always a +<EM>scalar</EM> type; although the formal definition of Pascal allows for +array expressions, there are no operations that act on arrays the way +operations like <CODE>+</CODE> act on scalars, and so an array expression can only +be the name of an array variable. (<EM>Members</EM> of arrays can, of +course, be part of a scalar expression.) <CODE>Passign</CODE>, the compiler +procedure that handles assignment statements, first checks for the special +case of an array assignment and then, only if the left side of the +assignment is a scalar, invokes <CODE>pexpr</CODE> to parse a scalar expression. + +<P>In order to understand the code generated by the compiler, you should also +know about the <EM>runtime</EM> data structures used by compiled programs. +First, certain registers are reserved for special purposes: + +<P><TABLE> +<TR><TH align="right">number<TH> <TH align="left">name<TH> <TH align="left">purpose +<TR><TD> +<TR><TD align="right">0<TD><TD><CODE>reg.zero</CODE><TD><TD>always contains zero +<TR><TD align="right">1<TD><TD><CODE>reg.retaddr</CODE><TD><TD>return address from procedure call +<TR><TD align="right">2<TD><TD><CODE>reg.stackptr</CODE><TD><TD>first free memory address +<TR><TD align="right">3<TD><TD><CODE>reg.globalptr</CODE><TD><TD>address of global frame +<TR><TD align="right">4<TD><TD><CODE>reg.frameptr</CODE><TD><TD>address of current frame +<TR><TD align="right">5<TD><TD><CODE>reg.newfp</CODE><TD><TD>address of frame being made for procedure call +<TR><TD align="right">6<TD><TD><CODE>reg.retval</CODE><TD><TD>return value from function +<TR><TD align="right">7<TD><TD><CODE>reg.firstfree</CODE><TD><TD>first register available for expressions +</TABLE> + +<P>We've already seen most of these while discussing stack frames. +A Pascal function returns its result in register 6; the caller immediately +copies the return value into some other register so that it won't be +lost if the program calls another function, for a case like + +<P><PRE>x := f(3)+f(4) +</PRE> + +<P>Whenever a register is needed to hold some computed value, the compiler +calls the Logo procedure <CODE>newregister</CODE>, which finds the first register +number starting from 7 that isn't currently in use. When the value in +a register is no longer needed, the compiler calls <CODE>regfree</CODE> to indicate +that that register can be reassigned by <CODE>newregister</CODE>. + +<P>The other noteworthy runtime data structure is the use of slots within each +frame for special purposes: + +<P><TABLE> +<TR><TH align="right">number<TH> <TH align="left">name<TH> <TH align="left">purpose +<TR><TD> +<TR><TD align="right">0<TD><TD><CODE>frame.retaddr</CODE><TD><TD>address from which this procedure was called +<TR><TD align="right">1<TD><TD><CODE>frame.save.newfp</CODE><TD><TD>saved register 3 while filling this new frame +<TR><TD align="right">2<TD><TD><CODE>frame.outerframe</CODE><TD><TD>the frame lexically enclosing this one +<TR><TD align="right">3<TD><TD><CODE>frame.prevframe</CODE><TD><TD>the frame from which this one was called +<TR><TD align="right">4-35<TD><TD><CODE>frame.regsave</CODE><TD><TD>space for saving registers +<TR><TD align="right">36<TD><TD><CODE>frame.retval</CODE><TD><TD>function return value +</TABLE> + +<P>Why is there both a register and a frame slot for a function's return +value? Remember that the way you indicate the return value in a Pascal +function is by assigning to the function's name as if it were a variable. +Such an assignment is not necessarily the last instruction in the function; +it may do more work after computing the return value. The compiler notices +as assignment to the function name and generates code to save the computed +value in slot 36 of the current frame. Then, when the function actually +returns, the compiler generates the instruction + +<P><PRE>rload 6 36(4) +</PRE> + +<P>to copy the return value into register 6. The function's frame +is about to be freed, so the caller can't look there +for the return value; that's why a register is used. + +<P>Each frame includes a block of space for saving registers when another +procedure is called. That's because each procedure allocates register +numbers independently; each starts with register 7 as the first free one. +So if the registers weren't saved before a procedure call and restored +after the call, the values in the registers would be lost. (Although +the frame has enough room to save all 32 registers, to make things simple, +not all 32 are actually saved. The compiler knows which registers contain +active expression values at the moment of the procedure call, and it +generates code to save and restore only the necessary ones.) + +<P>You might think it would be easier to have each procedure use a separate +set of registers, so saving wouldn't be necessary. But this +doesn't work for two reasons. First, there are only a few registers, and +in a large program we'd run out. Even more important, the compiled +code for a <EM>recursive</EM> procedure is going to use the same registers +in each invocation, so we certainly can't avoid saving registers in that +situation. + +<P><H2>Code Generation</H2> + + +<P>Let's look again at how the compiler handles a Pascal <CODE>if</CODE> statement: + +<P><PRE>to pif +local [cond elsetag endtag] +make "cond pboolean pexpr +make "elsetag gensym +make "endtag gensym +mustbe "then +code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +statement +code (list "jump (word "" :endtag)) +code :elsetag +ifbe "else [statement] +code :endtag +end +</PRE> + +<P>I showed you this procedure while talking about parsing, asking you to +ignore the parts about code generation. Now we'll come back to that part of +the process. + +<P>The format of the <CODE>if</CODE> statement is either of these: + +<P><PRE>if <EM>condition</EM> then <EM>statement</EM> +if <EM>condition</EM> then <EM>statement</EM> else <EM>statement</EM> +</PRE> + +<P>(There is probably a semicolon after the statement, but it's +not officially part of the <CODE>if</CODE>; it's part of the compound statement +that contains the <CODE>if</CODE>.) When we get to <CODE>pif</CODE>, the compiler has +already read the token <CODE>if</CODE>; the next thing to read is an expression, +which must be of type <CODE>boolean</CODE>, providing the condition part of the +statement. + +<P>In the instruction + +<P><PRE>make "cond pboolean pexpr +</PRE> + +<P>the call to <CODE>pexpr</CODE> generates code for the expression and +returns an expression list, in the format shown earlier. The procedure <CODE> +pboolean</CODE> does three things: First, it checks the mode of the expression; +if it's immediate, the value is loaded into a register. Second, it checks +the type of the expression to ensure that it really is boolean. Third, +<CODE>pboolean</CODE> returns just the register number, which will be used in code +generated by <CODE>pif</CODE>. + +<P><PRE>to pboolean :expr [:pval noimmediate :expr] +if equalp exp.type :pval "boolean [output exp.value :pval] +(throw "error sentence exp.type :pval [not true or false]) +end + +to noimmediate :value +if equalp exp.mode :value "immediate ~ + [localmake "reg newregister + code (list "addi :reg reg.zero exp.value :value) + output (list exp.type :value "register :reg)] +output :value +end +</PRE> + +<P>Overall, the code compiled for the <CODE>if</CODE> statement will look like this: + +<P><PRE> ... get condition into register <EM>cond</EM> ... + jumpf <EM>cond</EM> "g5 + ... code for <CODE>then</CODE> statement ... + jump "g6 +g5 + ... code for <CODE>else</CODE> statement ... +g6 +</PRE> + +<P>The labels <CODE>g5</CODE> and <CODE>g6</CODE> in this example are generated +symbols; they'll be different each time. The labels are generated by the +instructions + +<P><PRE>make "elsetag gensym +make "endtag gensym +</PRE> + +<P>in <CODE>pif</CODE>. After we call <CODE>pexpr</CODE> to generate the code +for the conditional expression, we explicitly generate the <CODE>jumpf</CODE> +instruction: + +<P><PRE>code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +</PRE> + +<P>Notice that once we've generated the <CODE>jumpf</CODE> instruction, we +no longer need the value in register <CODE>:cond</CODE>, and we call <CODE>regfree</CODE> +to say so. The rest of this code generation process should be easy to work +out. All of the structured statements (<CODE>for</CODE>, <CODE>while</CODE>, and <CODE> +repeat</CODE>) are similarly simple. + +<P>The code generation for expressions is all in <CODE>ppopop</CODE>. Most of the +complexity of dealing with expressions is in the parsing, not in the code +generation; by the time we get to <CODE>ppopop</CODE>, we know that we want to +carry out a single operation on two values, both of which are either in +registers or immediate values. The simple case is that both are in +registers; suppose, for example, that we are given the subtraction operation +and the two operands are in registers 8 and 9. Then we just generate the +instruction + +<P><PRE>sub 8 8 9 +</PRE> + +<P>and declare register 9 free. <CODE>Ppopop</CODE> is a little long, +because it has to check for special cases such as immediate operands. +Also, a unary minus is turned into a subtraction from register zero, +since there is no unary <CODE>minus</CODE> operation in our simulated machine. + +<P>Ironically, it's the "simple" statements that are hardest to +compile: assignment and procedure calling. For procedure (or function) +calling, the difficulty is in matching actual argument expressions with +formal parameters. Procedure <CODE>pproccall1</CODE> generates the instructions to +manipulate frame pointers, as described earlier, and procedure <CODE> +procargs</CODE> fills the newly-created frame with the actual argument values. +(If an argument is an array passed by value, each member of the array must +be copied into the new frame.) Assignment, handled by procedure <CODE> +passign</CODE> in the compiler, is similar to argument passing; a value must be +computed and then stored into a frame. I wouldn't be too upset if you +decide to stop here and take code generation for memory references on faith. + +<P>Suppose we are compiling the assignment + +<P><PRE>x := <EM>expression</EM> +</PRE> + +<P><CODE>Passign</CODE> reads the name <CODE>x</CODE> and uses <CODE>getid</CODE> to find +the information associated with that name. If the assignment is to an array +member, then <CODE>passign</CODE> must also read the array indices, but let's say +that we are assigning to a scalar variable, to keep it simple. + +<P><PRE>to passign +local [name id type index value pointer target] +<U>make "name token</U> +make "index [] +ifbe "|[| [make "index commalist [pexpr] mustbe "|]|] +mustbe "|:=| +<U>make "id getid :name</U> +<U>make "pointer id.pointer :id</U> +<U>make "type id.type :id</U> +passign1 +end +</PRE> + +<P>Procedure <CODE>passign1</CODE> contains the steps that are in common between +ordinary assignment (handled by <CODE>passign</CODE>) and assignment to the name of +the current function, to set the return value (handled by <CODE>pfunset</CODE>, +which you can read in the complete listing at the end of the chapter). + +<P><PRE>to passign1 +if and (listp :type) (emptyp :index) [parrayassign :id stop] +setindex "false +<U>make "value check.type :type pexpr</U> +<U>codestore :value (id.pointer :id) (id.varp :id) :index</U> +regfree :value +end +</PRE> + +<P>We call <CODE>pexpr</CODE> to generate the code to compute the +expression. <CODE>Check.type</CODE> is like <CODE>pboolean</CODE>, which you saw earlier, +except that it takes the desired type as an argument. It returns the +number of the register that contains the expression value. + +<P>The real work is done by <CODE>codestore</CODE>, which takes four inputs. +The first is the register number whose value should be stored; the other +three inputs indicate where in memory the value should go. First comes the +pointer from the identifier list; this, you'll recall, tells us the lexical +depth at which the variable was declared and the offset within its frame +where the variable is kept. Next is a true or false value indicating +whether or not this variable is a <CODE>var</CODE> parameter; if so, then its value +is a pointer to the variable whose value we <EM>really</EM> want to change. +Finally, the <EM>index</EM> input will be zero for a scalar variable, or the +number of a register containing the +array index for an array member. (Procedure <CODE>lindex</CODE>, whose name stands +for "linear index," has been called to generate code to convert the possible +multi-dimensional indices, with possibly varying starting values, into a +single number indicating the position within the array, starting from zero +for the first member.) + +<P><PRE>to codestore :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "store :reg targetaddr) +regfree last :target +end +</PRE> + +<P>(There is a similar procedure <CODE>codeload</CODE> used to generate +the code to load a variable's value into a register.) <CODE>Codestore</CODE> +invokes a subprocedure <CODE>memsetup</CODE> whose job is to work out an +appropriate operand for an <CODE>rload</CODE> or <CODE>store</CODE> machine instruction. +That operand must be an offset and an index register, such as <CODE>41(4)</CODE>. +What <CODE>memsetup</CODE> returns is a list of the two numbers, in this case +<CODE>[41 4]</CODE>. Procedure <CODE>targetaddr</CODE> turns that into the right notation +for use in the instruction. + +<P><CODE>Memsetup</CODE> is the most complicated procedure in the compiler, because +there are so many special cases. I'll describe the easy cases here. Suppose +that we are dealing with a scalar variable that isn't a <CODE>var</CODE> parameter. +Then there are three cases. If the lexical depth of that variable is equal +to the current lexical depth, then this variable is declared in the same +block that we're compiling. In that case, we use register 4 (the current +frame pointer) as the index register, and the variable's frame slot as the +offset. If the variable's lexical depth is zero, then it's a global +variable. In that case, we use register 3 (the global frame pointer) as +the index register, and the variable's frame slot as the offset. If the +variable's depth is something other than zero or the current depth, then +we have to find a pointer to the variable's own frame by looking in the +current frame's <CODE>frame.outerframe</CODE> slot, and perhaps in <EM>that</EM> +frame's <CODE>frame.outerframe</CODE> slot, as many times as the difference between +the current depth and the variable's depth. + +<P>If the variable is a <CODE>var</CODE> parameter, then we go through the same cases +just described, and then load the value of that variable (which is a pointer +to the variable we really want) into a register. We use that new register +as the index register, and zero as the offset. + +<P>If the variable is an array member, then we must add the linear index +(which is already in a register) to the offset as computed so far. + +<P>Perhaps an example will help sort this out. Here is the compiled version +of the <CODE>tower</CODE> program, with annotations: + +<P><PRE>[ [add 3 0 0] set up initial pointers + [add 4 0 0] + [addi 2 0 36] + [jump "g1] jump to main program + +%hanoi [store 1 0(4)] save return value + [jump "g2] jump to body of hanoi + +%movedisk [store 1 0(4)] + [jump "g3] + +g3 [putstr 1 [Move disk ]] body of movedisk + [rload 7 36(4)] + [putint 1 7] write(number:1) + [putstr 1 [ from ]] + [rload 7 37(4)] + [putch 1 7] write(from:1) + [putstr 1 [ to ]] + [rload 7 38(4)] + [putch 1 7] write(to:1) + [newline] + [rload 1 0(4)] reload return address + [add 2 4 0] free stack frame + [rload 4 3(2)] + [jr 1] return to caller + +g2 [rload 7 36(4)] body of hanoi + [neqi 7 7 0] if number <> 0 + [jumpf 7 "g4] + [store 5 1(2)] allocate new frame + [add 5 2 0] + [addi 2 2 40] + [store 4 3(5)] set previous frame + [rload 7 2(4)] + [store 7 2(5)] set enclosing frame + [rload 7 36(4)] + [subi 7 7 1] + [store 7 36(5)] first arg is number-1 + [rload 7 37(4)] + [store 7 37(5)] next arg is from + [rload 7 39(4)] + [store 7 38(5)] next arg is other + [rload 7 38(4)] + [store 7 39(5)] next arg is onto + [add 4 5 0] switch to new frame + [rload 5 1(4)] + [jal 1 "%hanoi] recursive call + + [store 5 1(2)] set up for movedisk + [add 5 2 0] + [addi 2 2 39] + [store 4 3(5)] + [store 4 2(5)] note different enclosing frame + [rload 7 36(4)] + [store 7 36(5)] copy args + [rload 7 37(4)] + [store 7 37(5)] + [rload 7 38(4)] + [store 7 38(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%movedisk] call movedisk + + [store 5 1(2)] second recursive call + [add 5 2 0] + [addi 2 2 40] + [store 4 3(5)] + [rload 7 2(4)] + [store 7 2(5)] + [rload 7 36(4)] + [subi 7 7 1] + [store 7 36(5)] + [rload 7 39(4)] + [store 7 37(5)] + [rload 7 38(4)] + [store 7 38(5)] + [rload 7 37(4)] + [store 7 39(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%hanoi] + [jump "g5] end of if...then + +g4 +g5 [rload 1 0(4)] return to caller + [add 2 4 0] + [rload 4 3(2)] + [jr 1] + +g1 [store 5 1(2)] body of main program + [add 5 2 0] prepare to call hanoi + [addi 2 2 40] + [store 4 3(5)] + [store 4 2(5)] + [addi 7 0 5] constant argument 5 + [store 7 36(5)] + [addi 7 0 97] ASCII code for 'a' + [store 7 37(5)] + [addi 7 0 98] ASCII code for 'b' + [store 7 38(5)] + [addi 7 0 99] ASCII code for 'c' + [store 7 39(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%hanoi] call hanoi + [exit] +] +</PRE> + +<P> +<TABLE width="100%"><TR><TD><A HREF="../v3-toc2.html">(back to Table of Contents)</A> +<TD align="right"><A HREF="https://people.eecs.berkeley.edu/~bh/v3ch4/v3ch4.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../v3ch6/v3ch6.html"><STRONG>NEXT</STRONG></A> +</TABLE> + +<P><H2>Program Listing</H2> + +<P><PRE> +to compile :file +if namep "peekchar [ern "peekchar] +if namep "peektoken [ern "peektoken] +if not namep "idlist [opsetup] +if not emptyp :file [openread :file] +setread :file +ignore error +catch "error [program] +localmake "error error +if not emptyp :error [print first butfirst :error] +setread [] +if not emptyp :file [close :file] +end + +;; Global setup + +to opsetup +make "numregs 32 +make "memsize 3000 +pprop "|=| "binary [eql 2 [boolean []] 1] +pprop "|<>| "binary [neq 2 [boolean []] 1] +pprop "|<| "binary [less 2 [boolean []] 1] +pprop "|>| "binary [gtr 2 [boolean []] 1] +pprop "|<=| "binary [leq 2 [boolean []] 1] +pprop "|>=| "binary [geq 2 [boolean []] 1] +pprop "|+| "binary [add 2 [[] []] 2] +pprop "|-| "binary [sub 2 [[] []] 2] +pprop "or "binary [lor 2 [boolean boolean] 2] +pprop "|*| "binary [mul 2 [[] []] 3] +pprop "|/| "binary [quo 2 [real []] 3] +pprop "div "binary [div 2 [integer integer] 3] +pprop "mod "binary [rem 2 [integer integer] 3] +pprop "and "binary [land 2 [boolean boolean] 3] +pprop "|+| "unary [plus 1 [[] []] 4] +pprop "|-| "unary [minus 1 [[] []] 4] +pprop "not "unary [lnot 1 [boolean boolean] 4] +make "idlist `[[trunc function int [1 ,[framesize.fun+1]]] + [round function round [1 ,[framesize.fun+1]]] + [random function random [1 ,[framesize.fun+1]]]] +make "int [integer real] +make "round [integer real] +make "random [integer integer] +end + +;; Block structure + +to program +mustbe "program +localmake "progname token +ifbe "|(| [ignore commalist [id] mustbe "|)|] +mustbe "|;| +localmake "lexical.depth 0 +localmake "namesused [] +localmake "needint "false +localmake "needround "false +localmake "needrandom "false +localmake "idlist :idlist +localmake "frame [0 0] +localmake "id (list :progname "program (newlname :progname) :frame) +push "idlist :id +localmake "codeinto word "% :progname +make :codeinto [] +localmake "framesize framesize.proc +program1 +mustbe ". +code [exit] +foreach [int round random] "plibrary +make :codeinto reverse thing :codeinto +end + +to program1 +localmake "regsused (array :numregs 0) +for [i reg.firstfree :numregs-1] [setitem :i :regsused "false] +ifbe "var [varpart] +.setfirst butfirst :frame :framesize +if :lexical.depth = 0 [code (list "add reg.globalptr reg.zero reg.zero) + code (list "add reg.frameptr reg.zero reg.zero) + code (list "addi reg.stackptr reg.zero :framesize)] +localmake "bodytag gensym +code (list "jump (word "" :bodytag)) +tryprocpart +code :bodytag +mustbe "begin +blockbody "end +end + +to plibrary :func +if not thing (word "need :func) [stop] +code :func +code (list "rload reg.firstfree (memaddr framesize.fun reg.frameptr)) +code (list (word "s :func) reg.retval reg.firstfree) +code (list "add reg.stackptr reg.frameptr reg.zero) +code (list "rload reg.frameptr (memaddr frame.prevframe reg.stackptr)) +code (list "jr reg.retaddr) +end + +;; Variable declarations + +to varpart +local [token namelist type] +make "token token +make "peektoken :token +if reservedp :token [stop] +vargroup +foreach :namelist [newvar ? :type] +mustbe "|;| +varpart +end + +to vargroup +make "namelist commalist [id] +mustbe ": +ifbe "packed [] +make "type token +ifelse equalp :type "array [make "type arraytype] [typecheck :type] +end + +to id +local "token +make "token token +if letterp ascii first :token [output :token] +make "peektoken :token +output [] +end + +to arraytype +local [ranges type] +mustbe "|[| +make "ranges commalist [range] +mustbe "|]| +mustbe "of +make "type token +typecheck :type +output list :type :ranges +end + +to range +local [first last] +make "first range1 +mustbe ".. +make "last range1 +if :first > :last ~ + [(throw "error (sentence [array bounds not increasing:] + :first ".. :last))] +output list :first (1 + :last - :first) +end + +to range1 +local "bound +make "bound token +if equalp first :bound "' [output ascii first butfirst :bound] +if equalp :bound "|-| [make "bound minus token] +if equalp :bound int :bound [output :bound] +(throw "error sentence [array bound not ordinal:] :bound) +end + +to typecheck :type +if memberp :type [real integer char boolean] [stop] +(throw "error sentence [undefined type] :type) +end + +to newvar :pname :type +if reservedp :pname [(throw "error sentence :pname [reserved word])] +push "idlist (list :pname :type (list :lexical.depth :framesize) "false) +make "framesize :framesize + ifelse listp :type [arraysize :type] [1] +end + +to arraysize :type +output reduce "product map [last ?] last :type +end + +;; Procedure and function declarations + +to tryprocpart +ifbeelse "procedure ~ + [procedure tryprocpart] ~ + [ifbe "function [function tryprocpart]] +end + +to procedure +proc1 "procedure framesize.proc +end + +to function +proc1 "function framesize.fun +end + +to proc1 :proctype :framesize +localmake "procname token +localmake "lexical.depth :lexical.depth+1 +localmake "frame (list :lexical.depth 0) +push "idlist (list :procname :proctype (newlname :procname) :frame) +localmake "idlist :idlist +make lname :procname [] +ifbe "|(| [arglist] +if equalp :proctype "function ~ + [mustbe ": + localmake "type token + typecheck :type + make lname :procname fput :type thing lname :procname] +mustbe "|;| +code lname :procname +code (list "store reg.retaddr (memaddr frame.retaddr reg.frameptr)) +program1 +if equalp :proctype "function ~ + [code (list "rload reg.retval (memaddr frame.retval reg.frameptr))] +code (list "rload reg.retaddr (memaddr frame.retaddr reg.frameptr)) +code (list "add reg.stackptr reg.frameptr reg.zero) +code (list "rload reg.frameptr (memaddr frame.prevframe reg.stackptr)) +code (list "jr reg.retaddr) +mustbe "|;| +end + +to arglist +local [token namelist type varflag] +make "varflag "false +ifbe "var [make "varflag "true] +vargroup +foreach :namelist [newarg ? :type :varflag] +ifbeelse "|;| [arglist] [mustbe "|)|] +end + +to newarg :pname :type :varflag +if reservedp :pname [(throw "error sentence :pname [reserved word])] +localmake "pointer (list :lexical.depth :framesize) +push "idlist (list :pname :type :pointer :varflag) +make "framesize :framesize + ifelse (and listp :type not :varflag) ~ + [arraysize :type] [1] +queue lname :procname ifelse :varflag [list "var :type] [:type] +end + +;; Statement part + +to blockbody :endword +statement +ifbeelse "|;| [blockbody :endword] [mustbe :endword] +end + +to statement +local [token type] +ifbe "begin [compound stop] +ifbe "for [pfor stop] +ifbe "if [pif stop] +ifbe "while [pwhile stop] +ifbe "repeat [prepeat stop] +ifbe "write [pwrite stop] +ifbe "writeln [pwriteln stop] +make "token token +make "peektoken :token +if memberp :token [|;| end until] [stop] +make "type gettype :token +if emptyp :type [(throw "error sentence :token [can't begin statement])] +if equalp :type "procedure [pproccall stop] +if equalp :type "function [pfunset stop] +passign +end + +;; Compound statement + +to compound +blockbody "end +end + +;; Structured statements + +to pfor +local [var init step final looptag endtag testreg] +make "var token +mustbe "|:=| +make "init pinteger pexpr +make "step 1 +ifbeelse "downto [make "step -1] [mustbe "to] +make "final pinteger pexpr +mustbe "do +make "looptag gensym +make "endtag gensym +code :looptag +localmake "id getid :var +codestore :init (id.pointer :id) (id.varp :id) 0 +make "testreg newregister +code (list (ifelse :step<0 ["less] ["gtr]) :testreg :init :final) +code (list "jumpt :testreg (word "" :endtag)) +regfree :testreg +statement +code (list "addi :init :init :step) +code (list "jump (word "" :looptag)) +code :endtag +regfree :init +regfree :final +end + +to prepeat +local [cond looptag] +make "looptag gensym +code :looptag +blockbody "until +make "cond pboolean pexpr +code (list "jumpf :cond (word "" :looptag)) +regfree :cond +end + +to pif +local [cond elsetag endtag] +make "cond pboolean pexpr +make "elsetag gensym +make "endtag gensym +mustbe "then +code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +statement +code (list "jump (word "" :endtag)) +code :elsetag +ifbe "else [statement] +code :endtag +end + +to pwhile +local [cond looptag endtag] +make "looptag gensym +make "endtag gensym +code :looptag +make "cond pboolean pexpr +code (list "jumpf :cond (word "" :endtag)) +regfree :cond +mustbe "do +statement +code (list "jump (word "" :looptag)) +code :endtag +end + +;; Simple statements: write and writeln + +to pwrite +mustbe "|(| +pwrite1 +end + +to pwrite1 +pwrite2 +ifbe "|)| [stop] +ifbeelse ", [pwrite1] [(throw "error [missing comma])] +end + +to pwrite2 +localmake "result pwrite3 +ifbe ": [.setfirst (butfirst :result) token] +code :result +if not equalp first :result "putstr [regfree last :result] +end + +to pwrite3 +localmake "token token +if equalp first :token "' ~ + [output (list "putstr 1 (list butlast butfirst :token))] +make "peektoken :token +localmake "result pexpr +if equalp first :result "char [output (list "putch 1 pchar :result)] +if equalp first :result "boolean [output (list "puttf 1 pboolean :result)] +if equalp first :result "integer [output (list "putint 10 pinteger :result)] +output (list "putreal 20 preal :result) +end + +to pwriteln +ifbe "|(| [pwrite1] +code [newline] +end + +;; Simple statements: procedure call + +to pproccall +localmake "pname token +localmake "id getid :pname +localmake "lname id.lname :id +localmake "vartypes thing :lname +pproccall1 framesize.proc +end + +to pproccall1 :offset +code (list "store reg.newfp (memaddr frame.save.newfp reg.stackptr)) +code (list "add reg.newfp reg.stackptr reg.zero) +code (list "addi reg.stackptr reg.stackptr (last id.frame :id)) +code (list "store reg.frameptr (memaddr frame.prevframe reg.newfp)) +localmake "newdepth first id.frame :id +ifelse :newdepth > :lexical.depth ~ + [code (list "store reg.frameptr + (memaddr frame.outerframe reg.newfp))] ~ + [localmake "tempreg newregister + code (list "rload :tempreg (memaddr frame.outerframe reg.frameptr)) + repeat (:lexical.depth - :newdepth) + [code (list "rload :tempreg + (memaddr frame.outerframe :tempreg))] + code (list "store :tempreg (memaddr frame.outerframe reg.newfp)) + regfree :tempreg] +if not emptyp :vartypes [mustbe "|(| procargs :vartypes :offset] +for [i reg.firstfree :numregs-1] ~ + [if item :i :regsused + [code (list "store :i (memaddr frame.regsave+:i reg.frameptr))]] +code (list "add reg.frameptr reg.newfp reg.zero) +code (list "rload reg.newfp (memaddr frame.save.newfp reg.frameptr)) +code (list "jal reg.retaddr (word "" :lname)) +for [i reg.firstfree :numregs-1] ~ + [if item :i :regsused + [code (list "rload :i (memaddr frame.regsave+:i reg.frameptr))]] +end + +to procargs :types :offset +if emptyp :types [mustbe "|)| stop] +localmake "next procarg first :types :offset +if not emptyp butfirst :types [mustbe ",] +procargs butfirst :types :offset+:next +end + +to procarg :type :offset +local "result +if equalp first :type "var [output procvararg last :type] +if listp :type [output procarrayarg :type] +make "result check.type :type pexpr +code (list "store :result (memaddr :offset reg.newfp)) +regfree :result +output 1 +end + +to procvararg :ftype +local [pname id type index] +make "pname token +make "id getid :pname +make "type id.type :id +ifelse wordp :ftype ~ + [setindex "true] ~ + [make "index 0] +if not equalp :type :ftype ~ + [(throw "error sentence :pname [arg wrong type])] +localmake "target memsetup (id.pointer :id) (id.varp :id) :index +localmake "tempreg newregister +code (list "addi :tempreg (last :target) (first :target)) +code (list "store :tempreg (memaddr :offset reg.newfp)) +regfree last :target +regfree :tempreg +output 1 +end + +to procarrayarg :type +localmake "pname token +localmake "id getid :pname +if not equalp :type (id.type :id) ~ + [(throw "error (sentence "array :pname [wrong type for arg]))] +localmake "size arraysize :type +localmake "rtarget memsetup (id.pointer :id) (id.varp :id) 0 +localmake "pointreg newregister +code (list "addi :pointreg reg.newfp :offset) +localmake "ltarget (list 0 :pointreg) +copyarray +output :size +end + +;; Simple statements: assignment statement (including function value) + +to passign +local [name id type index value pointer target] +make "name token +make "index [] +ifbe "|[| [make "index commalist [pexpr] mustbe "|]|] +mustbe "|:=| +make "id getid :name +make "pointer id.pointer :id +make "type id.type :id +passign1 +end + +to pfunset +local [name id type index value pointer target] +make "name token +make "index [] +if not equalp :name :procname ~ + [(throw "error sentence [assign to wrong function] :name)] +mustbe "|:=| +make "pointer (list :lexical.depth frame.retval) +make "type first thing lname :name +make "id (list :name :type :pointer "false) +passign1 +end + +to passign1 +if and (listp :type) (emptyp :index) [parrayassign :id stop] +setindex "false +make "value check.type :type pexpr +codestore :value (id.pointer :id) (id.varp :id) :index +regfree :value +end + +to noimmediate :value +if equalp exp.mode :value "immediate ~ + [localmake "reg newregister + code (list "addi :reg reg.zero exp.value :value) + output (list exp.type :value "register :reg)] +output :value +end + +to check.type :type :result +if equalp :type "real [output preal :result] +if equalp :type "integer [output pinteger :result] +if equalp :type "char [output pchar :result] +if equalp :type "boolean [output pboolean :result] +end + +to preal :expr [:pval noimmediate :expr] +if equalp exp.type :pval "real [output exp.value :pval] +output pinteger :pval +end + +to pinteger :expr [:pval noimmediate :expr] +local "type +make "type exp.type :pval +if memberp :type [integer boolean char] [output exp.value :pval] +(throw "error sentence exp.type :pval [isn't ordinal]) +end + +to pchar :expr [:pval noimmediate :expr] +if equalp exp.type :pval "char [output exp.value :pval] +(throw "error sentence exp.type :pval [not character value]) +end + +to pboolean :expr [:pval noimmediate :expr] +if equalp exp.type :pval "boolean [output exp.value :pval] +(throw "error sentence exp.type :pval [not true or false]) +end + +to parrayassign :id +localmake "right token +if equalp first :right "' ~ + [pstringassign :type (butlast butfirst :right) stop] +localmake "rid getid :right +if not equalp (id.type :id) (id.type :rid) ~ + [(throw "error (sentence "arrays :name "and :right [unequal types]))] +localmake "size arraysize id.type :id +localmake "ltarget memsetup (id.pointer :id) (id.varp :id) 0 +localmake "rtarget memsetup (id.pointer :rid) (id.varp :rid) 0 +copyarray +end + +to pstringassign :type :string +if not equalp first :type "char [stringlose] +if not emptyp butfirst last :type [stringlose] +if not equalp (last first last :type) (count :string) [stringlose] +localmake "ltarget memsetup (id.pointer :id) (id.varp :id) 0 +pstringassign1 newregister (first :ltarget) (last :ltarget) :string +regfree last :ltarget +end + +to pstringassign1 :tempreg :offset :reg :string +if emptyp :string [regfree :tempreg stop] +code (list "addi :tempreg reg.zero ascii first :string) +code (list "store :tempreg (memaddr :offset :reg)) +pstringassign1 :tempreg :offset+1 :reg (butfirst :string) +end + +to stringlose +(throw "error sentence :name [not string array or wrong size]) +end + +;; Multiple array indices to linear index computation + +to setindex :parseflag +ifelse listp :type ~ + [if :parseflag + [mustbe "|[| make "index commalist [pexpr] mustbe "|]| ] + make "index lindex last :type :index + make "type first :type] ~ + [make "index 0] +end + +to lindex :bounds :index +output lindex1 (offset pinteger noimmediate first :index + first first :bounds) ~ + butfirst :bounds butfirst :index +end + +to lindex1 :sofar :bounds :index +if emptyp :bounds [output :sofar] +output lindex1 (nextindex :sofar + last first :bounds + pinteger noimmediate first :index + first first :bounds) ~ + butfirst :bounds butfirst :index +end + +to nextindex :old :factor :new :offset +code (list "muli :old :old :factor) +localmake "newreg offset :new :offset +code (list "add :old :old :newreg) +regfree :newreg +output :old +end + +to offset :indexreg :lowbound +if not equalp :lowbound 0 [code (list "subi :indexreg :indexreg :lowbound)] +output :indexreg +end + +;; Memory interface: load and store instructions + +to codeload :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "rload :reg targetaddr) +regfree last :target +end + +to codestore :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "store :reg targetaddr) +regfree last :target +end + +to targetaddr +output memaddr (first :target) (last :target) +end + +to memaddr :offset :index +output (word :offset "\( :index "\)) +end + +to memsetup :pointer :varflag :index +localmake "depth first :pointer +localmake "offset last :pointer +local "newreg +ifelse equalp :depth 0 ~ + [make "newreg reg.globalptr] ~ + [ifelse equalp :depth :lexical.depth + [make "newreg reg.frameptr] + [make "newreg newregister + code (list "rload :newreg + (memaddr frame.outerframe reg.frameptr)) + repeat (:lexical.depth - :depth) - 1 + [code (list "rload :newreg + (memaddr frame.outerframe :newreg))]]] +if :varflag ~ + [ifelse :newreg = reg.frameptr + [make "newreg newregister + code (list "rload :newreg (memaddr :offset reg.frameptr))] + [code (list "rload :newreg (memaddr :offset :newreg))] + make "offset 0] +if not equalp :index 0 ~ + [code (list "add :index :index :newreg) + regfree :newreg + make "newreg :index] +output list :offset :newreg +end + +to copyarray +localmake "looptag gensym +localmake "sizereg newregister +code (list "addi :sizereg reg.zero :size) +code :looptag +localmake "tempreg newregister +code (list "rload :tempreg (memaddr (first :rtarget) (last :rtarget))) +code (list "store :tempreg (memaddr (first :ltarget) (last :ltarget))) +code (list "addi (last :rtarget) (last :rtarget) 1) +code (list "addi (last :ltarget) (last :ltarget) 1) +code (list "subi :sizereg :sizereg 1) +code (list "gtr :tempreg :sizereg reg.zero) +code (list "jumpt :tempreg (word "" :looptag)) +regfree :sizereg +regfree :tempreg +regfree last :ltarget +regfree last :rtarget +end + +;; Expressions + +to pexpr +local [opstack datastack parenlevel] +make "opstack [[popen 1 0]] +make "datastack [] +make "parenlevel 0 +output pexpr1 +end + +to pexpr1 +local [token op] +make "token token +while [equalp :token "|(|] [popen make "token token] +make "op pgetunary :token +if not emptyp :op [output pexprop :op] +push "datastack pdata :token +make "token token +while [and (:parenlevel > 0) (equalp :token "|)| )] ~ + [pclose make "token token] +make "op pgetbinary :token +if not emptyp :op [output pexprop :op] +make "peektoken :token +pclose +if not emptyp :opstack [(throw "error [too many operators])] +if not emptyp butfirst :datastack [(throw "error [too many operands])] +output pop "datastack +end + +to pexprop :op +while [(op.prec :op) < (1 + op.prec first :opstack)] [ppopop] +push "opstack :op +output pexpr1 +end + +to popen +push "opstack [popen 1 0] +make "parenlevel :parenlevel + 1 +end + +to pclose +while [(op.prec first :opstack) > 0] [ppopop] +ignore pop "opstack +make "parenlevel :parenlevel - 1 +end + +to pgetunary :token +output gprop :token "unary +end + +to pgetbinary :token +output gprop :token "binary +end + +to ppopop +local [op function args left right type reg] +make "op pop "opstack +make "function op.instr :op +if equalp :function "plus [stop] +make "args op.nargs :op +make "right pop "datastack +make "left (ifelse equalp :args 2 [pop "datastack] [[[] []]]) +make "type pnewtype :op exp.type :left exp.type :right +if equalp exp.mode :left "immediate ~ + [localmake "leftreg newregister + code (list "addi :leftreg reg.zero exp.value :left) + make "left (list exp.type :left "register :leftreg)] +ifelse equalp exp.mode :left "register ~ + [make "reg exp.value :left] ~ + [ifelse equalp exp.mode :right "register + [make "reg exp.value :right] + [make "reg newregister]] +if equalp :function "minus ~ + [make "left (list exp.type :right "register reg.zero) + make "function "sub + make "args 2] +if equalp exp.mode :right "immediate ~ + [make "function word :function "i] +ifelse equalp :args 2 ~ + [code (list :function :reg exp.value :left exp.value :right)] ~ + [code (list :function :reg exp.value :right)] +if not equalp :reg exp.value :left [regfree exp.value :left] +if (and (equalp exp.mode :right "register) + (not equalp :reg exp.value :right)) ~ + [regfree exp.value :right] +push "datastack (list :type "register :reg) +end + +to pnewtype :op :ltype :rtype +local "type +make "type op.types :op +if emptyp :ltype [make "ltype :rtype] +if not emptyp last :type [pchecktype last :type :ltype :rtype] +if and (equalp :ltype "real) (equalp :rtype "integer) [make "rtype "real] +if and (equalp :ltype "integer) (equalp :rtype "real) [make "ltype "real] +if not equalp :ltype :rtype [(throw "error [type clash])] +if emptyp last :type ~ + [if not memberp :rtype [integer real] + [(throw "error [nonarithmetic type])]] +if emptyp first :type [output :rtype] +output first :type +end + +to pchecktype :want :left :right +if not equalp :want :left [(throw "error (sentence :left "isn't :want))] +if not equalp :want :right [(throw "error (sentence :right "isn't :want))] +end + +;; Expression elements + +to pdata :token +if equalp :token "true [output [boolean immediate 1]] +if equalp :token "false [output [boolean immediate 0]] +if equalp first :token "' [output pchardata :token] +if numberp :token [output (list numtype :token "immediate :token)] +localmake "id getid :token +if emptyp :id [(throw "error sentence [undefined symbol] :token)] +localmake "type id.type :id +if equalp :type "function [output pfuncall :token] +local "index +setindex "true +localmake "reg newregister +codeload :reg (id.pointer :id) (id.varp :id) :index +output (list :type "register :reg) +end + +to pchardata :token +if not equalp count :token 3 ~ + [(throw "error sentence :token [not single character])] +output (list "char "immediate ascii first butfirst :token) +end + +to numtype :number +if memberp ". :number [output "real] +if memberp "e :number [output "real] +output "integer +end + +to pfuncall :pname +localmake "id getid :pname +localmake "lname id.lname :id +if namep (word "need :lname) [make (word "need :lname) "true] +localmake "vartypes thing :lname +localmake "returntype first :vartypes +make "vartypes butfirst :vartypes +pproccall1 framesize.fun +localmake "reg newregister +code (list "add :reg reg.retval reg.zero) +output (list :returntype "register :reg) +end + +;; Parsing assistance + +to code :stuff +if emptyp :stuff [stop] +push :codeinto :stuff +end + +to commalist :test [:sofar []] +local [result token] +make "result run :test +if emptyp :result [output :sofar] +ifbe ", [output (commalist :test (lput :result :sofar))] +output lput :result :sofar +end + +.macro ifbe :wanted :action +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output [] +end + +.macro ifbeelse :wanted :action :else +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output :else +end + +to mustbe :wanted +localmake "token token +if equalp :token :wanted [stop] +(throw "error (sentence "expected :wanted "got :token)) +end + +to newregister +for [i reg.firstfree :numregs-1] ~ + [if not item :i :regsused [setitem :i :regsused "true output :i]] +(throw "error [not enough registers available]) +end + +to regfree :reg +setitem :reg :regsused "false +end + +to reservedp :word +output memberp :word [and array begin case const div do downto else end ~ + file for forward function goto if in label mod nil ~ + not of packed procedure program record repeat set ~ + then to type until var while with] +end + +;; Lexical analysis + +to token +local [token char] +if namep "peektoken [make "token :peektoken + ern "peektoken output :token] +make "char getchar +if equalp :char "|{| [skipcomment output token] +if equalp :char char 32 [output token] +if equalp :char char 13 [output token] +if equalp :char char 10 [output token] +if equalp :char "' [output string "'] +if memberp :char [+ - * / = ( , ) |[| |]| |;|] [output :char] +if equalp :char "|<| [output twochar "|<| [= >]] +if equalp :char "|>| [output twochar "|>| [=]] +if equalp :char ". [output twochar ". [.]] +if equalp :char ": [output twochar ": [=]] +if numberp :char [output number :char] +if letterp ascii :char [output token1 lowercase :char] +(throw "error sentence [unrecognized character:] :char) +end + +to skipcomment +if equalp getchar "|}| [stop] +skipcomment +end + +to string :string +local "char +make "char getchar +if not equalp :char "' [output string word :string :char] +make "char getchar +if equalp :char "' [output string word :string :char] +make "peekchar :char +output word :string "' +end + +to twochar :old :ok +localmake "char getchar +if memberp :char :ok [output word :old :char] +make "peekchar :char +output :old +end + +to number :num +local "char +make "char getchar +if equalp :char ". ~ + [make "char getchar ~ + ifelse equalp :char ". ~ + [make "peektoken ".. output :num] ~ + [make "peekchar :char output number word :num ".]] +if equalp :char "e [output number word :num twochar "e [+ -]] +if numberp :char [output number word :num :char] +make "peekchar :char +output :num +end + +to token1 :token +local "char +make "char getchar +if or letterp ascii :char numberp :char ~ + [output token1 word :token lowercase :char] +make "peekchar :char +output :token +end + +to letterp :code +if and (:code > 64) (:code < 91) [output "true] +output and (:code > 96) (:code < 123) +end + +to getchar +local "char +if namep "peekchar [make "char :peekchar ern "peekchar output :char] +if eofp [output char 1] +output rc1 +end + +to rc1 +local "result +make "result readchar +type :result +output :result +end + +;; Data abstraction: ID List + +to newlname :word +if memberp :word :namesused [output gensym] +if namep word "% :word [output gensym] +push "namesused :word +output word "% :word +end + +to lname :word +local "result +make "result getid :word +if not emptyp :result [output item 3 :result] +(throw "error sentence [unrecognized identifier] :word) +end + +to gettype :word +local "result +make "result getid :word +if not emptyp :result [output item 2 :result] +(throw "error sentence [unrecognized identifier] :word) +end + +to getid :word [:list :idlist] +if emptyp :list [output []] +if equalp :word first first :list [output first :list] +output (getid :word butfirst :list) +end + +to id.type :id +output item 2 :id +end + +to id.pointer :id +output item 3 :id +end + +to id.lname :id +output item 3 :id +end + +to id.varp :id +output item 4 :id +end + +to id.frame :id +output item 4 :id +end + +;; Data abstraction: Frame slots + +to frame.retaddr +output 0 +end + +to frame.save.newfp +output 1 +end + +to frame.outerframe +output 2 +end + +to frame.prevframe +output 3 +end + +to frame.regsave +output 4 +end + +to framesize.proc +output 4+:numregs +end + +to frame.retval +output 4+:numregs +end + +to framesize.fun +output 5+:numregs +end + +;; Data abstraction: Operators + +to op.instr :op +output first :op +end + +to op.nargs :op +output first bf :op +end + +to op.types :op +output item 3 :op +end + +to op.prec :op +output last :op +end + +;; Data abstraction: Expressions + +to exp.type :exp +output first :exp +end + +to exp.mode :exp +output first butfirst :exp +end + +to exp.value :exp +output last :exp +end + +;; Data abstraction: Registers + +to reg.zero +output 0 +end + +to reg.retaddr +output 1 +end + +to reg.stackptr +output 2 +end + +to reg.globalptr +output 3 +end + +to reg.frameptr +output 4 +end + +to reg.newfp +output 5 +end + +to reg.retval +output 6 +end + +to reg.firstfree +output 7 +end + +;; Runtime (machine simulation) + +to prun :progname +localmake "prog thing word "% :progname +localmake "regs (array :numregs 0) +local filter "wordp :prog +foreach :prog [if wordp ? [make ? ?rest]] +localmake "memory (array :memsize 0) +setitem 0 :regs 0 +if not procedurep "add [runsetup] +prun1 :prog +end + +to prun1 :pc +if emptyp :pc [stop] +if listp first :pc [run first :pc] +prun1 butfirst :pc +end + +to rload :reg :offset :index +setitem :reg :regs (item (item :index :regs)+:offset :memory) +end + +to store :reg :offset :index +setitem (item :index :regs)+:offset :memory (item :reg :regs) +end + +to runsetup +foreach [[add sum] [sub difference] [mul product] [quo quotient] + [div [int quotient]] [rem remainder] [land product] + [lor [tobool lessp 0 sum]] [eql [tobool equalp]] + [neq [tobool not equalp]] [less [tobool lessp]] + [gtr [tobool greaterp]] [leq [tobool not greaterp]] + [geq [tobool not lessp]]] ~ + [define first ? + `[[dest src1 src2] + [setitem :dest :regs ,@[last ?] (item :src1 :regs) + (item :src2 :regs)]] + define word first ? "i + `[[dest src1 immed] + [setitem :dest :regs ,@[last ?] (item :src1 :regs) + :immed]]] +foreach [[lnot [difference 1]] [sint int] [sround round] [srandom random]] ~ + [define first ? + `[[dest src] + [setitem :dest :regs ,@[last ?] (item :src :regs)]] + define word first ? "i + `[[dest immed] + [setitem :dest :regs ,@[last ?] :immed]]] +end + +to tobool :tf +output ifelse :tf [1] [0] +end + +to jump :label +make "pc fput :label thing :label +end + +to jumpt :reg :label +if (item :reg :regs)=1 [jump :label] +end + +to jumpf :reg :label +if (item :reg :regs)=0 [jump :label] +end + +to jr :reg +make "pc item :reg :regs +end + +to jal :reg :label +setitem :reg :regs :pc +jump :label +end + +to putch :width :reg +spaces :width 1 +type char (item :reg :regs) +end + +to putstr :width :string +spaces :width (count first :string) +type :string +end + +to puttf :width :bool +spaces :width 1 +type ifelse (item :bool :regs)=0 ["F] ["T] +end + +to putint :width :reg +localmake "num (item :reg :regs) +spaces :width count :num +type :num +end + +to putreal :width :reg +putint :width :reg +end + +to spaces :width :count +if :width > :count [repeat :width - :count [type "| |]] +end + +to newline +print [] +end + +to exit +make "pc [exit] +end +</PRE> + +<P><A HREF="../v3-toc2.html">(back to Table of Contents)</A> +<P><A HREF="https://people.eecs.berkeley.edu/~bh/v3ch4/v3ch4.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../v3ch6/v3ch6.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> diff --git a/js/games/nluqo.github.io/~bh/v3ch5/memory.gif b/js/games/nluqo.github.io/~bh/v3ch5/memory.gif new file mode 100644 index 0000000..7922f1d --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/memory.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/pascal.lg b/js/games/nluqo.github.io/~bh/v3ch5/pascal.lg new file mode 100644 index 0000000..0aae43c --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/pascal.lg @@ -0,0 +1,1217 @@ +to compile :file +if namep "peekchar [ern "peekchar] +if namep "peektoken [ern "peektoken] +if not namep "idlist [opsetup] +if not emptyp :file [openread :file] +setread :file +ignore error +catch "error [program] +localmake "error error +if not emptyp :error [print first butfirst :error] +setread [] +if not emptyp :file [close :file] +end + +;; Global setup + +to opsetup +make "numregs 32 +make "memsize 3000 +pprop "|=| "binary [eql 2 [boolean []] 1] +pprop "|<>| "binary [neq 2 [boolean []] 1] +pprop "|<| "binary [less 2 [boolean []] 1] +pprop "|>| "binary [gtr 2 [boolean []] 1] +pprop "|<=| "binary [leq 2 [boolean []] 1] +pprop "|>=| "binary [geq 2 [boolean []] 1] +pprop "|+| "binary [add 2 [[] []] 2] +pprop "|-| "binary [sub 2 [[] []] 2] +pprop "or "binary [lor 2 [boolean boolean] 2] +pprop "|*| "binary [mul 2 [[] []] 3] +pprop "|/| "binary [quo 2 [real []] 3] +pprop "div "binary [div 2 [integer integer] 3] +pprop "mod "binary [rem 2 [integer integer] 3] +pprop "and "binary [land 2 [boolean boolean] 3] +pprop "|+| "unary [plus 1 [[] []] 4] +pprop "|-| "unary [minus 1 [[] []] 4] +pprop "not "unary [lnot 1 [boolean boolean] 4] +make "idlist `[[trunc function int [1 ,[framesize.fun+1]]] + [round function round [1 ,[framesize.fun+1]]] + [random function random [1 ,[framesize.fun+1]]]] +make "int [integer real] +make "round [integer real] +make "random [integer integer] +end + +;; Block structure + +to program +mustbe "program +localmake "progname token +ifbe "|(| [ignore commalist [id] mustbe "|)|] +mustbe "|;| +localmake "lexical.depth 0 +localmake "namesused [] +localmake "needint "false +localmake "needround "false +localmake "needrandom "false +localmake "idlist :idlist +localmake "frame [0 0] +localmake "id (list :progname "program (newlname :progname) :frame) +push "idlist :id +localmake "codeinto word "% :progname +make :codeinto [] +localmake "framesize framesize.proc +program1 +mustbe ". +code [exit] +foreach [int round random] "plibrary +make :codeinto reverse thing :codeinto +end + +to program1 +localmake "regsused (array :numregs 0) +for [i reg.firstfree :numregs-1] [setitem :i :regsused "false] +ifbe "var [varpart] +.setfirst butfirst :frame :framesize +if :lexical.depth = 0 [code (list "add reg.globalptr reg.zero reg.zero) + code (list "add reg.frameptr reg.zero reg.zero) + code (list "addi reg.stackptr reg.zero :framesize)] +localmake "bodytag gensym +code (list "jump (word "" :bodytag)) +tryprocpart +code :bodytag +mustbe "begin +blockbody "end +end + +to plibrary :func +if not thing (word "need :func) [stop] +code :func +code (list "rload reg.firstfree (memaddr framesize.fun reg.frameptr)) +code (list (word "s :func) reg.retval reg.firstfree) +code (list "add reg.stackptr reg.frameptr reg.zero) +code (list "rload reg.frameptr (memaddr frame.prevframe reg.stackptr)) +code (list "jr reg.retaddr) +end + +;; Variable declarations + +to varpart +local [token namelist type] +make "token token +make "peektoken :token +if reservedp :token [stop] +vargroup +foreach :namelist [newvar ? :type] +mustbe "|;| +varpart +end + +to vargroup +make "namelist commalist [id] +mustbe ": +ifbe "packed [] +make "type token +ifelse equalp :type "array [make "type arraytype] [typecheck :type] +end + +to id +local "token +make "token token +if letterp ascii first :token [output :token] +make "peektoken :token +output [] +end + +to arraytype +local [ranges type] +mustbe "|[| +make "ranges commalist [range] +mustbe "|]| +mustbe "of +make "type token +typecheck :type +output list :type :ranges +end + +to range +local [first last] +make "first range1 +mustbe ".. +make "last range1 +if :first > :last ~ + [(throw "error (sentence [array bounds not increasing:] + :first ".. :last))] +output list :first (1 + :last - :first) +end + +to range1 +local "bound +make "bound token +if equalp first :bound "' [output ascii first butfirst :bound] +if equalp :bound "|-| [make "bound minus token] +if equalp :bound int :bound [output :bound] +(throw "error sentence [array bound not ordinal:] :bound) +end + +to typecheck :type +if memberp :type [real integer char boolean] [stop] +(throw "error sentence [undefined type] :type) +end + +to newvar :pname :type +if reservedp :pname [(throw "error sentence :pname [reserved word])] +push "idlist (list :pname :type (list :lexical.depth :framesize) "false) +make "framesize :framesize + ifelse listp :type [arraysize :type] [1] +end + +to arraysize :type +output reduce "product map [last ?] last :type +end + +;; Procedure and function declarations + +to tryprocpart +ifbeelse "procedure ~ + [procedure tryprocpart] ~ + [ifbe "function [function tryprocpart]] +end + +to procedure +proc1 "procedure framesize.proc +end + +to function +proc1 "function framesize.fun +end + +to proc1 :proctype :framesize +localmake "procname token +localmake "lexical.depth :lexical.depth+1 +localmake "frame (list :lexical.depth 0) +push "idlist (list :procname :proctype (newlname :procname) :frame) +localmake "idlist :idlist +make lname :procname [] +ifbe "|(| [arglist] +if equalp :proctype "function ~ + [mustbe ": + localmake "type token + typecheck :type + make lname :procname fput :type thing lname :procname] +mustbe "|;| +code lname :procname +code (list "store reg.retaddr (memaddr frame.retaddr reg.frameptr)) +program1 +if equalp :proctype "function ~ + [code (list "rload reg.retval (memaddr frame.retval reg.frameptr))] +code (list "rload reg.retaddr (memaddr frame.retaddr reg.frameptr)) +code (list "add reg.stackptr reg.frameptr reg.zero) +code (list "rload reg.frameptr (memaddr frame.prevframe reg.stackptr)) +code (list "jr reg.retaddr) +mustbe "|;| +end + +to arglist +local [token namelist type varflag] +make "varflag "false +ifbe "var [make "varflag "true] +vargroup +foreach :namelist [newarg ? :type :varflag] +ifbeelse "|;| [arglist] [mustbe "|)|] +end + +to newarg :pname :type :varflag +if reservedp :pname [(throw "error sentence :pname [reserved word])] +localmake "pointer (list :lexical.depth :framesize) +push "idlist (list :pname :type :pointer :varflag) +make "framesize :framesize + ifelse (and listp :type not :varflag) ~ + [arraysize :type] [1] +queue lname :procname ifelse :varflag [list "var :type] [:type] +end + +;; Statement part + +to blockbody :endword +statement +ifbeelse "|;| [blockbody :endword] [mustbe :endword] +end + +to statement +local [token type] +ifbe "begin [compound stop] +ifbe "for [pfor stop] +ifbe "if [pif stop] +ifbe "while [pwhile stop] +ifbe "repeat [prepeat stop] +ifbe "write [pwrite stop] +ifbe "writeln [pwriteln stop] +make "token token +make "peektoken :token +if memberp :token [|;| end until] [stop] +make "type gettype :token +if emptyp :type [(throw "error sentence :token [can't begin statement])] +if equalp :type "procedure [pproccall stop] +if equalp :type "function [pfunset stop] +passign +end + +;; Compound statement + +to compound +blockbody "end +end + +;; Structured statements + +to pfor +local [var init step final looptag endtag testreg] +make "var token +mustbe "|:=| +make "init pinteger pexpr +make "step 1 +ifbeelse "downto [make "step -1] [mustbe "to] +make "final pinteger pexpr +mustbe "do +make "looptag gensym +make "endtag gensym +code :looptag +localmake "id getid :var +codestore :init (id.pointer :id) (id.varp :id) 0 +make "testreg newregister +code (list (ifelse :step<0 ["less] ["gtr]) :testreg :init :final) +code (list "jumpt :testreg (word "" :endtag)) +regfree :testreg +statement +code (list "addi :init :init :step) +code (list "jump (word "" :looptag)) +code :endtag +regfree :init +regfree :final +end + +to prepeat +local [cond looptag] +make "looptag gensym +code :looptag +blockbody "until +make "cond pboolean pexpr +code (list "jumpf :cond (word "" :looptag)) +regfree :cond +end + +to pif +local [cond elsetag endtag] +make "cond pboolean pexpr +make "elsetag gensym +make "endtag gensym +mustbe "then +code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +statement +code (list "jump (word "" :endtag)) +code :elsetag +ifbe "else [statement] +code :endtag +end + +to pwhile +local [cond looptag endtag] +make "looptag gensym +make "endtag gensym +code :looptag +make "cond pboolean pexpr +code (list "jumpf :cond (word "" :endtag)) +regfree :cond +mustbe "do +statement +code (list "jump (word "" :looptag)) +code :endtag +end + +;; Simple statements: write and writeln + +to pwrite +mustbe "|(| +pwrite1 +end + +to pwrite1 +pwrite2 +ifbe "|)| [stop] +ifbeelse ", [pwrite1] [(throw "error [missing comma])] +end + +to pwrite2 +localmake "result pwrite3 +ifbe ": [.setfirst (butfirst :result) token] +code :result +if not equalp first :result "putstr [regfree last :result] +end + +to pwrite3 +localmake "token token +if equalp first :token "' ~ + [output (list "putstr 1 (list butlast butfirst :token))] +make "peektoken :token +localmake "result pexpr +if equalp first :result "char [output (list "putch 1 pchar :result)] +if equalp first :result "boolean [output (list "puttf 1 pboolean :result)] +if equalp first :result "integer [output (list "putint 10 pinteger :result)] +output (list "putreal 20 preal :result) +end + +to pwriteln +ifbe "|(| [pwrite1] +code [newline] +end + +;; Simple statements: procedure call + +to pproccall +localmake "pname token +localmake "id getid :pname +localmake "lname id.lname :id +localmake "vartypes thing :lname +pproccall1 framesize.proc +end + +to pproccall1 :offset +code (list "store reg.newfp (memaddr frame.save.newfp reg.stackptr)) +code (list "add reg.newfp reg.stackptr reg.zero) +code (list "addi reg.stackptr reg.stackptr (last id.frame :id)) +code (list "store reg.frameptr (memaddr frame.prevframe reg.newfp)) +localmake "newdepth first id.frame :id +ifelse :newdepth > :lexical.depth ~ + [code (list "store reg.frameptr + (memaddr frame.outerframe reg.newfp))] ~ + [localmake "tempreg newregister + code (list "rload :tempreg (memaddr frame.outerframe reg.frameptr)) + repeat (:lexical.depth - :newdepth) + [code (list "rload :tempreg + (memaddr frame.outerframe :tempreg))] + code (list "store :tempreg (memaddr frame.outerframe reg.newfp)) + regfree :tempreg] +if not emptyp :vartypes [mustbe "|(| procargs :vartypes :offset] +for [i reg.firstfree :numregs-1] ~ + [if item :i :regsused + [code (list "store :i (memaddr frame.regsave+:i reg.frameptr))]] +code (list "add reg.frameptr reg.newfp reg.zero) +code (list "rload reg.newfp (memaddr frame.save.newfp reg.frameptr)) +code (list "jal reg.retaddr (word "" :lname)) +for [i reg.firstfree :numregs-1] ~ + [if item :i :regsused + [code (list "rload :i (memaddr frame.regsave+:i reg.frameptr))]] +end + +to procargs :types :offset +if emptyp :types [mustbe "|)| stop] +localmake "next procarg first :types :offset +if not emptyp butfirst :types [mustbe ",] +procargs butfirst :types :offset+:next +end + +to procarg :type :offset +local "result +if equalp first :type "var [output procvararg last :type] +if listp :type [output procarrayarg :type] +make "result check.type :type pexpr +code (list "store :result (memaddr :offset reg.newfp)) +regfree :result +output 1 +end + +to procvararg :ftype +local [pname id type index] +make "pname token +make "id getid :pname +make "type id.type :id +ifelse wordp :ftype ~ + [setindex "true] ~ + [make "index 0] +if not equalp :type :ftype ~ + [(throw "error sentence :pname [arg wrong type])] +localmake "target memsetup (id.pointer :id) (id.varp :id) :index +localmake "tempreg newregister +code (list "addi :tempreg (last :target) (first :target)) +code (list "store :tempreg (memaddr :offset reg.newfp)) +regfree last :target +regfree :tempreg +output 1 +end + +to procarrayarg :type +localmake "pname token +localmake "id getid :pname +if not equalp :type (id.type :id) ~ + [(throw "error (sentence "array :pname [wrong type for arg]))] +localmake "size arraysize :type +localmake "rtarget memsetup (id.pointer :id) (id.varp :id) 0 +localmake "pointreg newregister +code (list "addi :pointreg reg.newfp :offset) +localmake "ltarget (list 0 :pointreg) +copyarray +output :size +end + +;; Simple statements: assignment statement (including function value) + +to passign +local [name id type index value pointer target] +make "name token +make "index [] +ifbe "|[| [make "index commalist [pexpr] mustbe "|]|] +mustbe "|:=| +make "id getid :name +make "pointer id.pointer :id +make "type id.type :id +passign1 +end + +to pfunset +local [name id type index value pointer target] +make "name token +make "index [] +if not equalp :name :procname ~ + [(throw "error sentence [assign to wrong function] :name)] +mustbe "|:=| +make "pointer (list :lexical.depth frame.retval) +make "type first thing lname :name +make "id (list :name :type :pointer "false) +passign1 +end + +to passign1 +if and (listp :type) (emptyp :index) [parrayassign :id stop] +setindex "false +make "value check.type :type pexpr +codestore :value (id.pointer :id) (id.varp :id) :index +regfree :value +end + +to noimmediate :value +if equalp exp.mode :value "immediate ~ + [localmake "reg newregister + code (list "addi :reg reg.zero exp.value :value) + output (list exp.type :value "register :reg)] +output :value +end + +to check.type :type :result +if equalp :type "real [output preal :result] +if equalp :type "integer [output pinteger :result] +if equalp :type "char [output pchar :result] +if equalp :type "boolean [output pboolean :result] +end + +to preal :expr [:pval noimmediate :expr] +if equalp exp.type :pval "real [output exp.value :pval] +output pinteger :pval +end + +to pinteger :expr [:pval noimmediate :expr] +local "type +make "type exp.type :pval +if memberp :type [integer boolean char] [output exp.value :pval] +(throw "error sentence exp.type :pval [isn't ordinal]) +end + +to pchar :expr [:pval noimmediate :expr] +if equalp exp.type :pval "char [output exp.value :pval] +(throw "error sentence exp.type :pval [not character value]) +end + +to pboolean :expr [:pval noimmediate :expr] +if equalp exp.type :pval "boolean [output exp.value :pval] +(throw "error sentence exp.type :pval [not true or false]) +end + +to parrayassign :id +localmake "right token +if equalp first :right "' ~ + [pstringassign :type (butlast butfirst :right) stop] +localmake "rid getid :right +if not equalp (id.type :id) (id.type :rid) ~ + [(throw "error (sentence "arrays :name "and :right [unequal types]))] +localmake "size arraysize id.type :id +localmake "ltarget memsetup (id.pointer :id) (id.varp :id) 0 +localmake "rtarget memsetup (id.pointer :rid) (id.varp :rid) 0 +copyarray +end + +to pstringassign :type :string +if not equalp first :type "char [stringlose] +if not emptyp butfirst last :type [stringlose] +if not equalp (last first last :type) (count :string) [stringlose] +localmake "ltarget memsetup (id.pointer :id) (id.varp :id) 0 +pstringassign1 newregister (first :ltarget) (last :ltarget) :string +regfree last :ltarget +end + +to pstringassign1 :tempreg :offset :reg :string +if emptyp :string [regfree :tempreg stop] +code (list "addi :tempreg reg.zero ascii first :string) +code (list "store :tempreg (memaddr :offset :reg)) +pstringassign1 :tempreg :offset+1 :reg (butfirst :string) +end + +to stringlose +(throw "error sentence :name [not string array or wrong size]) +end + +;; Multiple array indices to linear index computation + +to setindex :parseflag +ifelse listp :type ~ + [if :parseflag + [mustbe "|[| make "index commalist [pexpr] mustbe "|]| ] + make "index lindex last :type :index + make "type first :type] ~ + [make "index 0] +end + +to lindex :bounds :index +output lindex1 (offset pinteger noimmediate first :index + first first :bounds) ~ + butfirst :bounds butfirst :index +end + +to lindex1 :sofar :bounds :index +if emptyp :bounds [output :sofar] +output lindex1 (nextindex :sofar + last first :bounds + pinteger noimmediate first :index + first first :bounds) ~ + butfirst :bounds butfirst :index +end + +to nextindex :old :factor :new :offset +code (list "muli :old :old :factor) +localmake "newreg offset :new :offset +code (list "add :old :old :newreg) +regfree :newreg +output :old +end + +to offset :indexreg :lowbound +if not equalp :lowbound 0 [code (list "subi :indexreg :indexreg :lowbound)] +output :indexreg +end + +;; Memory interface: load and store instructions + +to codeload :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "rload :reg targetaddr) +regfree last :target +end + +to codestore :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "store :reg targetaddr) +regfree last :target +end + +to targetaddr +output memaddr (first :target) (last :target) +end + +to memaddr :offset :index +output (word :offset "\( :index "\)) +end + +to memsetup :pointer :varflag :index +localmake "depth first :pointer +localmake "offset last :pointer +local "newreg +ifelse equalp :depth 0 ~ + [make "newreg reg.globalptr] ~ + [ifelse equalp :depth :lexical.depth + [make "newreg reg.frameptr] + [make "newreg newregister + code (list "rload :newreg + (memaddr frame.outerframe reg.frameptr)) + repeat (:lexical.depth - :depth) - 1 + [code (list "rload :newreg + (memaddr frame.outerframe :newreg))]]] +if :varflag ~ + [ifelse :newreg = reg.frameptr + [make "newreg newregister + code (list "rload :newreg (memaddr :offset reg.frameptr))] + [code (list "rload :newreg (memaddr :offset :newreg))] + make "offset 0] +if not equalp :index 0 ~ + [code (list "add :index :index :newreg) + regfree :newreg + make "newreg :index] +output list :offset :newreg +end + +to copyarray +localmake "looptag gensym +localmake "sizereg newregister +code (list "addi :sizereg reg.zero :size) +code :looptag +localmake "tempreg newregister +code (list "rload :tempreg (memaddr (first :rtarget) (last :rtarget))) +code (list "store :tempreg (memaddr (first :ltarget) (last :ltarget))) +code (list "addi (last :rtarget) (last :rtarget) 1) +code (list "addi (last :ltarget) (last :ltarget) 1) +code (list "subi :sizereg :sizereg 1) +code (list "gtr :tempreg :sizereg reg.zero) +code (list "jumpt :tempreg (word "" :looptag)) +regfree :sizereg +regfree :tempreg +regfree last :ltarget +regfree last :rtarget +end + +;; Expressions + +to pexpr +local [opstack datastack parenlevel] +make "opstack [[popen 1 0]] +make "datastack [] +make "parenlevel 0 +output pexpr1 +end + +to pexpr1 +local [token op] +make "token token +while [equalp :token "|(|] [popen make "token token] +make "op pgetunary :token +if not emptyp :op [output pexprop :op] +push "datastack pdata :token +make "token token +while [and (:parenlevel > 0) (equalp :token "|)| )] ~ + [pclose make "token token] +make "op pgetbinary :token +if not emptyp :op [output pexprop :op] +make "peektoken :token +pclose +if not emptyp :opstack [(throw "error [too many operators])] +if not emptyp butfirst :datastack [(throw "error [too many operands])] +output pop "datastack +end + +to pexprop :op +while [(op.prec :op) < (1 + op.prec first :opstack)] [ppopop] +push "opstack :op +output pexpr1 +end + +to popen +push "opstack [popen 1 0] +make "parenlevel :parenlevel + 1 +end + +to pclose +while [(op.prec first :opstack) > 0] [ppopop] +ignore pop "opstack +make "parenlevel :parenlevel - 1 +end + +to pgetunary :token +output gprop :token "unary +end + +to pgetbinary :token +output gprop :token "binary +end + +to ppopop +local [op function args left right type reg] +make "op pop "opstack +make "function op.instr :op +if equalp :function "plus [stop] +make "args op.nargs :op +make "right pop "datastack +make "left (ifelse equalp :args 2 [pop "datastack] [[[] []]]) +make "type pnewtype :op exp.type :left exp.type :right +if equalp exp.mode :left "immediate ~ + [localmake "leftreg newregister + code (list "addi :leftreg reg.zero exp.value :left) + make "left (list exp.type :left "register :leftreg)] +ifelse equalp exp.mode :left "register ~ + [make "reg exp.value :left] ~ + [ifelse equalp exp.mode :right "register + [make "reg exp.value :right] + [make "reg newregister]] +if equalp :function "minus ~ + [make "left (list exp.type :right "register reg.zero) + make "function "sub + make "args 2] +if equalp exp.mode :right "immediate ~ + [make "function word :function "i] +ifelse equalp :args 2 ~ + [code (list :function :reg exp.value :left exp.value :right)] ~ + [code (list :function :reg exp.value :right)] +if not equalp :reg exp.value :left [regfree exp.value :left] +if (and (equalp exp.mode :right "register) + (not equalp :reg exp.value :right)) ~ + [regfree exp.value :right] +push "datastack (list :type "register :reg) +end + +to pnewtype :op :ltype :rtype +local "type +make "type op.types :op +if emptyp :ltype [make "ltype :rtype] +if not emptyp last :type [pchecktype last :type :ltype :rtype] +if and (equalp :ltype "real) (equalp :rtype "integer) [make "rtype "real] +if and (equalp :ltype "integer) (equalp :rtype "real) [make "ltype "real] +if not equalp :ltype :rtype [(throw "error [type clash])] +if emptyp last :type ~ + [if not memberp :rtype [integer real] + [(throw "error [nonarithmetic type])]] +if emptyp first :type [output :rtype] +output first :type +end + +to pchecktype :want :left :right +if not equalp :want :left [(throw "error (sentence :left "isn't :want))] +if not equalp :want :right [(throw "error (sentence :right "isn't :want))] +end + +;; Expression elements + +to pdata :token +if equalp :token "true [output [boolean immediate 1]] +if equalp :token "false [output [boolean immediate 0]] +if equalp first :token "' [output pchardata :token] +if numberp :token [output (list numtype :token "immediate :token)] +localmake "id getid :token +if emptyp :id [(throw "error sentence [undefined symbol] :token)] +localmake "type id.type :id +if equalp :type "function [output pfuncall :token] +local "index +setindex "true +localmake "reg newregister +codeload :reg (id.pointer :id) (id.varp :id) :index +output (list :type "register :reg) +end + +to pchardata :token +if not equalp count :token 3 ~ + [(throw "error sentence :token [not single character])] +output (list "char "immediate ascii first butfirst :token) +end + +to numtype :number +if memberp ". :number [output "real] +if memberp "e :number [output "real] +output "integer +end + +to pfuncall :pname +localmake "id getid :pname +localmake "lname id.lname :id +if namep (word "need :lname) [make (word "need :lname) "true] +localmake "vartypes thing :lname +localmake "returntype first :vartypes +make "vartypes butfirst :vartypes +pproccall1 framesize.fun +localmake "reg newregister +code (list "add :reg reg.retval reg.zero) +output (list :returntype "register :reg) +end + +;; Parsing assistance + +to code :stuff +if emptyp :stuff [stop] +push :codeinto :stuff +end + +to commalist :test [:sofar []] +local [result token] +make "result run :test +if emptyp :result [output :sofar] +ifbe ", [output (commalist :test (lput :result :sofar))] +output lput :result :sofar +end + +.macro ifbe :wanted :action +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output [] +end + +.macro ifbeelse :wanted :action :else +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output :else +end + +to mustbe :wanted +localmake "token token +if equalp :token :wanted [stop] +(throw "error (sentence "expected :wanted "got :token)) +end + +to newregister +for [i reg.firstfree :numregs-1] ~ + [if not item :i :regsused [setitem :i :regsused "true output :i]] +(throw "error [not enough registers available]) +end + +to regfree :reg +setitem :reg :regsused "false +end + +to reservedp :word +output memberp :word [and array begin case const div do downto else end ~ + file for forward function goto if in label mod nil ~ + not of packed procedure program record repeat set ~ + then to type until var while with] +end + +;; Lexical analysis + +to token +local [token char] +if namep "peektoken [make "token :peektoken + ern "peektoken output :token] +make "char getchar +if equalp :char "|{| [skipcomment output token] +if equalp :char char 32 [output token] +if equalp :char char 13 [output token] +if equalp :char char 10 [output token] +if equalp :char "' [output string "'] +if memberp :char [+ - * / = ( , ) |[| |]| |;|] [output :char] +if equalp :char "|<| [output twochar "|<| [= >]] +if equalp :char "|>| [output twochar "|>| [=]] +if equalp :char ". [output twochar ". [.]] +if equalp :char ": [output twochar ": [=]] +if numberp :char [output number :char] +if letterp ascii :char [output token1 lowercase :char] +(throw "error sentence [unrecognized character:] :char) +end + +to skipcomment +if equalp getchar "|}| [stop] +skipcomment +end + +to string :string +local "char +make "char getchar +if not equalp :char "' [output string word :string :char] +make "char getchar +if equalp :char "' [output string word :string :char] +make "peekchar :char +output word :string "' +end + +to twochar :old :ok +localmake "char getchar +if memberp :char :ok [output word :old :char] +make "peekchar :char +output :old +end + +to number :num +local "char +make "char getchar +if equalp :char ". ~ + [make "char getchar ~ + ifelse equalp :char ". ~ + [make "peektoken ".. output :num] ~ + [make "peekchar :char output number word :num ".]] +if equalp :char "e [output number word :num twochar "e [+ -]] +if numberp :char [output number word :num :char] +make "peekchar :char +output :num +end + +to token1 :token +local "char +make "char getchar +if or letterp ascii :char numberp :char ~ + [output token1 word :token lowercase :char] +make "peekchar :char +output :token +end + +to letterp :code +if and (:code > 64) (:code < 91) [output "true] +output and (:code > 96) (:code < 123) +end + +to getchar +local "char +if namep "peekchar [make "char :peekchar ern "peekchar output :char] +if eofp [output char 1] +output rc1 +end + +to rc1 +local "result +make "result readchar +type :result +output :result +end + +;; Data abstraction: ID List + +to newlname :word +if memberp :word :namesused [output gensym] +if namep word "% :word [output gensym] +push "namesused :word +output word "% :word +end + +to lname :word +local "result +make "result getid :word +if not emptyp :result [output item 3 :result] +(throw "error sentence [unrecognized identifier] :word) +end + +to gettype :word +local "result +make "result getid :word +if not emptyp :result [output item 2 :result] +(throw "error sentence [unrecognized identifier] :word) +end + +to getid :word [:list :idlist] +if emptyp :list [output []] +if equalp :word first first :list [output first :list] +output (getid :word butfirst :list) +end + +to id.type :id +output item 2 :id +end + +to id.pointer :id +output item 3 :id +end + +to id.lname :id +output item 3 :id +end + +to id.varp :id +output item 4 :id +end + +to id.frame :id +output item 4 :id +end + +;; Data abstraction: Frame slots + +to frame.retaddr +output 0 +end + +to frame.save.newfp +output 1 +end + +to frame.outerframe +output 2 +end + +to frame.prevframe +output 3 +end + +to frame.regsave +output 4 +end + +to framesize.proc +output 4+:numregs +end + +to frame.retval +output 4+:numregs +end + +to framesize.fun +output 5+:numregs +end + +;; Data abstraction: Operators + +to op.instr :op +output first :op +end + +to op.nargs :op +output first bf :op +end + +to op.types :op +output item 3 :op +end + +to op.prec :op +output last :op +end + +;; Data abstraction: Expressions + +to exp.type :exp +output first :exp +end + +to exp.mode :exp +output first butfirst :exp +end + +to exp.value :exp +output last :exp +end + +;; Data abstraction: Registers + +to reg.zero +output 0 +end + +to reg.retaddr +output 1 +end + +to reg.stackptr +output 2 +end + +to reg.globalptr +output 3 +end + +to reg.frameptr +output 4 +end + +to reg.newfp +output 5 +end + +to reg.retval +output 6 +end + +to reg.firstfree +output 7 +end + +;; Runtime (machine simulation) + +to prun :progname +localmake "prog thing word "% :progname +localmake "regs (array :numregs 0) +local filter "wordp :prog +foreach :prog [if wordp ? [make ? ?rest]] +localmake "memory (array :memsize 0) +setitem 0 :regs 0 +if not procedurep "add [runsetup] +prun1 :prog +end + +to prun1 :pc +if emptyp :pc [stop] +if listp first :pc [run first :pc] +prun1 butfirst :pc +end + +to rload :reg :offset :index +setitem :reg :regs (item (item :index :regs)+:offset :memory) +end + +to store :reg :offset :index +setitem (item :index :regs)+:offset :memory (item :reg :regs) +end + +to runsetup +foreach [[add sum] [sub difference] [mul product] [quo quotient] + [div [int quotient]] [rem remainder] [land product] + [lor [tobool lessp 0 sum]] [eql [tobool equalp]] + [neq [tobool not equalp]] [less [tobool lessp]] + [gtr [tobool greaterp]] [leq [tobool not greaterp]] + [geq [tobool not lessp]]] ~ + [define first ? + `[[dest src1 src2] + [setitem :dest :regs ,@[last ?] (item :src1 :regs) + (item :src2 :regs)]] + define word first ? "i + `[[dest src1 immed] + [setitem :dest :regs ,@[last ?] (item :src1 :regs) + :immed]]] +foreach [[lnot [difference 1]] [sint int] [sround round] [srandom random]] ~ + [define first ? + `[[dest src] + [setitem :dest :regs ,@[last ?] (item :src :regs)]] + define word first ? "i + `[[dest immed] + [setitem :dest :regs ,@[last ?] :immed]]] +end + +to tobool :tf +output ifelse :tf [1] [0] +end + +to jump :label +make "pc fput :label thing :label +end + +to jumpt :reg :label +if (item :reg :regs)=1 [jump :label] +end + +to jumpf :reg :label +if (item :reg :regs)=0 [jump :label] +end + +to jr :reg +make "pc item :reg :regs +end + +to jal :reg :label +setitem :reg :regs :pc +jump :label +end + +to putch :width :reg +spaces :width 1 +type char (item :reg :regs) +end + +to putstr :width :string +spaces :width (count first :string) +type :string +end + +to puttf :width :bool +spaces :width 1 +type ifelse (item :bool :regs)=0 ["F] ["T] +end + +to putint :width :reg +localmake "num (item :reg :regs) +spaces :width count :num +type :num +end + +to putreal :width :reg +putint :width :reg +end + +to spaces :width :count +if :width > :count [repeat :width - :count [type "| |]] +end + +to newline +print [] +end + +to exit +make "pc [exit] +end diff --git a/js/games/nluqo.github.io/~bh/v3ch5/pif.gif b/js/games/nluqo.github.io/~bh/v3ch5/pif.gif new file mode 100644 index 0000000..19e4153 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/pif.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/rtn1.gif b/js/games/nluqo.github.io/~bh/v3ch5/rtn1.gif new file mode 100644 index 0000000..1fc03b4 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/rtn1.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/rtn2.gif b/js/games/nluqo.github.io/~bh/v3ch5/rtn2.gif new file mode 100644 index 0000000..c6ef962 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/rtn2.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/stack1.gif b/js/games/nluqo.github.io/~bh/v3ch5/stack1.gif new file mode 100644 index 0000000..74f8640 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/stack1.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/stack2.gif b/js/games/nluqo.github.io/~bh/v3ch5/stack2.gif new file mode 100644 index 0000000..28fd54f --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/stack2.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/tokenized.gif b/js/games/nluqo.github.io/~bh/v3ch5/tokenized.gif new file mode 100644 index 0000000..d94ac01 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/tokenized.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/unambiguous.gif b/js/games/nluqo.github.io/~bh/v3ch5/unambiguous.gif new file mode 100644 index 0000000..b944d22 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/unambiguous.gif Binary files differdiff --git a/js/games/nluqo.github.io/~bh/v3ch5/v3ch5.html b/js/games/nluqo.github.io/~bh/v3ch5/v3ch5.html new file mode 100644 index 0000000..927e3cf --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/v3ch5.html @@ -0,0 +1,3297 @@ +<HTML> +<HEAD> +<TITLE>Computer Science Logo Style vol 3 ch 5: Programming Language Implementation</TITLE> +</HEAD> +<BODY> +<CITE>Computer Science Logo Style</CITE> volume 3: +<CITE>Beyond Programming</CITE> 2/e Copyright (C) 1997 MIT +<H1>Programming Language Implementation</H1> + +<TABLE width="100%"><TR><TD> +<IMG SRC="../csls3.jpg" ALT="cover photo"> +<TD><TABLE> +<TR><TD align="right"><CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian +Harvey</A><BR>University of California, Berkeley</CITE> +<TR><TD align="right"><BR> +<TR><TD align="right"><A HREF="../pdf/v3ch05.pdf">Download PDF version</A> +<TR><TD align="right"><A HREF="../v3-toc2.html">Back to Table of Contents</A> +<TR><TD align="right"><A HREF="https://people.eecs.berkeley.edu/~bh/v3ch4/v3ch4.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../v3ch6/v3ch6.html"><STRONG>NEXT</STRONG></A> +<TR><TD align="right"><A HREF="https://mitpress.mit.edu/books/computer-science-logo-style-second-edition-volume-3">MIT +Press web page for <CITE>Computer Science Logo Style</CITE></A> +</TABLE></TABLE> + +<HR><P>Program file for this chapter: <A HREF="pascal.lg"><CODE>pascal</CODE></A> + +<P>We are now ready to turn from the questions of language design to those of +compiler implementation. A Pascal compiler is a much larger programming +project than most of the ones we've explored so far. You might well ask, +"where do we <EM>begin</EM> in writing a compiler?" My goal in this chapter +is to show some of the parts that go into a compiler design. + +<P>A compiler translates programs from a language like Pascal into the +machine language of some particular computer model. My compiler +translates into a simplified, simulated machine language; the compiled +programs are actually carried out by another Logo program, the simulator, +rather than directly by the computer hardware. The advantage of using this +simulated machine language is that this compiler will work no matter what +kind of computer you have; also, the simplifications in this simulated +machine allow me to leave out many confusing details of a practical compiler. +Our machine language is, however, realistic enough to give you a good sense +of what compiling into a real machine language would be like; it's loosely +based on the MIPS microprocessor design. You'll see in a moment that most +of the structure of the compiler is independent of the target language, +anyway. + +<P> + +<P>Here is a short, uninteresting Pascal program: + +<P><PRE>program test; + +procedure doit(n:integer); + begin + writeln(n,n*n) + end; + +begin + doit(3) +end. +</PRE> + +<P>If you type this program into a disk file and then compile it +using <CODE>compile</CODE> as described in Chapter 4, the compiler will translate +the program into this sequence of instructions, contained in a list in the +variable named <CODE>%test</CODE>: + +<P> + +<P><PRE>[ [add 3 0 0] + [add 4 0 0] + [addi 2 0 36] + [jump "g1] +%doit [store 1 0(4)] + [jump "g2] +g2 [rload 7 36(4)] + [putint 10 7] + [rload 7 36(4)] + [rload 8 36(4)] + [mul 7 7 8] + [putint 10 7] + [newline] + [rload 1 0(4)] + [add 2 4 0] + [rload 4 3(2)] + [jr 1] +g1 [store 5 1(2)] + [add 5 2 0] + [addi 2 2 37] + [store 4 3(5)] + [store 4 2(5)] + [addi 7 0 3] + [store 7 36(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%doit] + [exit] +] +</PRE> + +<P>I've displayed this list of instructions with some extra spacing +thrown in to make it look somewhat like a typical <EM>assembler</EM> listing. +(An assembler is a program that translates a notation like <CODE>add 3 0 0</CODE> +into a binary number, the form in which the machine hardware actually +recognizes these instructions.) A real assembler listing wouldn't have the +square brackets that Logo uses to mark each sublist, but would instead +depend on the convention that each instruction occupies one line. + +<P>The first three instructions carry out initialization that would be the same +for any compiled Pascal program; the fourth is a <CODE>jump</CODE> instruction that +tells the (simulated) computer to skip to the instruction following the +<EM>label</EM> <CODE>g1</CODE> that appears later in the program. (A word that +isn't part of a sublist is a label.) In Pascal, the body of the main +program comes after the declarations of procedures; this <CODE>jump</CODE> +instruction allows the compiler to translate the parts of the program in the +order in which they appear. + +<P>(Two instructions later, you'll notice a <CODE>jump</CODE> to a label that comes +right after the jump instruction! The compiler issues this useless +instruction just in case some internal procedures were declared within +the procedure <CODE>doit</CODE>. A better compiler would include an <EM> +optimizer</EM> that would go through the compiled program looking for +ways to eliminate unnecessary instructions such as this one. The optimizer +is the most important thing that I've left out of my compiler.) + +<P>We're not ready yet to talk in detail about how the compiled instructions +represent the Pascal program, but you might be able to guess certain things. +For example, the variable <CODE>n</CODE> in procedure <CODE>doit</CODE> seems to be +represented as <CODE>36(4)</CODE> in the compiled program; you can see where <CODE> +36(4)</CODE> is printed and then multiplied by itself, although it may not yet be +clear to you what the numbers <CODE>7</CODE> and <CODE>8</CODE> have to do with anything. +Before we get into those details, I want to give a broader overview of +the organization of the compiler. + +<P>The compilation process is divided into three main pieces. First and +simplest is <EM>tokenization.</EM> The compiler initially sees the +source program as a string of characters: <CODE>p</CODE>, then <CODE>r</CODE>, and so on, +including spaces and line separators. The first step in compilation is to +turn these characters into symbols, so that the later stages of compilation +can deal with the word <CODE>program</CODE> as a unit. The second piece of the +compiler is the <EM>parser,</EM> the part that recognizes certain +patterns of symbols as representing meaningful units. "Oh," says the +parser, "I've just seen the word <CODE>procedure</CODE> so what comes next must be +a procedure header and then a <CODE>begin</CODE>-<CODE>end</CODE> block for the body of +the procedure." Finally, there is the process of <EM> +code generation,</EM> in which each unit that was recognized by the +parser is actually translated into the equivalent machine language +instructions. + +<P>(I don't mean that parsing and code generation happen separately, one after +the other, in the compiler's algorithm. In fact each meaningful unit is +translated as it's encountered, and the translation of a large unit like a +procedure includes, recursively, the translation of smaller units like +statements. But parsing and code generation are conceptually two different +tasks, and we'll talk about them separately.) + +<P><H2>Formal Syntax Definition</H2> + +<P>One common starting place is to develop a formal definition for the language +we're trying to compile. The regular expressions of Chapter 1 are an +example of what I mean by a formal definition. A +regular expression tells +us unambiguously that certain strings of characters are accepted as members +of the category defined by the expression, while other strings aren't. A +language like Pascal is too complicated to be described by a regular +expression, but other kinds of formal definition can be used. + +<P>The formal systems of Chapter 1 just gave a yes-or-no decision for any input +string: Is it, or is it not, accepted in the language under discussion? +That's not quite good enough for a compiler. We don't just want to know +whether a Pascal program is syntactically correct; we want a translation of +the program into some executable form. Nevertheless, it turns out to be +worthwhile to begin by designing a formal acceptor for Pascal. That part of +the compiler--the part that determines the syntactic structure of the +source program--is called the <EM>parser.</EM> Later we'll add provisions for +<EM>code generation:</EM> the translation of each syntactic unit of the +source program into a piece of <EM>object</EM> (executable) program that +carries out the meaning (the <EM>semantics</EM>) of that unit. + +<P>One common form in which programming languages are described is the +<EM>production rule</EM> notation mentioned briefly in Chapter 1. For +example, here is part of a specification for Pascal: + +<P><PRE>program: program <EM>identifier</EM> <EM>filenames</EM> ; <EM>block</EM> . + +filenames: | ( <EM>idlist</EM> ) + +idlist: <EM>identifier</EM> | <EM>idlist</EM> , <EM>identifier</EM> + +block: <EM>varpart</EM> <EM>procpart</EM> <EM>compound</EM> + +varpart: | var <EM>varlist</EM> + +procpart: | <EM>procpart</EM> <EM>procedure</EM> | <EM>procpart</EM> <EM>function</EM> + +compound: begin <EM>statements</EM> end + +statements: <EM>statement</EM> | <EM>statements</EM> ; <EM>statement</EM> + +procedure: procedure <EM>identifier</EM> <EM>args</EM> ; <EM>block</EM> ; + +function: function <EM>identifier</EM> <EM>args</EM> : <EM>type</EM> ; <EM>block</EM> ; +</PRE> + +<P>A program consists of six components. Some of these components +are particular words (like <CODE>program</CODE>) or punctuation marks; other +components are defined in terms of even smaller units by other +rules.<SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>The <EM>filenames</EM> component is an optional list of +names of files, part of Pascal's input/output capability; my compiler doesn't +handle file input or output, so it ignores this list if there is one.</SMALL></BLOCKQUOTE></SMALL><P>A vertical bar (<CODE>|</CODE>) in a rule separates alternatives; an idlist +(identifier list) is either a single identifier or a smaller idlist followed +by a comma and another identifier. Sometimes one of the alternatives in a +rule is empty; for example, a varpart can be empty because a block need not +declare any local variables. + +<P>The goal in designing a formal specification is to capture the +syntactic hierarchy +of the language you're describing. For example, you could define +a Pascal type as + +<P><PRE>type: integer | real | char | boolean | array <EM>range</EM> of integer | + packed array <EM>range</EM> of integer | array <EM>range</EM> of real | + ... +</PRE> + +<P>but it's better to say + +<P><PRE>type: <EM>scalar</EM> | array <EM>range</EM> of <EM>scalar</EM> | + packed array <EM>range</EM> of <EM>scalar</EM> + +scalar: integer | real | char | boolean +</PRE> + +<P>Try completing the syntactic description of my subset of Pascal along these +lines. You might also try a similar syntactic description of Logo. Which +is easier? + +<P>Another kind of formal description is + +the <EM>recursive transition network</EM> (RTN). +An RTN is like a finite-state machine except that instead +of each arrow representing a single symbol in the machine's alphabet, an +arrow can be labeled with the name of another RTN; such an arrow represents +any string of symbols accepted by that RTN. + +<P>Below I show two RTNs, one for a program and one for a +sequence of statements (the body of a compound statement). +In the former, the transition from state 5 to state 6 is followed if what +comes next in the Pascal program is a string of symbols accepted by the RTN +named "block." In these diagrams, a word in <CODE>typewriter</CODE> style like +<CODE>program</CODE> represents a single symbol, as in a finite-state machine diagram, +while a word in <EM>italics</EM> like <EM>block</EM> represents any +string accepted by the RTN of that name. The <EM>statements</EM> RTN +is recursive; one path through the network involves a transition that +requires parsing a smaller <EM>statements</EM> unit. + +<P><CENTER><IMG SRC="rtn1.gif" ALT="figure: rtn1"></CENTER> +<P><CENTER><IMG SRC="rtn2.gif" ALT="figure: rtn2"></CENTER> + +<P><H2>Tokenization</H2> + + +<P>In both the production rules and the RTNs I've treated words like <CODE> +program</CODE> as a single symbol of the "alphabet" of the language. It would +be possible, of course, to use single characters as the alphabetic symbols +and describe the language in this form: + +<P><CENTER><IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/tokenrtn.gif" ALT="figure: tokenrtn"></CENTER> + +<P>Extending the formal description down to that level, though, makes +it hard to see the forest for the trees; the important structural patterns +get lost in details about, for instance, where spaces are required between +words (as in <CODE>program tower</CODE>), where they're optional (as in <CODE> +2 + 3</CODE>), and where they're not allowed at all (<CODE>prog +ram</CODE>). A similar complication is that a comment in braces can +be inserted anywhere in the program; it would be enormously complicated if +every state of every RTN had to have a transition for a left brace beginning +a comment. + +<P>Most language processors therefore group the characters of the source +program into <EM>tokens</EM> used as the alphabet for the formal +grammar. A token may be a single character, such as a punctuation mark, or +a group of characters, such as a word or a number. Spaces do not ordinarily +form part of tokens, although in the Pascal compiler one kind of token is a +quoted character string that can include spaces. Comments are also removed +during tokenization. Here's what the <CODE>tower</CODE> program from Chapter 4 +looks like in token form: + +<P><CENTER><IMG SRC="tokenized.gif"></CENTER> + +<P>Tokenization is what the Logo <CODE>readlist</CODE> operation does when +it uses spaces and brackets to turn the string of characters you type into +a sequence of words and lists. + +<P>Tokenization is also called <EM>lexical analysis.</EM> This term has +nothing to do with lexical scope; the word "lexical" is used not to remind +us of a dictionary but because the root "lex" means <EM>word</EM> and +lexical analysis divides the source program into words. + +<P>I've been talking as if the Pascal compiler first went through the entire +source file tokenizing it and then went back and parsed the result. That's +not actually how it works; instead, the parser just calls a procedure named +<CODE>token</CODE> whenever it wants to see the next token in the source file. +I've already mentioned that Pascal was designed to allow the compiler to +read straight through the source program without jumping around and +re-reading parts of it. + +<P><H2>Lookahead</H2> + + +<P>Consider the situation when the parser has recognized the first token +(<CODE>program</CODE>) as the beginning of a program and it invokes <CODE>token</CODE> to +read the second token, the program name. In the <CODE>tower</CODE> program, the +desired token is <CODE>tower</CODE>. <CODE>Token</CODE> reads the letter <CODE>t</CODE>; since +it's a letter, it must be the beginning of an identifier. Any number of +letters or digits following the <CODE>t</CODE> will be part of the identifier, but +the first non-alphanumeric character ends the token. (In this case, the +character that ends the token will be a semicolon.) + +<P>What this means is that <CODE>token</CODE> has to read one character too many in +order to find the end of the word <CODE>tower</CODE>. The semicolon isn't +part of that token; it's part of the <EM>following</EM> token. (In fact it's +the entire following token, but in other situations that need not be true.) +Ordinarily <CODE>token</CODE> begins its work by reading a character from the +source file, but the next time we call <CODE>token</CODE> it has to deal with the +character it's already read. It would simplify things enormously if <CODE> +token</CODE> could "un-read" the semicolon that ends the token <CODE> +tower</CODE>. It's possible to allow something like un-reading by using a +technique called <EM>lookahead.</EM> + +<P><PRE>to getchar +local "char +if namep "peekchar + [make "char :peekchar + ern "peekchar + output :char] +output readchar +end +</PRE> + +<P><CODE>Getchar</CODE> is the procedure that <CODE>token</CODE> calls to read the next +character from the source file. Ordinarily <CODE>getchar</CODE> just invokes the +primitive <CODE>readchar</CODE> to read a character from the file.<SUP>*</SUP> But if there is a variable named <CODE>peekchar</CODE>, then <CODE> +getchar</CODE> just outputs whatever is in that variable without looking at the +file. <CODE>Token</CODE> can now un-read a character by saying + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>I'm lying. +The real <CODE>getchar</CODE> is slightly more complicated because it checks for an +unexpected end of file and because it prints the characters that it reads +onto the screen. The program listing at the end of the chapter tells the +whole story.</SMALL></BLOCKQUOTE></SMALL><P><PRE>make "peekchar :char +</PRE> + +<P>This technique only allows <CODE>token</CODE> to un-read a single +character at a time. It would be possible to replace <CODE>peekchar</CODE> with a +<EM>list</EM> of pre-read characters to be recycled. But in fact one is +enough. When a program "peeks at" characters before they're read "for +real," the technique is called <EM>lookahead.</EM> <CODE>Getchar</CODE> uses +<EM>one-character lookahead</EM> because <CODE>peekchar</CODE> only stores a single +character. + +<P>It turns out that, for similar reasons, the Pascal parser will occasionally +find it convenient to peek at a <EM>token</EM> and re-read it later. <CODE> +Token</CODE> therefore provides for one-<EM>token</EM> lookahead using a similar +mechanism: + +<P><PRE>to token +local [token char] +if namep "peektoken [make "token :peektoken + ern "peektoken output :token] +make "char getchar +if equalp :char "|{| [skipcomment output token] +if equalp :char char 32 [output token] +if equalp :char char 13 [output token] +if equalp :char char 10 [output token] +if equalp :char "' [output string "'] +if memberp :char [+ - * / = ( , ) |[| |]| |;|] [output :char] +if equalp :char "|<| [output twochar "|<| [= >]] +if equalp :char "|>| [output twochar "|>| [=]] +if equalp :char ". [output twochar ". [.]] +if equalp :char ": [output twochar ": [=]] +if numberp :char [output number :char] +if letterp ascii :char [output token1 lowercase :char] +(throw "error sentence [unrecognized character:] :char) +end + +to twochar :old :ok +localmake "char getchar +if memberp :char :ok [output word :old :char] +make "peekchar :char +output :old +end +</PRE> + +<P>As you can see, <CODE>token</CODE> is mainly a selection of special cases. +<CODE>Char 32</CODE> is a space; <CODE>char 13</CODE> or <CODE>char 10</CODE> is the end-of-line +character. <CODE>Skipcomment</CODE> skips over characters until it sees a right +brace. <CODE>String</CODE> accumulates characters up to and including a single +quote (apostrophe), except that two single quotes in a row become one single +quote inside the string and don't end the string. <CODE>Number</CODE> is a little +tricky because of decimal points (the string of characters <CODE>1.10</CODE> is a +single token, but the string <CODE>1..10</CODE> is three tokens!) and exponent +notation. I'm not showing you all the details because the compiler is a +very large program and we'll never get through it if I annotate every +procedure. But I did want to show you <CODE>twochar</CODE> because it's a good, +simple example of character lookahead at work. If the character <CODE><</CODE> is +seen in the source program, it may be a token by itself or it may be part of +the two-character tokens <CODE><=</CODE> or <CODE><></CODE>. <CODE>Twochar</CODE> takes a peek +at the next character in the file to decide. + +<P>If the character that <CODE>token</CODE> reads isn't part of any recognizable +token, the procedure generates an error. (The error is caught by the +toplevel procedure <CODE>compile</CODE> so that it can close the source file.) +This extremely primitive error handling is one of the most serious +deficiencies in my compiler; it would be better if the compilation process +continued, despite the error, so that any other errors in the program could +also be discovered. In a real compiler, more than half of the parsing +effort goes into error handling; it's relatively trivial to parse a correct +source program. + +<P><H2>Parsing</H2> + + +<P>There are general techniques for turning a formal language specification, +such as a set of production rules, into an algorithm for parsing the +language so specified. These techniques are analogous to the program in +Chapter 1 that translates a regular expression into a finite-state machine. +A program that turns a formal specification into a parser is called a +<EM>parser generator.</EM> + +<P>The trouble is that the techniques that work for <EM>any</EM> set of rules +are quite slow. The time required to parse a sequence of length <EM>n</EM> is +O(<EM>n</EM><SUP><SMALL>2</SMALL></SUP>) if the grammar is unambiguous or O(<EM>n</EM><SUP><SMALL>3</SMALL></SUP>) if it's +ambiguous. A grammar is <EM>ambiguous</EM> if the same input sequence +can be parsed correctly in more than one way. For example, if the +production rule + +<P><PRE>idlist: <EM>identifier</EM> | <EM>idlist</EM> , <EM>identifier</EM> +</PRE> + +<P>is applied to the string + +<P><PRE>Beatles,Who,Zombies,Kinks +</PRE> + +<P>then the only possible application of the rule to accept the +string produces this left-to-right grouping: + +<P><CENTER><IMG SRC="unambiguous.gif" ALT="figure: unambiguous"></CENTER> + +<P>However, if the rule were + +<P><PRE>idlist: <EM>identifier</EM> | <EM>idlist</EM> , <EM>idlist</EM> +</PRE> + +<P>this new rule would accept the same strings, but would allow +alternative groupings like + +<P><CENTER><IMG SRC="ambiguous.gif" ALT="figure: ambiguous"></CENTER> + +<P>The former rule could be part of an unambiguous grammar; the new +rule makes the grammar that contains it ambiguous. + +<P> + +<P>It's usually not hard to devise an unambiguous grammar for any practical +programming language, but even a quadratic algorithm is too slow. Luckily, +most programming languages have <EM>deterministic grammars,</EM> +which is a condition even stricter than being unambiguous. It means that a +parser can read a program from left to right, and can figure out what to do +with the next token using only a fixed amount of lookahead. A parser for a +deterministic grammar can run in linear time, which is a lot better than +quadratic. + +<P>When I said "figure out what to do with the next token," I was being +deliberately vague. A deterministic parser doesn't necessarily know exactly +how a token will fit into the complete program--which production rules will +be branch nodes in a parse tree having this token as a leaf node--as soon +as it reads the token. As a somewhat silly example, pretend that the word +<CODE>if</CODE> is not a "reserved word" in Pascal; suppose it could be the +name of a variable. Then, when the parser is expecting the beginning of a +new statement and the next token is the word <CODE>if</CODE>, the parser doesn't +know whether it is seeing the beginning of a conditional statement such as +<CODE>if x > 0 then writeln('positive')</CODE> or the beginning of an assignment +statement such as <CODE>if := 87</CODE>. But the parser could still be deterministic. +Upon seeing the word <CODE>if</CODE>, it would enter a state (as in a finite state +machine) from which there are two exits. If the next token turned out to be +the <CODE>:=</CODE> assignment operator, the parser would follow one transition; if +the next token was a variable or constant value, the parser would choose a +different next state. + +<P>The real Pascal, though, contains no such syntactic cliffhangers. A Pascal +compiler can always tell which production rule the next token requires. +That's why the language includes keywords like <CODE>var</CODE>, <CODE> +procedure</CODE>, and <CODE>function</CODE>. For the most part, you could figure out +which kind of declaration you're reading without those keywords by looking +for clues like whether or not there are parentheses after the identifier +being declared. (If so, it's a procedure or a function.) But the keywords +let you know from the beginning what to expect next. That means we can +write what's called a <EM>predictive grammar</EM> for Pascal, +even simpler to implement than a deterministic one. + +<P>There are general algorithms for parsing deterministic languages, and there +are parser generators using these algorithms. One widely used example is +the YACC (Yet Another Compiler Compiler) program that translates +production rules into a parser in the C programming language.<SUP>*</SUP> But because Pascal's +grammar is so simple I found it just as easy to do the translation by hand. +For each production rule in a formal description of Pascal, the compiler +includes a Logo procedure that parses each component part of the production +rule. A parser written in this way is called a <EM> +recursive descent parser.</EM> Here's a sample: + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>A parser +generator is also called a <EM>compiler compiler</EM> because it treats +the formal specification as a kind of source program and produces a compiler +as the object program. But the name isn't quite accurate because, as you +know, there's more to a compiler than the parser.</SMALL></BLOCKQUOTE></SMALL><P><PRE>to statement +local [token type] +ifbe "begin [compound stop] +ifbe "for [pfor stop] +ifbe "if [pif stop] +ifbe "while [pwhile stop] +ifbe "repeat [prepeat stop] +ifbe "write [pwrite stop] +ifbe "writeln [pwriteln stop] +make "token token +make "peektoken :token +if memberp :token [|;| end until] [stop] +make "type gettype :token +if emptyp :type [(throw "error sentence :token [can't begin statement])] +if equalp :type "procedure [pproccall stop] +if equalp :type "function [pfunset stop] +passign +end + +to pif +local [cond elsetag endtag] +make "cond pboolean pexpr +make "elsetag gensym +make "endtag gensym +mustbe "then +code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +statement +code (list "jump (word "" :endtag)) +code :elsetag +ifbe "else [statement] +code :endtag +end +</PRE> + +<P>Many of the details of <CODE>pif</CODE> have to do with code +generation, but never mind those parts now. For the +moment, my concern is with the parsing aspect of these procedures: how they +decide what to accept. + +<P><CODE>Statement</CODE> is an important part of the parser; it is invoked whenever a +Pascal statement is expected. It begins by checking the next token from the +source file. If that token is <CODE>begin</CODE>, <CODE>for</CODE>, <CODE>if</CODE>, <CODE>while</CODE>, +or <CODE>repeat</CODE> then we're finished with the token and <CODE>statement</CODE> turns +to a subprocedure to handle the syntax of whatever structured statement type +we've found. If the token isn't one of those, then the statement has to be +a simple statement and the token has to be an identifier, i.e., the name of +a procedure, a function, or a variable. (One other trivial possibility is +that this is an <EM>empty</EM> statement, if we're already up to the +semicolon, <CODE>end</CODE>, or <CODE>until</CODE> that marks the end of a statement.) +In any of these cases, the token we've just read is important to the parsing +procedure that will handle the simple statement, so <CODE>statement</CODE> un-reads +it before deciding what to do next. <CODE>Gettype</CODE> outputs the type of the +identifier, either a variable type like <CODE>real</CODE> or else <CODE>procedure</CODE> +or <CODE>function</CODE>. (The compiler data structures that underlie the work of +<CODE>gettype</CODE> will be discussed later.) If the token is a +procedure name, then this is a procedure call statement. If the token is a +function name, then this is the special kind of assignment inside a function +definition that provides the return value for the function. Otherwise, the +token must be a variable name and this is an ordinary assignment statement. + +<P>The procedure <CODE>pif</CODE> parses <CODE>if</CODE> statements. (The letter <CODE>p</CODE> in +its name stands for "Pascal"; many procedures in the compiler have such +names to avoid conflicts with Logo procedures with similar purposes.) The +syntax of Pascal <CODE>if</CODE> is + +<P><PRE>ifstatement: if <EM>boolean</EM> then <EM>statement</EM> | + if <EM>boolean</EM> then <EM>statement</EM> else <EM>statement</EM> +</PRE> + +<P>When <CODE>pif</CODE> begins, the token <CODE>if</CODE> has just been read by +<CODE>statement</CODE>. So the first thing that's required is a boolean expression. +<CODE>Pexpr</CODE> parses an expression; that task is relatively complicated and +will be discussed in more detail later. <CODE>Pboolean</CODE> ensures that the +expression just parsed does indeed produce a value of type <CODE>boolean</CODE>. + +<P>The next token in the source file <EM>must</EM> be the word <CODE>then</CODE>. The +instruction + +<P><PRE>mustbe "then +</PRE> + +<P>in <CODE>pif</CODE> ensures that. Here's <CODE>mustbe</CODE>: + +<P><PRE>to mustbe :wanted +localmake "token token +if equalp :token :wanted [stop] +(throw "error (sentence "expected :wanted "got :token)) +end +</PRE> + +<P>If <CODE>mustbe</CODE> returns successfully, <CODE>pif</CODE> then invokes +<CODE>statement</CODE> recursively to parse the true branch of the conditional. +The production rule tells us that there is then an <EM>optional</EM> false +branch, signaled by the reserved word <CODE>else</CODE>. The instruction + +<P><PRE>ifbe "else [statement] +</PRE> + +<P>handles that possibility. If the next token matches the first +input to <CODE>ifbe</CODE> then the second input, an instruction list, is carried +out. Otherwise the token is un-read. There is also an <CODE>ifbeelse</CODE> that +takes a third input, an instruction list to be carried out if the next token +isn't equal to the first input. (<CODE>Ifbeelse</CODE> still un-reads the token in +that case, before it runs the third input.) These must be macros so that +the instruction list inputs can include <CODE>output</CODE> or <CODE>stop</CODE> +instructions (as discussed in Volume 2), as in the invocations of <CODE>ifbe</CODE> +in <CODE>statement</CODE> seen a moment ago. + +<P><PRE>.macro ifbe :wanted :action +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output [] +end + +.macro ifbeelse :wanted :action :else +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output :else +end +</PRE> + +<P>If there were no code generation involved, <CODE>pif</CODE> would be written this +way: + +<P><PRE>to pif +pboolean pexpr +mustbe "then +statement +ifbe "else [statement] +end +</PRE> + +<P>This simplified procedure is a straightforward translation of the +RTN + +<P><CENTER><IMG SRC="pif.gif" ALT="figure: pif"></CENTER> + +<P>The need to generate object code complicates the parser. But +don't let that distract you; in general you can see the formal structure of +Pascal syntax reflected in the sequence of instructions used to parse that +syntax. + +<P>The procedures that handle other structured statements, such as <CODE>pfor</CODE> +and <CODE>pwhile</CODE>, are a lot like <CODE>pif</CODE>. Procedure and function +declarations (procedures <CODE>procedure</CODE>, <CODE>function</CODE>, and <CODE>proc1</CODE> in +the compiler) also use the same straightforward parsing technique, but are a +little more complicated because of the need to keep track of type +declarations for each procedure's parameters and local variables. +Ironically, the hardest thing to compile is the "simple" assignment +statement, partly because of operator precedence (multiplication before +addition) in expressions (procedure <CODE>pexpr</CODE> in the compiler) and partly +because of the need to deal with the complexity of variables, including +special cases such as assignments to <CODE>var</CODE> parameters and array elements. + +<P>I haven't yet showed you <CODE>pboolean</CODE> because you have to understand how +the compiler handles expressions first. But it's worth noticing that Pascal +can check <EM>at compile time</EM> whether or not an expression is going to +produce a <CODE>boolean</CODE> value even though the program hasn't been run yet +and the variables in the expression don't have values yet. It's the strict +variable typing of Pascal that makes this compile-time checking possible. +If we were writing a Logo compiler, the checking would have to be postponed +until run time because you can't, in general, know what type of datum will +be computed by a Logo expression until it's actually evaluated. + +<P><H2>Expressions and Precedence</H2> + +<P> + +<P>Arithmetic or boolean expressions appear not only on the right side of +assignment statements but also as actual parameters, array index values, and +as "phrases" in structured statements. One of the classic problems in +compiler construction is the translation of these expressions to executable +form. The interesting difficulty concerns <EM> +operator precedence</EM>--the rule that in a string of alternating +operators and operands, multiplications are done before additions, so + +<P><PRE>a + b * c + d +</PRE> + +<P>means + +<P><PRE>a + (b * c) + d +</PRE> + +<P>Pascal has four levels of operator precedence. The highest level, number 4, + +is the <EM>unary</EM> operators <CODE>+</CODE>, <CODE>-</CODE>, and <CODE>not</CODE>. (The first + +two can be used as unary operators (<CODE>-3</CODE>) or <EM>binary</EM> ones (<CODE> +6-3</CODE>); it's only in the unary case that they have this +precedence.)<SUP>*</SUP> Then comes multiplication, division, and +logical <CODE>and</CODE> at level 3. Level 2 has binary addition, subtraction, and +<CODE>or</CODE>. And level 1 includes the relational operators like +<CODE>=</CODE>. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>It's unfortunate that the word "binary" is used in +computer science both for base-2 numbers and for two-input operations. + Kenneth Iverson, in his documentation for the +language APL, used the words <EM>monadic</EM> and <EM> +dyadic</EM> instead of unary and binary to avoid that ambiguity. But +those terms haven't caught on.</SMALL></BLOCKQUOTE></SMALL><P> + +<P>The formalization of precedence could be done using the mechanisms we've +already seen. For example, here is a production rule grammar for +expressions using only the four basic arithmetic operations. + +<P><PRE>expression: <EM>term</EM> | <EM>expression</EM> + <EM>term</EM> | <EM>expression</EM> - <EM>term</EM> +term: <EM>factor</EM> | <EM>term</EM> * <EM>factor</EM> | <EM>term</EM> / <EM>factor</EM> +factor: <EM>variable</EM> | <EM>number</EM> | ( <EM>expression</EM> ) +</PRE> + +<P>This grammar also introduces into the discussion the fact that the +precedence of operations can be changed by using parentheses. + +<P>This grammar, although formally correct, is not so easy to use in a +recursive descent parser. +One subtle but important problem is that it's <EM>left recursive:</EM> Some +of the alternative forms for an <CODE>expression</CODE> start with an <CODE> +expression</CODE>. If we tried to translate this into a Logo procedure it would +naturally start out + +<P><PRE>to expression +local [left op right] +make "left expression +ifbe "+ + [make "op "+ + make "right term] + [ifbe "- + [make "op "- + make "right term] + [make "op []] ] +... +</PRE> + +<P>But this procedure will never get past the first <CODE>make</CODE>; it's +an infinite loop. It will never actually read a token from the source file; +instead it keeps invoking itself recursively. + +<P>Left association is a problem for automatic compiler compilers, too. There +are ways to solve the problem but I don't want to get into that because in +fact arithmetic expressions are generally handled by an entirely different +scheme, which I'll show you in a moment. The problem wouldn't come up if +the order of the operands were reversed, so the rules said + +<P><PRE>expression: <EM>term</EM> | <EM>term</EM> + <EM>expression</EM> | <EM>term</EM> - <EM>expression</EM> +</PRE> + +<P>and so on. Unfortunately this changes the meaning, and the rules +of Pascal say that equal-precedence operations are performed left to right. + +<P>In any case, the formalization of precedence with production rules gets more +complicated as the number of levels of precedence increases. I showed you a +grammar with two levels. Pascal, with four levels, might reasonably be done +in a similar way, but think about the C programming language, which has 15 +levels of precedence! + +<P><H2>The Two-Stack Algorithm for Expressions</H2> + + +<P>What we're after is an algorithm that will allow the compiler to read an +expression once, left to right, and group operators and operands correctly. +The algorithm involves the use of two stacks, one for operations and one for +data. For each operation we need to know whether it's unary or binary and +what its precedence level is. I'll use the notation "<CODE>*</CODE><SUB><SMALL>2,3</SMALL></SUB>" to +represent binary <CODE>*</CODE> at precedence level 3. So the expression + +<P><PRE>a + b * - c - d +</PRE> + +<P>will be represented in this algorithm as + +<P> +<IMG SRC="vdash.gif"><SUB><SMALL>0</SMALL></SUB> a +<SUB><SMALL>2,2</SMALL></SUB> b *<SUB><SMALL>2,3</SMALL></SUB> -<SUB><SMALL>1,4</SMALL></SUB> c -<SUB><SMALL>2,2</SMALL></SUB> d <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"><SUB><SMALL>0</SMALL></SUB> + + +<P>The symbols <IMG SRC="vdash.gif"> and <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"> aren't really part of the source +expression; they're imaginary markers for the beginning and end of the +expression. When we read a token that doesn't make sense as part of an +expression, we can un-read that token and pretend we read a <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"> instead. +These markers are given precedence level zero because they form a boundary +for <EM>any</EM> operators inside them, just as a low-precedence operator like +<CODE>+</CODE> is a boundary for the operands of a higher-precedence operator like +<CODE>*</CODE>. (For the same reason, you'll see that parentheses are considered +precedence zero.) + +<P>The two minus signs in this expression have two different meanings. As you +read the following algorithm description, you'll see how the algorithm knows +whether an operation symbol is unary or binary. + +<P><STRONG>Step 1.</STRONG> We initialize the two stacks this way: + +<P><PRE>operation: [ <IMG SRC="vdash.gif"><SUB><SMALL>0</SMALL></SUB> ] + +data: [ ] +</PRE> + +<P><STRONG>Step 2.</STRONG> We are now expecting a datum, such as a variable. Read a +token. If it's an operation, it must be unary; subscript it accordingly and +go to step 4. If it's a datum, push it onto the data stack. (If it's +neither an operation nor a datum, something's wrong.) + +<P><STRONG>Step 3.</STRONG> We are now expecting a binary operation. Read a token. If +it's an operation, subscript it as binary and go to step 4. If not, we've +reached the end of the expression. Un-read the token, and go to step 4 with +the token <IMG SRC="https://people.eecs.berkeley.edu/~bh/v3ch5/dashv.gif"><SUB><SMALL>0</SMALL></SUB>. + +<P><STRONG>Step 4.</STRONG> We have an operation in hand at this point and we know its +precedence level and how many arguments it needs. Compare its precedence +level with that of the topmost (most recently pushed) operation on the stack. +If the precedence of the new operation is less than or equal to that of the +one on the stack, go to step 5. If it's greater, go to step 6. + +<P><STRONG>Step 5.</STRONG> The topmost operation on the stack has higher precedence than +the one we just read, so we should do it right away. (For example, we've +just read the <CODE>+</CODE> in <CODE>a*b+c</CODE>; the multiplication operation and both +of its operands are ready on the stacks.) Pop the operation off +the stack, pop either one or two items off the data stack depending on the +first subscript of the popped operation, then compile machine instructions to +perform the indicated computation. Push the result on the data stack +as a single quantity. However, if the operation we popped is <IMG SRC="vdash.gif">, then +we're finished. There should be only one thing on the data stack, and it's +the completely compiled expression. Otherwise, we still have the new +operation waiting to be processed, so return to step 4. + +<P><STRONG>Step 6.</STRONG> The topmost operation on the stack has lower precedence than +the one we just read, so we can't do it yet because we're still reading its +right operand. (For example, we've just read the <CODE>*</CODE> in <CODE>a+b*c</CODE>; +we're not ready to do either operation until we read the <CODE>c</CODE> later.) +Push the new operation onto the operation stack, then return to step 2. + +<P>Here's how this algorithm works out with the sample expression above. In +the data stack, a boxed entry like <IMG SRC="a+b.gif"> means the result from +translating that subexpression into the object language. + +<P><IMG SRC="stack1.gif"> +<BR><IMG SRC="stack2.gif"> + +<P>The final value on the data stack is the translation of the +entire expression. + +<P>The algorithm so far does not deal with parentheses. They're handled +somewhat like operations, but with slightly different rules. A left +parenthesis is stored on the operation stack as <CODE>(</CODE><SUB><SMALL>0</SMALL></SUB>, like the +special marker at the beginning of the expression, but it does not invoke +step 5 of the algorithm before being pushed on the stack. A right +parenthesis <EM>does</EM> invoke step 5, but only as far down the stack as +the first matching left parenthesis; if it were an ordinary operation of +precedence zero it would pop everything off the stack. You might try to +express precisely how to modify the algorithm to allow for parentheses. + +<P>Here are the procedures that embody this algorithm in the compiler. +<CODE>Pgetunary</CODE> and <CODE>pgetbinary</CODE> output a list like + +<P><PRE>[sub 2 2] +</PRE> + +<P>for binary <CODE>-</CODE> or + +<P><PRE>[minus 1 4] +</PRE> + +<P>for unary minus. (I'm leaving out some complications +having to do with type checking.) They work by looking for a <CODE>unary</CODE> or +<CODE>binary</CODE> property on the property list of the operation symbol. +Procedures with names like <CODE>op.prec</CODE> are selectors for the members +of these lists. + +<P>In this algorithm, only step 5 actually generates any instructions in the +object program. This is the step in which an operation is removed from +the operation stack and actually performed. Step 5 is carried out by the +procedure <CODE>ppopop</CODE> (Pascal pop operation); most of that procedure deals +with code generation, but I've omitted that part of the procedure in the +following listing because right now we're concerned with the parsing +algorithm. We'll return to code generation shortly. + +<P><CODE>Pexpr1</CODE> invokes <CODE>pdata</CODE> when it expects to read an operand, which +could be a number, a variable, or a function call. <CODE>Pdata</CODE>, which I'm +not showing here, generates code to make the operand available and +outputs the location of the result in the simulated computer, in a form +that can be used by <CODE>pexpr</CODE>. + +<P><PRE>to pexpr +local [opstack datastack parenlevel] +make "opstack [[popen 1 0]] ; step 1 +make "datastack [] +make "parenlevel 0 +output pexpr1 +end + +to pexpr1 +local [token op] +make "token token ; step 2 +while [equalp :token "|(|] [popen make "token token] +make "op pgetunary :token +if not emptyp :op [output pexprop :op] +push "datastack pdata :token +make "token token ; step 3 +while [and (:parenlevel > 0) (equalp :token "|)| )] + [pclose make "token token] +make "op pgetbinary :token +if not emptyp :op [output pexprop :op] +make "peektoken :token +pclose +if not emptyp :opstack [(throw "error [too many operators])] +if not emptyp butfirst :datastack [(throw "error [too many operands])] +output pop "datastack +end + +to pexprop :op ; step 4 +while [(op.prec :op) < (1 + op.prec first :opstack)] [ppopop] +push "opstack :op ; step 6 +output pexpr1 +end + +to ppopop ; step 5 +local [op function args left right type reg] +make "op pop "opstack +make "function op.instr :op +if equalp :function "plus [stop] +make "args op.nargs :op +make "right pop "datastack +make "left (ifelse equalp :args 2 [pop "datastack] [[[] []]]) +make "type pnewtype :op exp.type :left exp.type :right +... code generation omitted ... +push "datastack (list :type "register :reg) +end + +to popen +push "opstack [popen 1 0] +make "parenlevel :parenlevel+1 +end + +to pclose +while [(op.prec first :opstack) > 0] [ppopop] +ignore pop "opstack +make "parenlevel :parenlevel - 1 +end +</PRE> + +<P><H2>The Simulated Machine</H2> + +<P>We're ready to move from parsing to code generation, but first you must +understand what a computer's native language is like. Most computer models +in use today have a very similar structure, although there are differences +in details. My simulated computer design makes these detail choices in +favor of simplicity rather than efficiency. (It wouldn't be very efficient +no matter what, compared to real computers. This "computer" is actually an +interpreter, written in Logo, which is itself an interpreter. So we have +two levels of interpretation involved in each simulated instruction, whereas +on a real computer, each instruction is carried out directly by the +hardware. Our compiled Pascal programs, as you've probably already noticed, +run very slowly. That's not Pascal's fault, and it's not even primarily my +compiler's fault, even though the compiler doesn't include optimization +techniques. The main slowdown is in the interpretation of the machine +instructions.) + +<P>Every computer includes a <EM>processor,</EM> which decodes +instructions and carries out the indicated arithmetic operations, and a +<EM>memory,</EM> in which information (such as the values of +variables) is stored. In modern computers, the processor is generally +a single <EM>integrated circuit,</EM> nicknamed a <EM>chip,</EM> +which is a rectangular black plastic housing one or two inches on a side +that contains thousands or even millions of tiny components made of silicon. +The memory is usually a <EM>circuit board</EM> containing several memory +chips. Computers also include circuitry to connect with input and output +devices, but we're not going to have to think about those. What makes one +computer model different from another is mainly the processor. If you +have a PC, its processor is probably an Intel design with a name like +80486 or Pentium; if you have a Macintosh, the processor might be a Motorola +68040 or a Power PC chip. + +<P>It turns out that the wiring connecting the processor to the memory is +often the main limiting factor on the speed of a computer. Things happen at +great speed within the processor, and within the memory, but only one value +at a time can travel from one to the other. Computer designers have invented +several ways to get around this problem, but the important one for our +purposes is that every modern processor includes a little bit of memory +within the processor chip itself. By "a little bit" I mean that a typical +processor has enough memory in it to hold 32 values, compared to several +million values that can be stored in the computer's main memory. The 32 +memory slots within the processor are called <EM> +registers.</EM><SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>One current topic in computer architecture research +is the development of <EM>parallel</EM> computers with many processors working +together. In some of these designs, each processor includes its own +medium-size memory within the processor chip.</SMALL></BLOCKQUOTE></SMALL><P>Whenever you want to perform an arithmetic operation, the operands must +already be within the processor, in registers. So, for example, the Pascal +instruction + +<P><PRE>c := a + b +</PRE> + +<P>isn't compiled into a single machine instruction. First we must +<EM>load</EM> the values of <CODE>a</CODE> and <CODE>b</CODE> from memory into registers, +then add the two registers, then <EM>store</EM> the result back into memory: + +<P><PRE>rload 8 a +rload 9 b +add 10 8 9 +store 10 c +</PRE> + +<P>The first <CODE>rload</CODE> instruction loads the value from memory +location <CODE>a</CODE> into register 8.<SUP>*</SUP> The <CODE>add</CODE> instruction adds +the numbers in registers 8 and 9, putting the result into register 10. (In +practice, you'll see that the compiler would be more likely to conserve +registers by reusing one of the operand registers for the result, but for +this first example I wanted to keep things simple.) Finally we store the +result into the variable <CODE>c</CODE> in memory. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>Really I should have called this +instruction <CODE>load</CODE>, but my machine simulator uses Logo procedures to +carry out the machine instructions, and I had to pick a name that wouldn't +conflict with the Logo <CODE>load</CODE> primitive.</SMALL></BLOCKQUOTE></SMALL><P>The instructions above are actually not machine language instructions, but +rather <EM>assembly language</EM> instructions, a kind of shorthand. +A program called an <EM>assembler</EM> translates assembly language into +machine language, in which each instruction is represented as a number. +For example, if the instruction code for <CODE>add</CODE> is <CODE>0023</CODE>, then +the <CODE>add</CODE> instruction above might be translated into <CODE>0023100809</CODE>, +with four digits for the instruction code and two digits for each of the three +register numbers. (In reality the encoding would use binary numbers rather +than the decimal numbers I've shown in this example.) Since a machine +language instruction is just a number, the instructions that make up a +computer program are stored in memory along with the program's data values. +But one of the simplifications I've made in my simulated computer is that +the simulator deals directly with assembly language instructions, and those +instructions are stored in a Logo list, separate from the program's data memory. + +<P>The simulated computer has 32 processor registers plus 3000 locations of +main memory; it's a very small computer, but big enough for my sample +Pascal programs. (You can change these sizes by editing procedure <CODE> +opsetup</CODE> in the compiler.) The registers are numbered from 0 to 31, and +the memory locations are numbered from 0 to 2999. The number of a memory +location is called its <EM>address.</EM> Each memory location can hold +one numeric value.<SUP>*</SUP> A Pascal array will be represented by a contiguous block of +memory locations, one for each member of the array. Each register, too, can +hold one numeric value. In this machine, as in some real computers, register +number 0 is special; it always contains the value zero. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>This, too, is a simplification. In real computers, +different data types require different amounts of memory. A character +value, for example, fits into eight <EM>bits</EM> (binary digits) of memory, +whereas an integer requires 32 bits in most current computers. Instead of a +single <CODE>load</CODE> instruction, a real computer has a separate one for each +datum size.</SMALL></BLOCKQUOTE></SMALL><P>The simulated computer understands 50 instruction codes, fewer than most +real computers. The first group we'll consider are the 14 binary arithmetic +instructions: <CODE>add</CODE>, <CODE>sub</CODE>, <CODE>mul</CODE>, <CODE>div</CODE> (real quotient), +<CODE>quo</CODE> (integer quotient), <CODE>rem</CODE> (remainder), <CODE>land</CODE> (logical +and), <CODE>lor</CODE> (logical or), <CODE>eql</CODE> (compare two operands for equality), +<CODE>neq</CODE> (not equal), <CODE>less</CODE>, <CODE>gtr</CODE> (greater than), <CODE>leq</CODE> (less +than or equal), and <CODE>geq</CODE> (greater than or equal). The result of each +of the six comparison operators is <CODE>0</CODE> for false or <CODE>1</CODE> for true. +The machine also has four unary arithmetic instructions: <CODE>lnot</CODE> +(logical not), <CODE>sint</CODE> (truncate to integer), <CODE>sround</CODE> (round to +integer), and <CODE>srandom</CODE>. Each of these 18 arithmetic instructions takes +its operands from registers and puts its result into a register. + +<P>All but the last three of these are typical instructions of real +computers.<SUP>*</SUP> +(Not every computer has all of them; for example, if a computer has <CODE>eql</CODE> +and <CODE>lnot</CODE>, then it doesn't really need a <CODE>neq</CODE> instruction because +the same value can be computed by a sequence of two instructions.) The +operations <CODE>sint</CODE>, <CODE>sround</CODE>, and <CODE>srandom</CODE> are less likely to be +machine instructions on actual computers. On the other hand, most real +computers have a <EM>system call</EM> mechanism, which is a machine +instruction that switches the computer from the user's program to a part of +the operating system that performs some task on behalf of the user. System +calls are used mainly for input and output, but we can pretend that there are +system calls to compute these Pascal library functions. (The letter <CODE>s</CODE> +in the instruction names stands for "system call" to remind us.) + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>One important simplification is that in the simulated +computer, the same instructions are used for all kinds of numbers. A typical +computer has three <CODE>add</CODE> instructions: one for integers, one for short +reals (32 bits), and one for long reals (64 bits).</SMALL></BLOCKQUOTE></SMALL><P>The simulated computer also has another set of 18 <EM>immediate</EM> +instructions, with the letter <CODE>i</CODE> added to the instruction name: <CODE> +addi</CODE>, <CODE>subi</CODE>, and so on. In these instructions, the rightmost operand +in the instruction is the actual value desired, rather than the number of +a register containing the operand. For example, + +<P><PRE>add 10 8 9 +</PRE> + +<P>means, "add the number in register 8 and the number in register 9, +putting the result into register 10." But + +<P><PRE>addi 10 8 9 +</PRE> + +<P>means, "add the number in register 8 to the value 9, putting +the result in register 10." + +<P>It's only the right operand that can be made immediate. So, for example, +the Pascal assignment + +<P><PRE>y := x - 5 +</PRE> + +<P>can be translated into + +<P><PRE>rload 8 x +subi 8 8 5 +store 8 y +</PRE> + +<P>but the Pascal assignment + +<P><PRE>y := 5 - x +</PRE> + +<P>must be translated as + +<P><PRE>addi 8 0 5 +rload 9 x +sub 8 8 9 +store 8 y +</PRE> + +<P>This example illustrates one situation in which it's useful to +have register 0 guaranteed to contain the value 0. + +<P>Our simulated machine has six more system call instructions having to do +with printing results. One of them, <CODE>newline</CODE>, uses no operands and +simply prints a newline character, moving to the beginning of a new line +on the screen. Four more are for printing the value in a register; the +instruction used depends on the data type of the value in the register. +The instructions are <CODE>putch</CODE> for a character, <CODE>puttf</CODE> for a boolean +(true or false) value, <CODE>putint</CODE> for an integer, and <CODE>putreal</CODE> for a +real number. Each takes two operands; the first, an immediate value, gives +the minimum width in which to print the value, and the second is a register +number. So the instruction + +<P><PRE>putint 10 8 +</PRE> + +<P>means, "print the integer value in register 8, using at least +10 character positions on the line." The sixth printing instruction, +<CODE>putstr</CODE>, is used only for constant character strings in the Pascal +program; its first operand is a width, as for the others, but its second +is a Logo list containing the string to print: + +<P><PRE>putstr 1 [The shuffled deck:] +</PRE> + +<P>This is, of course, unrealistic; in a real computer the second +operand would have to be the memory address of the beginning of the +array of characters to print. But the way I handle printing isn't very +realistic in any case; I wanted to do the simplest possible thing, because +worrying about printing really doesn't add anything to your understanding +of the process of compilation, which is the point of this chapter. + +<P>The next group of instructions has to do with the flow of control in the +computer program. Ordinarily the computer carries out its instructions +in sequence, that is, in the order in which they appear in the program. +But in order to implement conditionals (such as <CODE>if</CODE>), loops (such as +<CODE>while</CODE>), and procedure calls, we must be able to jump out of sequence. +The <CODE>jump</CODE> instruction takes a single operand, a <EM>label</EM> +that appears somewhere in the program. When the computer carries out a jump +instruction, it looks for the specified label and starts reading instructions +just after where that label appears in the program. (We saw an example of +labels at the beginning of this chapter.) + +<P>The <CODE>jump</CODE> instruction is used for unconditional jumps. In order to +implement conditionals and loops, we need a way to jump if some condition +is true. The instruction <CODE>jumpt</CODE> (jump if true) has two operands, +a register number and a label. It jumps to the specified label if and only +if the given register contains a true value. (Since registers hold only +numbers, we use the value 1 to represent true, and 0 to represent false.) +Similarly, <CODE>jumpf</CODE> jumps if the value in the given register is false. + +<P>For procedure and function calls, we need a different mechanism. The jump +is unconditional, but the computer must remember where it came from, so that +it can continue where it left off once the called procedure or function +returns. The instruction <CODE>jal</CODE> (jump and link) takes two operands, a +register and a label. It puts into the register the address of the +instruction following the <CODE>jal</CODE> instruction.<SUP>*</SUP> Then it jumps to the specified label. +To return from the called procedure, we use the <CODE>jr</CODE> (jump register) +instruction. It has one operand, a register number; it jumps to the +instruction whose address is in the register. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>In a real computer, +each instruction is stored in a particular memory location, so the address +of an instruction is the address of the memory location in which it's stored. +In this simulated computer, I keep the program in the form of a Logo list, +and so I cheat and put the sublist starting at the next instruction into +the register. This isn't quite as much of a cheat as it may seem, though, +since you know from Chapter 3 that Logo represents a list with the memory +address of the first pair of the list.</SMALL></BLOCKQUOTE></SMALL><P>One final instruction that affects the flow of control is the <CODE>exit</CODE> +system call. It requires no operands; it terminates the running of the +program. In this simulated computer, it returns to a Logo prompt; in a +real computer, the operating system would start running another user program. + +<P>The only remaining instructions are <CODE>rload</CODE> and <CODE>store</CODE>. You already +know what these do, but I've been showing them in oversimplified form so far. +The second operand can't just be a variable name, because that variable might +not be in the same place in memory every time the procedure is called. Think, +for example, about a recursive procedure. Several invocations may be in +progress at once, all of them carrying out the same compiled instructions, +but each referring to a separate set of local variables. The solution to +this problem is that the compiler arranges to load into a register +the address of a block of memory containing all the local variables for a +given procedure call. If the variable <CODE>c</CODE>, for example, is in the sixth +memory location of that block, an instruction to load or store that variable +must be able to say "the memory location whose address is the contents of +register 4 (let's say) plus five." So each load and store instruction +contains an <EM>index register</EM> in parentheses following an +<EM>offset</EM> to be added to the contents of that register. We'd say + +<P><PRE>store 8 5(4) +</PRE> + +<P>to store the contents of register 8 into the variable <CODE>c</CODE>, +provided that register 4 points to the correct procedure invocation's +local variables and that <CODE>c</CODE> is in the sixth position in the block. +(The first position in the block would have offset 0, and so on.) + +<P><H2>Stack Frames</H2> + +<P>The first step in invoking a procedure or function is to set aside, or +<EM>allocate,</EM> a block of memory locations for use by that invocation. +This block will include the procedure's local variables, its arguments, and +room to save the values of registers as needed. The compiler's data +structures include, for each procedure, how much memory that procedure needs +when it's invoked. That block of memory is called a <EM>frame.</EM> + +<P>In most programming languages, including Pascal and Logo (but not, as it +turns out, Lisp), the frame allocated when a procedure invocation begins can +be released, or <EM>deallocated,</EM> when that invocation returns to its +caller. In other words, the procedure's local variables no longer exist +once the invocation is finished. In these languages, the frames for all the +active procedure invocations can be viewed as a <EM>stack,</EM> a data structure +to which new elements are added by a Push operation, and elements are removed +using a Pop operation that removes the most recently pushed element. (In +this case, the elements are the frames.) That is, suppose that procedure A +invokes B, which invokes C, which invokes D. For each of these invocations +a new frame is pushed onto the stack. Which procedure finishes first? It +has to be D, the last one invoked. When D returns, its frame can be popped +off the stack. Procedure C returns next, and its frame is popped, and so on. +The phrase <EM>stack frame</EM> is used to refer to frames that +behave like elements of a stack. + +<P>My Pascal compiler allocates memory starting at location 0 and working +upward. At the beginning of the program, a <EM>global frame</EM> is allocated +to hold the program's global variables. Register 3, the +<EM>global pointer,</EM> always contains the +address of the beginning of the global frame, so that every procedure can +easily make use of global variables. (Since the global frame is the first +thing in memory, its address is always zero, so the value in register 3 is +always 0. But in a more realistic implementation the program itself would +appear in memory before the global frame, so its address would be greater +than zero.) + +<P>At any point in the program, register 4, the <EM>frame pointer,</EM> +contains the address of the beginning of the current frame, that is, the +frame that was created for the current procedure invocation. Register 2, +the <EM>stack pointer,</EM> contains the address of the first +currently unused location in memory. + +<P>My compiler is a little unusual in that when a procedure is called, the +stack frame for the new invocation is allocated by the caller, not by the +called procedure. This simplifies things because the procedure's arguments +can be stored in its own frame; if each procedure allocates its own frame, +then the caller must store argument values in its (the caller's) frame, +because the callee's frame doesn't exist yet. So, in my compiler, the +first step in a procedure call is to set register 5, the <EM>new frame +pointer,</EM> to point to the first free memory location, and change the +stack pointer to allocate the needed space. If <CODE>N</CODE> memory locations +are needed for the new frame, the calling procedure will contain the +following instructions: + +<P><PRE>add 5 2 0 +addi 2 2 N +</PRE> + +<P>The first instruction copies the value from register 2 (the +first free memory location) into register 5; the second adds <CODE>N</CODE> to +register 2. (I've left out a complication, which is that the old value +in register 5 must be saved somewhere before putting this new value +into it. You can read the code generation instructions at the beginning +of <CODE>pproccall1</CODE>, in the program listing at the end of the chapter, +for all the details.) The current frame pointer is also saved in +location 3 of the new frame: + +<P><PRE>store 4 3(5) +</PRE> + +<P>The compiler uses data abstraction to refer to these register +numbers and frame slots; for example, the procedure <CODE>reg.frameptr</CODE> takes +no arguments and always outputs 4, while <CODE>frame.prevframe</CODE> outputs 3. + +<P>The next step is to put the argument values into the new frame. During +this process, the calling procedure must use register 4 to refer to its own +variables, and register 5 to refer to the callee's variables. The final +step, just before calling the procedure, is to make the +frame pointer (register 4) point to the new frame: + +<P><PRE>add 4 5 0 +</PRE> + +<P>Once the caller has set up the new frame and saved the necessary registers, +it can call the desired procedure, putting the return address in register 1: + +<P><PRE>jal 1 "proclabel +</PRE> + +<P>The first step in the called procedure is to save the return +address in location zero of its frame: + +<P><PRE>store 1 0(4) +</PRE> + +<P>The procedure then carries out the instructions in its body. When it's +ready to return, it must load the saved return address back into register +1, then restore the old stack pointer and frame pointer to deallocate +its frame, and finally return to the caller: + +<P><PRE>rload 1 0(4) +add 2 4 0 +rload 4 3(2) +jr 1 +</PRE> + +<P>(Procedure <CODE>proc1</CODE> in the compiler generates these +instructions for each procedure.) + +<P>One final complication about stack frames comes from Pascal's block +structure. Suppose we have a program with internal procedures arranged +in this structure: + +<P><CENTER><IMG SRC="blocks.gif" ALT="figure: blocks"></CENTER> + +<P>Then suppose that the main program calls procedure A, which +calls B, which calls C, which calls itself recursively. The current (inner) +invocation of C has access to its own variables, those of procedure A, and +the global variables, but not to procedure B's variables. How does +procedure C know where procedure A's stack frame is located? The answer is +that every frame, in addition to saving a pointer to the previous frame, +must include a pointer to the <EM>lexically enclosing</EM> frame. The +calling procedure sets this up; it can do this because it knows its own +lexical depth and that of the called procedure. For example, when procedure +B calls procedure C, C's lexically enclosing frame will be the same as B's +(namely, the frame for the invocation of A), because B and C are at the +same lexical depth. (They are both declared inside A.) But when procedure +A calls procedure B, which is declared within itself, A must store its own +frame pointer as B's lexically enclosing frame. Here is a picture of what's +where in memory: + +<P><CENTER><IMG SRC="memory.gif" ALT="figure: memory"></CENTER> + +<P>If all these pointers between frames confuse you, it might help to keep in +mind that the two kinds of pointers have very different purposes. The +pointer to the previous frame is used only when a procedure returns, to +help in putting everything back the way it was before the procedure was +called (in particular, restoring the old value of register 4). The pointer +to the lexically enclosing frame is used while the procedure is running, +whenever the procedure makes reference to a variable that belongs to some +outer procedure (for example, a reference in procedure B or C to a variable +that belongs to procedure A).<SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>If procedures used the previous-frame +pointers to make variable references, we would be compiling a dynamically +scoped language! In this example, because Pascal is lexically scoped, +procedure C can't refer to procedure B's variables, even though B called C.</SMALL></BLOCKQUOTE></SMALL><P><H2>Data Structures</H2> + +<P>In this section I'll describe the main data structures used during +compilation (abstract data types for identifiers and for expressions) +and during the running of the program (registers and frames). + +<P>The main body of information that the compiler must maintain is the list of +Pascal identifiers (variable, procedure, and function names). Since Pascal +is lexically scoped, some attention is necessary to ensure that each compiled +Pascal procedure has access to precisely the variables that it should. +At any point during the compilation, the value of <CODE>:idlist</CODE> is a list of +just those identifiers that may be used in the part of the program being +compiled. We'll see in a moment how that's accomplished. + +<P>There are two main categories of identifier: procedure names (including the +main program and functions in this category) and variable names. The +information maintained for a procedure name looks like this example: + +<P><PRE>[myproc procedure %myproc [2 46]] +</PRE> + +<P>The first member of this list is the Pascal name of the program, +procedure, or function. The second member is the type indicator, which +will be one of the words <CODE>program</CODE>, <CODE>procedure</CODE>, or <CODE>function</CODE>. +The third member is the procedure's "Logo name," the unique name used within +the compiler to represent this program or procedure. The program's Logo name +is used as the variable name whose value will be the compiled program; the +Logo names for procedures and functions are used as the labels in the +compiled program at which each procedure or function begins. The fourth +member of the list contains the frame information for the procedure; it's +a list of two numbers, the lexical depth and the frame size. The lexical +depth is 0 for the main program, 1 for a procedure declared inside the +main program, 2 for a procedure declared inside a depth-1 procedure, and +so on. The frame size indicates how many memory locations must be allocated +for each invocation of the procedure. (For the main program, the frame size +indicates the size of the global frame.) + +<P>Because of the Pascal scope rules, there can be two procedures with the same +name, each declared within a different region of the program. But there is +no scoping of labels in the compiled program; each label must be unique. +The simplest solution would be to use a distinct program-generated name for +every Pascal procedure; the Pascal <CODE>doit</CODE> would become the +Logo <CODE>g14</CODE>. In fact I chose to modify this approach somewhat. When an +identifier <CODE>symbol</CODE> is declared in the source program, the compiler +looks to see whether another identifier with the same name has appeared +anywhere in the program. If not, the Logo name <CODE>%symbol</CODE> is used; if +so, a generated symbol is used. This rule makes the compiled +program a little easier to read, while preserving the rule that all Logo +names must be unique. The percent sign in <CODE>%symbol</CODE> ensures that this +Logo name doesn't conflict with any names used in the +compiler itself. Procedure <CODE>newlname</CODE> in the compiler takes a Pascal +identifier as input and generates a new Logo name to correspond. + +<P>The selectors <CODE>id.type</CODE>, <CODE>id.lname</CODE>, and <CODE>id.frame</CODE> are used +for the second through fourth members of these lists. There's no selector +for the first member, the Pascal name, because the compiler never +extracts this information explicitly. Instead, the Pascal name is used +by procedure <CODE>getid</CODE>, which takes a Pascal name as its input and +returns the corresponding identifier list. + +<P>For variable names, the identifier information looks a little different: + +<P><PRE>[i integer [1 41] false] +</PRE> + +<P>The first two members of this list are the Pascal name and the +type, the same as for a procedure. The third member is the <EM>pointer</EM> +information for the variable: its lexical depth and the offset within +a frame where it should be kept. The compiler will use this information +to issue instructions to load or store the value of the variable. The +fourth member of the list is <CODE>true</CODE> if this variable is a <CODE>var</CODE> +(call by reference) parameter, <CODE>false</CODE> otherwise. + +<P>The variable <CODE>i</CODE> above has a scalar type, so its type indicator is +a word. Had it been an array, the type indicator would be a list such as + +<P><PRE>[integer [0 6] [5 3]] +</PRE> + +<P>for a variable declared as <CODE>array [0..5, 5..7] of integer</CODE>. + +<P>For each dimension of the array, the first number in the list +is the smallest possible index, while the second number is the number of +possible index values in this dimension. That is, the range <CODE>[3..7]</CODE> +is represented by the list <CODE>[3 5]</CODE> because there are five possible +values starting from 3. Notice that there is no "Logo name" for a +variable; in the compiled program, a variable is represented as an +offset and an index register, such as <CODE>41(4)</CODE>. + +<P>For variables, the selectors used are <CODE>id.type</CODE>, <CODE>id.pointer</CODE>, +and <CODE>id.varp</CODE>. + +<P>The information about currently accessible identifiers is kept in the list +<CODE>idlist</CODE>. This variable holds a list of lists; each Pascal identifier +is represented by a list as indicated above. <CODE>Idlist</CODE> is a local +variable in the compiler procedures <CODE>program</CODE>, <CODE>procedure</CODE>, and <CODE> +function</CODE>. That is, there is a separate version for each block of the +Pascal source program. Each local version starts out with the same value as +the higher-level version; identifiers declared within a block are added to +the local version but not to the outer one. When the compiler finishes a +block, the (Logo) procedure in charge of that block stops and the outer <CODE> +idlist</CODE> becomes current again. + +<P>This arrangement may or may not seem strange to you. Recall that we had to +invent this <CODE>idlist</CODE> mechanism because Pascal's +lexical scope is different from Logo's dynamic scope. The reason we have +these different versions of <CODE>idlist</CODE> is to keep track of which +identifiers are lexically available to which blocks. And yet we are using +Logo's dynamic scope to determine which <CODE>idlist</CODE> is available at any +point in the compilation. The reason this works is that <EM>the +dynamic environment at compile time reflects the +lexical environment at run time.</EM> For example, in the <CODE>tower</CODE> +program, the fact that <CODE>tower</CODE> <EM>contains</EM> <CODE>hanoi</CODE>, which in +turn contains <CODE>movedisk</CODE>, is reflected in the fact that <CODE>program</CODE> +(compiling <CODE>tower</CODE>) <EM>invokes</EM> <CODE>procedure</CODE> (compiling <CODE> +hanoi</CODE>), which in turn <EM>invokes</EM> <CODE>procedure</CODE> recursively +(compiling <CODE>movedisk</CODE>). Earlier I said that lexical scope is easier for +a compiler than dynamic scope; this paragraph may help you see why that's +true. Even dynamically scoped Logo naturally falls into providing lexical +scope for a Pascal compiler. + +<P>Here is how procedure and function declarations are compiled: + +<P><PRE>to procedure +proc1 "procedure framesize.proc +end + +to function +proc1 "function framesize.fun +end + +to proc1 :proctype :framesize +localmake "procname token +localmake "lexical.depth :lexical.depth+1 +localmake "frame (list :lexical.depth 0) +push "idlist (list :procname :proctype (newlname :procname) :frame) +localmake "idlist :idlist + ... +end +</PRE> + +<P>(I'm leaving out the code generation part for now.) What I want +to be sure you understand is that the <CODE>push</CODE> instruction adds the new +procedure name to the <EM>outer</EM> <CODE>idlist</CODE>; after that, it creates a +new <CODE>idlist</CODE> whose initial value is the same as the old one. It's very +important that the instruction + +<P><PRE>localmake "idlist :idlist +</PRE> + +<P>comes where it does and not at the beginning of the procedure. +<CODE>Proc1</CODE> needs access to the outer <CODE>idlist</CODE> when it starts, and +then later it "shadows" that variable with its own local version. This +example shows that Logo's <CODE>local</CODE> command really is an executable +command and not a declaration like Pascal's <CODE>var</CODE> declaration. In +Pascal it would be unthinkable to declare a new local variable in the middle +of a block. + +<P><CODE>Getid</CODE> depends on Logo's dynamic scope to give it access to the right +version of <CODE>idlist</CODE>. Think about writing a Pascal compiler in Pascal. +There would be a large block for <CODE>program</CODE> with many other procedures +inside it. Two of those inner procedures would be the ones for <CODE> +procedure</CODE> and <CODE>function</CODE>. (Of course they couldn't have those names, +because they're Pascal reserved words. They'd be called <CODE> +compileprocedure</CODE> or some such thing. But I think this will be easier to +follow if I stick with the names used in the Logo version of the compiler.) +Those two procedures should be at the same level of block structure; neither +should be lexically within the other. That's because a Pascal procedure +block can include a function definition or vice versa. Now, where in the +lexical structure does <CODE>getid</CODE> belong? It needs access to the local +<CODE>idlist</CODE> of either <CODE>procedure</CODE> or <CODE>function</CODE>, whichever is +currently active. Similarly, things like <CODE>statement</CODE> need to be +lexically within both <CODE>procedure</CODE> and <CODE>function</CODE>, and actually also +within <CODE>program</CODE> because the outermost program block has statements too. +It would theoretically be possible to solve the problem by writing three +identical versions of each of these subprocedures, but that solution is too +horrible to contemplate. Instead a more common technique is to have only +one <CODE>idlist</CODE> variable, a global one, and write the compiler so that it +explicitly maintains a stack of old values of that variable. The Pascal +programmer has to do the work that the programming language should be doing +automatically. This is an example in which dynamic scope, while not +absolutely essential, makes the program much easier to write and more +straightforward to understand. + +<P>For every procedure or function in the Pascal source program, the compiler +creates a global Logo variable with the same name as the corresponding +label--that is, either a percent-prefix name or a generated symbol. The +value of this variable is a list of types, one for each argument to the +procedure or function. (For a function, the first member of the list is the +type of the function itself; the butfirst is the list of types of its +arguments.) The compiler examines this "type signature" variable when a +procedure or function is invoked, to make sure that the types of the actual +arguments match the types of the formal parameters. + +<P>The other important compile-time data structure is the one that represents +a compiled expression. When the compiler calls <CODE>pexpr</CODE>, its job is to +parse an expression from the Pascal source program and generate code to +compute (when the compiled program runs!) the value of the expression. + +The generated code leaves the computed value in some register. What <CODE> +pexpr</CODE> returns to its caller is a data structure indicating which register +and what type the expression has, like this: + +<P> + +<P><PRE>[real register 8] +</PRE> + +<P>The first member of this list is the type of the expression. +Most of the time, the second member is the word <CODE>register</CODE> and the +third member is the register number in which the expression's value +can be found. The only exception is for a constant expression; if +the expression is, for example, <CODE>15</CODE> then <CODE>pexpr</CODE> will output + +<P><PRE>[integer immediate 15] +</PRE> + +<P>For the most part, these immediate expressions are useful +only within recursive calls to <CODE>pexpr</CODE>. In compiling the Pascal +assignment + +<P><PRE>x := 15 +</PRE> + +<P>we're going to have to get the value 15 into a register anyway +in order to be able to store it into <CODE>x</CODE>; the generated code will be +something like + +<P><PRE>addi 7 0 15 +store 7 48(4) +</PRE> + +<P>An immediate expression is most useful in compiling something like + +<P><PRE>x := a+15 +</PRE> + +<P>in which we can avoid loading the value 15 into a register, but +can directly add it to the register containing <CODE>a</CODE>: + +<P><PRE>rload 7 53(4) +addi 7 7 15 +store 7 48(4) +</PRE> + +<P>The members of an expression list are examined using the selectors <CODE> +exp.type</CODE>, <CODE>exp.mode</CODE> (the word <CODE>register</CODE> or <CODE>immediate</CODE>), +and <CODE>exp.value</CODE> (the register number or immediate value). + +<P>In this compiler an "expression" is always a +<EM>scalar</EM> type; although the formal definition of Pascal allows for +array expressions, there are no operations that act on arrays the way +operations like <CODE>+</CODE> act on scalars, and so an array expression can only +be the name of an array variable. (<EM>Members</EM> of arrays can, of +course, be part of a scalar expression.) <CODE>Passign</CODE>, the compiler +procedure that handles assignment statements, first checks for the special +case of an array assignment and then, only if the left side of the +assignment is a scalar, invokes <CODE>pexpr</CODE> to parse a scalar expression. + +<P>In order to understand the code generated by the compiler, you should also +know about the <EM>runtime</EM> data structures used by compiled programs. +First, certain registers are reserved for special purposes: + +<P><TABLE> +<TR><TH align="right">number<TH> <TH align="left">name<TH> <TH align="left">purpose +<TR><TD> +<TR><TD align="right">0<TD><TD><CODE>reg.zero</CODE><TD><TD>always contains zero +<TR><TD align="right">1<TD><TD><CODE>reg.retaddr</CODE><TD><TD>return address from procedure call +<TR><TD align="right">2<TD><TD><CODE>reg.stackptr</CODE><TD><TD>first free memory address +<TR><TD align="right">3<TD><TD><CODE>reg.globalptr</CODE><TD><TD>address of global frame +<TR><TD align="right">4<TD><TD><CODE>reg.frameptr</CODE><TD><TD>address of current frame +<TR><TD align="right">5<TD><TD><CODE>reg.newfp</CODE><TD><TD>address of frame being made for procedure call +<TR><TD align="right">6<TD><TD><CODE>reg.retval</CODE><TD><TD>return value from function +<TR><TD align="right">7<TD><TD><CODE>reg.firstfree</CODE><TD><TD>first register available for expressions +</TABLE> + +<P>We've already seen most of these while discussing stack frames. +A Pascal function returns its result in register 6; the caller immediately +copies the return value into some other register so that it won't be +lost if the program calls another function, for a case like + +<P><PRE>x := f(3)+f(4) +</PRE> + +<P>Whenever a register is needed to hold some computed value, the compiler +calls the Logo procedure <CODE>newregister</CODE>, which finds the first register +number starting from 7 that isn't currently in use. When the value in +a register is no longer needed, the compiler calls <CODE>regfree</CODE> to indicate +that that register can be reassigned by <CODE>newregister</CODE>. + +<P>The other noteworthy runtime data structure is the use of slots within each +frame for special purposes: + +<P><TABLE> +<TR><TH align="right">number<TH> <TH align="left">name<TH> <TH align="left">purpose +<TR><TD> +<TR><TD align="right">0<TD><TD><CODE>frame.retaddr</CODE><TD><TD>address from which this procedure was called +<TR><TD align="right">1<TD><TD><CODE>frame.save.newfp</CODE><TD><TD>saved register 3 while filling this new frame +<TR><TD align="right">2<TD><TD><CODE>frame.outerframe</CODE><TD><TD>the frame lexically enclosing this one +<TR><TD align="right">3<TD><TD><CODE>frame.prevframe</CODE><TD><TD>the frame from which this one was called +<TR><TD align="right">4-35<TD><TD><CODE>frame.regsave</CODE><TD><TD>space for saving registers +<TR><TD align="right">36<TD><TD><CODE>frame.retval</CODE><TD><TD>function return value +</TABLE> + +<P>Why is there both a register and a frame slot for a function's return +value? Remember that the way you indicate the return value in a Pascal +function is by assigning to the function's name as if it were a variable. +Such an assignment is not necessarily the last instruction in the function; +it may do more work after computing the return value. The compiler notices +as assignment to the function name and generates code to save the computed +value in slot 36 of the current frame. Then, when the function actually +returns, the compiler generates the instruction + +<P><PRE>rload 6 36(4) +</PRE> + +<P>to copy the return value into register 6. The function's frame +is about to be freed, so the caller can't look there +for the return value; that's why a register is used. + +<P>Each frame includes a block of space for saving registers when another +procedure is called. That's because each procedure allocates register +numbers independently; each starts with register 7 as the first free one. +So if the registers weren't saved before a procedure call and restored +after the call, the values in the registers would be lost. (Although +the frame has enough room to save all 32 registers, to make things simple, +not all 32 are actually saved. The compiler knows which registers contain +active expression values at the moment of the procedure call, and it +generates code to save and restore only the necessary ones.) + +<P>You might think it would be easier to have each procedure use a separate +set of registers, so saving wouldn't be necessary. But this +doesn't work for two reasons. First, there are only a few registers, and +in a large program we'd run out. Even more important, the compiled +code for a <EM>recursive</EM> procedure is going to use the same registers +in each invocation, so we certainly can't avoid saving registers in that +situation. + +<P><H2>Code Generation</H2> + + +<P>Let's look again at how the compiler handles a Pascal <CODE>if</CODE> statement: + +<P><PRE>to pif +local [cond elsetag endtag] +make "cond pboolean pexpr +make "elsetag gensym +make "endtag gensym +mustbe "then +code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +statement +code (list "jump (word "" :endtag)) +code :elsetag +ifbe "else [statement] +code :endtag +end +</PRE> + +<P>I showed you this procedure while talking about parsing, asking you to +ignore the parts about code generation. Now we'll come back to that part of +the process. + +<P>The format of the <CODE>if</CODE> statement is either of these: + +<P><PRE>if <EM>condition</EM> then <EM>statement</EM> +if <EM>condition</EM> then <EM>statement</EM> else <EM>statement</EM> +</PRE> + +<P>(There is probably a semicolon after the statement, but it's +not officially part of the <CODE>if</CODE>; it's part of the compound statement +that contains the <CODE>if</CODE>.) When we get to <CODE>pif</CODE>, the compiler has +already read the token <CODE>if</CODE>; the next thing to read is an expression, +which must be of type <CODE>boolean</CODE>, providing the condition part of the +statement. + +<P>In the instruction + +<P><PRE>make "cond pboolean pexpr +</PRE> + +<P>the call to <CODE>pexpr</CODE> generates code for the expression and +returns an expression list, in the format shown earlier. The procedure <CODE> +pboolean</CODE> does three things: First, it checks the mode of the expression; +if it's immediate, the value is loaded into a register. Second, it checks +the type of the expression to ensure that it really is boolean. Third, +<CODE>pboolean</CODE> returns just the register number, which will be used in code +generated by <CODE>pif</CODE>. + +<P><PRE>to pboolean :expr [:pval noimmediate :expr] +if equalp exp.type :pval "boolean [output exp.value :pval] +(throw "error sentence exp.type :pval [not true or false]) +end + +to noimmediate :value +if equalp exp.mode :value "immediate ~ + [localmake "reg newregister + code (list "addi :reg reg.zero exp.value :value) + output (list exp.type :value "register :reg)] +output :value +end +</PRE> + +<P>Overall, the code compiled for the <CODE>if</CODE> statement will look like this: + +<P><PRE> ... get condition into register <EM>cond</EM> ... + jumpf <EM>cond</EM> "g5 + ... code for <CODE>then</CODE> statement ... + jump "g6 +g5 + ... code for <CODE>else</CODE> statement ... +g6 +</PRE> + +<P>The labels <CODE>g5</CODE> and <CODE>g6</CODE> in this example are generated +symbols; they'll be different each time. The labels are generated by the +instructions + +<P><PRE>make "elsetag gensym +make "endtag gensym +</PRE> + +<P>in <CODE>pif</CODE>. After we call <CODE>pexpr</CODE> to generate the code +for the conditional expression, we explicitly generate the <CODE>jumpf</CODE> +instruction: + +<P><PRE>code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +</PRE> + +<P>Notice that once we've generated the <CODE>jumpf</CODE> instruction, we +no longer need the value in register <CODE>:cond</CODE>, and we call <CODE>regfree</CODE> +to say so. The rest of this code generation process should be easy to work +out. All of the structured statements (<CODE>for</CODE>, <CODE>while</CODE>, and <CODE> +repeat</CODE>) are similarly simple. + +<P>The code generation for expressions is all in <CODE>ppopop</CODE>. Most of the +complexity of dealing with expressions is in the parsing, not in the code +generation; by the time we get to <CODE>ppopop</CODE>, we know that we want to +carry out a single operation on two values, both of which are either in +registers or immediate values. The simple case is that both are in +registers; suppose, for example, that we are given the subtraction operation +and the two operands are in registers 8 and 9. Then we just generate the +instruction + +<P><PRE>sub 8 8 9 +</PRE> + +<P>and declare register 9 free. <CODE>Ppopop</CODE> is a little long, +because it has to check for special cases such as immediate operands. +Also, a unary minus is turned into a subtraction from register zero, +since there is no unary <CODE>minus</CODE> operation in our simulated machine. + +<P>Ironically, it's the "simple" statements that are hardest to +compile: assignment and procedure calling. For procedure (or function) +calling, the difficulty is in matching actual argument expressions with +formal parameters. Procedure <CODE>pproccall1</CODE> generates the instructions to +manipulate frame pointers, as described earlier, and procedure <CODE> +procargs</CODE> fills the newly-created frame with the actual argument values. +(If an argument is an array passed by value, each member of the array must +be copied into the new frame.) Assignment, handled by procedure <CODE> +passign</CODE> in the compiler, is similar to argument passing; a value must be +computed and then stored into a frame. I wouldn't be too upset if you +decide to stop here and take code generation for memory references on faith. + +<P>Suppose we are compiling the assignment + +<P><PRE>x := <EM>expression</EM> +</PRE> + +<P><CODE>Passign</CODE> reads the name <CODE>x</CODE> and uses <CODE>getid</CODE> to find +the information associated with that name. If the assignment is to an array +member, then <CODE>passign</CODE> must also read the array indices, but let's say +that we are assigning to a scalar variable, to keep it simple. + +<P><PRE>to passign +local [name id type index value pointer target] +<U>make "name token</U> +make "index [] +ifbe "|[| [make "index commalist [pexpr] mustbe "|]|] +mustbe "|:=| +<U>make "id getid :name</U> +<U>make "pointer id.pointer :id</U> +<U>make "type id.type :id</U> +passign1 +end +</PRE> + +<P>Procedure <CODE>passign1</CODE> contains the steps that are in common between +ordinary assignment (handled by <CODE>passign</CODE>) and assignment to the name of +the current function, to set the return value (handled by <CODE>pfunset</CODE>, +which you can read in the complete listing at the end of the chapter). + +<P><PRE>to passign1 +if and (listp :type) (emptyp :index) [parrayassign :id stop] +setindex "false +<U>make "value check.type :type pexpr</U> +<U>codestore :value (id.pointer :id) (id.varp :id) :index</U> +regfree :value +end +</PRE> + +<P>We call <CODE>pexpr</CODE> to generate the code to compute the +expression. <CODE>Check.type</CODE> is like <CODE>pboolean</CODE>, which you saw earlier, +except that it takes the desired type as an argument. It returns the +number of the register that contains the expression value. + +<P>The real work is done by <CODE>codestore</CODE>, which takes four inputs. +The first is the register number whose value should be stored; the other +three inputs indicate where in memory the value should go. First comes the +pointer from the identifier list; this, you'll recall, tells us the lexical +depth at which the variable was declared and the offset within its frame +where the variable is kept. Next is a true or false value indicating +whether or not this variable is a <CODE>var</CODE> parameter; if so, then its value +is a pointer to the variable whose value we <EM>really</EM> want to change. +Finally, the <EM>index</EM> input will be zero for a scalar variable, or the +number of a register containing the +array index for an array member. (Procedure <CODE>lindex</CODE>, whose name stands +for "linear index," has been called to generate code to convert the possible +multi-dimensional indices, with possibly varying starting values, into a +single number indicating the position within the array, starting from zero +for the first member.) + +<P><PRE>to codestore :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "store :reg targetaddr) +regfree last :target +end +</PRE> + +<P>(There is a similar procedure <CODE>codeload</CODE> used to generate +the code to load a variable's value into a register.) <CODE>Codestore</CODE> +invokes a subprocedure <CODE>memsetup</CODE> whose job is to work out an +appropriate operand for an <CODE>rload</CODE> or <CODE>store</CODE> machine instruction. +That operand must be an offset and an index register, such as <CODE>41(4)</CODE>. +What <CODE>memsetup</CODE> returns is a list of the two numbers, in this case +<CODE>[41 4]</CODE>. Procedure <CODE>targetaddr</CODE> turns that into the right notation +for use in the instruction. + +<P><CODE>Memsetup</CODE> is the most complicated procedure in the compiler, because +there are so many special cases. I'll describe the easy cases here. Suppose +that we are dealing with a scalar variable that isn't a <CODE>var</CODE> parameter. +Then there are three cases. If the lexical depth of that variable is equal +to the current lexical depth, then this variable is declared in the same +block that we're compiling. In that case, we use register 4 (the current +frame pointer) as the index register, and the variable's frame slot as the +offset. If the variable's lexical depth is zero, then it's a global +variable. In that case, we use register 3 (the global frame pointer) as +the index register, and the variable's frame slot as the offset. If the +variable's depth is something other than zero or the current depth, then +we have to find a pointer to the variable's own frame by looking in the +current frame's <CODE>frame.outerframe</CODE> slot, and perhaps in <EM>that</EM> +frame's <CODE>frame.outerframe</CODE> slot, as many times as the difference between +the current depth and the variable's depth. + +<P>If the variable is a <CODE>var</CODE> parameter, then we go through the same cases +just described, and then load the value of that variable (which is a pointer +to the variable we really want) into a register. We use that new register +as the index register, and zero as the offset. + +<P>If the variable is an array member, then we must add the linear index +(which is already in a register) to the offset as computed so far. + +<P>Perhaps an example will help sort this out. Here is the compiled version +of the <CODE>tower</CODE> program, with annotations: + +<P><PRE>[ [add 3 0 0] set up initial pointers + [add 4 0 0] + [addi 2 0 36] + [jump "g1] jump to main program + +%hanoi [store 1 0(4)] save return value + [jump "g2] jump to body of hanoi + +%movedisk [store 1 0(4)] + [jump "g3] + +g3 [putstr 1 [Move disk ]] body of movedisk + [rload 7 36(4)] + [putint 1 7] write(number:1) + [putstr 1 [ from ]] + [rload 7 37(4)] + [putch 1 7] write(from:1) + [putstr 1 [ to ]] + [rload 7 38(4)] + [putch 1 7] write(to:1) + [newline] + [rload 1 0(4)] reload return address + [add 2 4 0] free stack frame + [rload 4 3(2)] + [jr 1] return to caller + +g2 [rload 7 36(4)] body of hanoi + [neqi 7 7 0] if number <> 0 + [jumpf 7 "g4] + [store 5 1(2)] allocate new frame + [add 5 2 0] + [addi 2 2 40] + [store 4 3(5)] set previous frame + [rload 7 2(4)] + [store 7 2(5)] set enclosing frame + [rload 7 36(4)] + [subi 7 7 1] + [store 7 36(5)] first arg is number-1 + [rload 7 37(4)] + [store 7 37(5)] next arg is from + [rload 7 39(4)] + [store 7 38(5)] next arg is other + [rload 7 38(4)] + [store 7 39(5)] next arg is onto + [add 4 5 0] switch to new frame + [rload 5 1(4)] + [jal 1 "%hanoi] recursive call + + [store 5 1(2)] set up for movedisk + [add 5 2 0] + [addi 2 2 39] + [store 4 3(5)] + [store 4 2(5)] note different enclosing frame + [rload 7 36(4)] + [store 7 36(5)] copy args + [rload 7 37(4)] + [store 7 37(5)] + [rload 7 38(4)] + [store 7 38(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%movedisk] call movedisk + + [store 5 1(2)] second recursive call + [add 5 2 0] + [addi 2 2 40] + [store 4 3(5)] + [rload 7 2(4)] + [store 7 2(5)] + [rload 7 36(4)] + [subi 7 7 1] + [store 7 36(5)] + [rload 7 39(4)] + [store 7 37(5)] + [rload 7 38(4)] + [store 7 38(5)] + [rload 7 37(4)] + [store 7 39(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%hanoi] + [jump "g5] end of if...then + +g4 +g5 [rload 1 0(4)] return to caller + [add 2 4 0] + [rload 4 3(2)] + [jr 1] + +g1 [store 5 1(2)] body of main program + [add 5 2 0] prepare to call hanoi + [addi 2 2 40] + [store 4 3(5)] + [store 4 2(5)] + [addi 7 0 5] constant argument 5 + [store 7 36(5)] + [addi 7 0 97] ASCII code for 'a' + [store 7 37(5)] + [addi 7 0 98] ASCII code for 'b' + [store 7 38(5)] + [addi 7 0 99] ASCII code for 'c' + [store 7 39(5)] + [add 4 5 0] + [rload 5 1(4)] + [jal 1 "%hanoi] call hanoi + [exit] +] +</PRE> + +<P> +<TABLE width="100%"><TR><TD><A HREF="../v3-toc2.html">(back to Table of Contents)</A> +<TD align="right"><A HREF="https://people.eecs.berkeley.edu/~bh/v3ch4/v3ch4.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../v3ch6/v3ch6.html"><STRONG>NEXT</STRONG></A> +</TABLE> + +<P><H2>Program Listing</H2> + +<P><PRE> +to compile :file +if namep "peekchar [ern "peekchar] +if namep "peektoken [ern "peektoken] +if not namep "idlist [opsetup] +if not emptyp :file [openread :file] +setread :file +ignore error +catch "error [program] +localmake "error error +if not emptyp :error [print first butfirst :error] +setread [] +if not emptyp :file [close :file] +end + +;; Global setup + +to opsetup +make "numregs 32 +make "memsize 3000 +pprop "|=| "binary [eql 2 [boolean []] 1] +pprop "|<>| "binary [neq 2 [boolean []] 1] +pprop "|<| "binary [less 2 [boolean []] 1] +pprop "|>| "binary [gtr 2 [boolean []] 1] +pprop "|<=| "binary [leq 2 [boolean []] 1] +pprop "|>=| "binary [geq 2 [boolean []] 1] +pprop "|+| "binary [add 2 [[] []] 2] +pprop "|-| "binary [sub 2 [[] []] 2] +pprop "or "binary [lor 2 [boolean boolean] 2] +pprop "|*| "binary [mul 2 [[] []] 3] +pprop "|/| "binary [quo 2 [real []] 3] +pprop "div "binary [div 2 [integer integer] 3] +pprop "mod "binary [rem 2 [integer integer] 3] +pprop "and "binary [land 2 [boolean boolean] 3] +pprop "|+| "unary [plus 1 [[] []] 4] +pprop "|-| "unary [minus 1 [[] []] 4] +pprop "not "unary [lnot 1 [boolean boolean] 4] +make "idlist `[[trunc function int [1 ,[framesize.fun+1]]] + [round function round [1 ,[framesize.fun+1]]] + [random function random [1 ,[framesize.fun+1]]]] +make "int [integer real] +make "round [integer real] +make "random [integer integer] +end + +;; Block structure + +to program +mustbe "program +localmake "progname token +ifbe "|(| [ignore commalist [id] mustbe "|)|] +mustbe "|;| +localmake "lexical.depth 0 +localmake "namesused [] +localmake "needint "false +localmake "needround "false +localmake "needrandom "false +localmake "idlist :idlist +localmake "frame [0 0] +localmake "id (list :progname "program (newlname :progname) :frame) +push "idlist :id +localmake "codeinto word "% :progname +make :codeinto [] +localmake "framesize framesize.proc +program1 +mustbe ". +code [exit] +foreach [int round random] "plibrary +make :codeinto reverse thing :codeinto +end + +to program1 +localmake "regsused (array :numregs 0) +for [i reg.firstfree :numregs-1] [setitem :i :regsused "false] +ifbe "var [varpart] +.setfirst butfirst :frame :framesize +if :lexical.depth = 0 [code (list "add reg.globalptr reg.zero reg.zero) + code (list "add reg.frameptr reg.zero reg.zero) + code (list "addi reg.stackptr reg.zero :framesize)] +localmake "bodytag gensym +code (list "jump (word "" :bodytag)) +tryprocpart +code :bodytag +mustbe "begin +blockbody "end +end + +to plibrary :func +if not thing (word "need :func) [stop] +code :func +code (list "rload reg.firstfree (memaddr framesize.fun reg.frameptr)) +code (list (word "s :func) reg.retval reg.firstfree) +code (list "add reg.stackptr reg.frameptr reg.zero) +code (list "rload reg.frameptr (memaddr frame.prevframe reg.stackptr)) +code (list "jr reg.retaddr) +end + +;; Variable declarations + +to varpart +local [token namelist type] +make "token token +make "peektoken :token +if reservedp :token [stop] +vargroup +foreach :namelist [newvar ? :type] +mustbe "|;| +varpart +end + +to vargroup +make "namelist commalist [id] +mustbe ": +ifbe "packed [] +make "type token +ifelse equalp :type "array [make "type arraytype] [typecheck :type] +end + +to id +local "token +make "token token +if letterp ascii first :token [output :token] +make "peektoken :token +output [] +end + +to arraytype +local [ranges type] +mustbe "|[| +make "ranges commalist [range] +mustbe "|]| +mustbe "of +make "type token +typecheck :type +output list :type :ranges +end + +to range +local [first last] +make "first range1 +mustbe ".. +make "last range1 +if :first > :last ~ + [(throw "error (sentence [array bounds not increasing:] + :first ".. :last))] +output list :first (1 + :last - :first) +end + +to range1 +local "bound +make "bound token +if equalp first :bound "' [output ascii first butfirst :bound] +if equalp :bound "|-| [make "bound minus token] +if equalp :bound int :bound [output :bound] +(throw "error sentence [array bound not ordinal:] :bound) +end + +to typecheck :type +if memberp :type [real integer char boolean] [stop] +(throw "error sentence [undefined type] :type) +end + +to newvar :pname :type +if reservedp :pname [(throw "error sentence :pname [reserved word])] +push "idlist (list :pname :type (list :lexical.depth :framesize) "false) +make "framesize :framesize + ifelse listp :type [arraysize :type] [1] +end + +to arraysize :type +output reduce "product map [last ?] last :type +end + +;; Procedure and function declarations + +to tryprocpart +ifbeelse "procedure ~ + [procedure tryprocpart] ~ + [ifbe "function [function tryprocpart]] +end + +to procedure +proc1 "procedure framesize.proc +end + +to function +proc1 "function framesize.fun +end + +to proc1 :proctype :framesize +localmake "procname token +localmake "lexical.depth :lexical.depth+1 +localmake "frame (list :lexical.depth 0) +push "idlist (list :procname :proctype (newlname :procname) :frame) +localmake "idlist :idlist +make lname :procname [] +ifbe "|(| [arglist] +if equalp :proctype "function ~ + [mustbe ": + localmake "type token + typecheck :type + make lname :procname fput :type thing lname :procname] +mustbe "|;| +code lname :procname +code (list "store reg.retaddr (memaddr frame.retaddr reg.frameptr)) +program1 +if equalp :proctype "function ~ + [code (list "rload reg.retval (memaddr frame.retval reg.frameptr))] +code (list "rload reg.retaddr (memaddr frame.retaddr reg.frameptr)) +code (list "add reg.stackptr reg.frameptr reg.zero) +code (list "rload reg.frameptr (memaddr frame.prevframe reg.stackptr)) +code (list "jr reg.retaddr) +mustbe "|;| +end + +to arglist +local [token namelist type varflag] +make "varflag "false +ifbe "var [make "varflag "true] +vargroup +foreach :namelist [newarg ? :type :varflag] +ifbeelse "|;| [arglist] [mustbe "|)|] +end + +to newarg :pname :type :varflag +if reservedp :pname [(throw "error sentence :pname [reserved word])] +localmake "pointer (list :lexical.depth :framesize) +push "idlist (list :pname :type :pointer :varflag) +make "framesize :framesize + ifelse (and listp :type not :varflag) ~ + [arraysize :type] [1] +queue lname :procname ifelse :varflag [list "var :type] [:type] +end + +;; Statement part + +to blockbody :endword +statement +ifbeelse "|;| [blockbody :endword] [mustbe :endword] +end + +to statement +local [token type] +ifbe "begin [compound stop] +ifbe "for [pfor stop] +ifbe "if [pif stop] +ifbe "while [pwhile stop] +ifbe "repeat [prepeat stop] +ifbe "write [pwrite stop] +ifbe "writeln [pwriteln stop] +make "token token +make "peektoken :token +if memberp :token [|;| end until] [stop] +make "type gettype :token +if emptyp :type [(throw "error sentence :token [can't begin statement])] +if equalp :type "procedure [pproccall stop] +if equalp :type "function [pfunset stop] +passign +end + +;; Compound statement + +to compound +blockbody "end +end + +;; Structured statements + +to pfor +local [var init step final looptag endtag testreg] +make "var token +mustbe "|:=| +make "init pinteger pexpr +make "step 1 +ifbeelse "downto [make "step -1] [mustbe "to] +make "final pinteger pexpr +mustbe "do +make "looptag gensym +make "endtag gensym +code :looptag +localmake "id getid :var +codestore :init (id.pointer :id) (id.varp :id) 0 +make "testreg newregister +code (list (ifelse :step<0 ["less] ["gtr]) :testreg :init :final) +code (list "jumpt :testreg (word "" :endtag)) +regfree :testreg +statement +code (list "addi :init :init :step) +code (list "jump (word "" :looptag)) +code :endtag +regfree :init +regfree :final +end + +to prepeat +local [cond looptag] +make "looptag gensym +code :looptag +blockbody "until +make "cond pboolean pexpr +code (list "jumpf :cond (word "" :looptag)) +regfree :cond +end + +to pif +local [cond elsetag endtag] +make "cond pboolean pexpr +make "elsetag gensym +make "endtag gensym +mustbe "then +code (list "jumpf :cond (word "" :elsetag)) +regfree :cond +statement +code (list "jump (word "" :endtag)) +code :elsetag +ifbe "else [statement] +code :endtag +end + +to pwhile +local [cond looptag endtag] +make "looptag gensym +make "endtag gensym +code :looptag +make "cond pboolean pexpr +code (list "jumpf :cond (word "" :endtag)) +regfree :cond +mustbe "do +statement +code (list "jump (word "" :looptag)) +code :endtag +end + +;; Simple statements: write and writeln + +to pwrite +mustbe "|(| +pwrite1 +end + +to pwrite1 +pwrite2 +ifbe "|)| [stop] +ifbeelse ", [pwrite1] [(throw "error [missing comma])] +end + +to pwrite2 +localmake "result pwrite3 +ifbe ": [.setfirst (butfirst :result) token] +code :result +if not equalp first :result "putstr [regfree last :result] +end + +to pwrite3 +localmake "token token +if equalp first :token "' ~ + [output (list "putstr 1 (list butlast butfirst :token))] +make "peektoken :token +localmake "result pexpr +if equalp first :result "char [output (list "putch 1 pchar :result)] +if equalp first :result "boolean [output (list "puttf 1 pboolean :result)] +if equalp first :result "integer [output (list "putint 10 pinteger :result)] +output (list "putreal 20 preal :result) +end + +to pwriteln +ifbe "|(| [pwrite1] +code [newline] +end + +;; Simple statements: procedure call + +to pproccall +localmake "pname token +localmake "id getid :pname +localmake "lname id.lname :id +localmake "vartypes thing :lname +pproccall1 framesize.proc +end + +to pproccall1 :offset +code (list "store reg.newfp (memaddr frame.save.newfp reg.stackptr)) +code (list "add reg.newfp reg.stackptr reg.zero) +code (list "addi reg.stackptr reg.stackptr (last id.frame :id)) +code (list "store reg.frameptr (memaddr frame.prevframe reg.newfp)) +localmake "newdepth first id.frame :id +ifelse :newdepth > :lexical.depth ~ + [code (list "store reg.frameptr + (memaddr frame.outerframe reg.newfp))] ~ + [localmake "tempreg newregister + code (list "rload :tempreg (memaddr frame.outerframe reg.frameptr)) + repeat (:lexical.depth - :newdepth) + [code (list "rload :tempreg + (memaddr frame.outerframe :tempreg))] + code (list "store :tempreg (memaddr frame.outerframe reg.newfp)) + regfree :tempreg] +if not emptyp :vartypes [mustbe "|(| procargs :vartypes :offset] +for [i reg.firstfree :numregs-1] ~ + [if item :i :regsused + [code (list "store :i (memaddr frame.regsave+:i reg.frameptr))]] +code (list "add reg.frameptr reg.newfp reg.zero) +code (list "rload reg.newfp (memaddr frame.save.newfp reg.frameptr)) +code (list "jal reg.retaddr (word "" :lname)) +for [i reg.firstfree :numregs-1] ~ + [if item :i :regsused + [code (list "rload :i (memaddr frame.regsave+:i reg.frameptr))]] +end + +to procargs :types :offset +if emptyp :types [mustbe "|)| stop] +localmake "next procarg first :types :offset +if not emptyp butfirst :types [mustbe ",] +procargs butfirst :types :offset+:next +end + +to procarg :type :offset +local "result +if equalp first :type "var [output procvararg last :type] +if listp :type [output procarrayarg :type] +make "result check.type :type pexpr +code (list "store :result (memaddr :offset reg.newfp)) +regfree :result +output 1 +end + +to procvararg :ftype +local [pname id type index] +make "pname token +make "id getid :pname +make "type id.type :id +ifelse wordp :ftype ~ + [setindex "true] ~ + [make "index 0] +if not equalp :type :ftype ~ + [(throw "error sentence :pname [arg wrong type])] +localmake "target memsetup (id.pointer :id) (id.varp :id) :index +localmake "tempreg newregister +code (list "addi :tempreg (last :target) (first :target)) +code (list "store :tempreg (memaddr :offset reg.newfp)) +regfree last :target +regfree :tempreg +output 1 +end + +to procarrayarg :type +localmake "pname token +localmake "id getid :pname +if not equalp :type (id.type :id) ~ + [(throw "error (sentence "array :pname [wrong type for arg]))] +localmake "size arraysize :type +localmake "rtarget memsetup (id.pointer :id) (id.varp :id) 0 +localmake "pointreg newregister +code (list "addi :pointreg reg.newfp :offset) +localmake "ltarget (list 0 :pointreg) +copyarray +output :size +end + +;; Simple statements: assignment statement (including function value) + +to passign +local [name id type index value pointer target] +make "name token +make "index [] +ifbe "|[| [make "index commalist [pexpr] mustbe "|]|] +mustbe "|:=| +make "id getid :name +make "pointer id.pointer :id +make "type id.type :id +passign1 +end + +to pfunset +local [name id type index value pointer target] +make "name token +make "index [] +if not equalp :name :procname ~ + [(throw "error sentence [assign to wrong function] :name)] +mustbe "|:=| +make "pointer (list :lexical.depth frame.retval) +make "type first thing lname :name +make "id (list :name :type :pointer "false) +passign1 +end + +to passign1 +if and (listp :type) (emptyp :index) [parrayassign :id stop] +setindex "false +make "value check.type :type pexpr +codestore :value (id.pointer :id) (id.varp :id) :index +regfree :value +end + +to noimmediate :value +if equalp exp.mode :value "immediate ~ + [localmake "reg newregister + code (list "addi :reg reg.zero exp.value :value) + output (list exp.type :value "register :reg)] +output :value +end + +to check.type :type :result +if equalp :type "real [output preal :result] +if equalp :type "integer [output pinteger :result] +if equalp :type "char [output pchar :result] +if equalp :type "boolean [output pboolean :result] +end + +to preal :expr [:pval noimmediate :expr] +if equalp exp.type :pval "real [output exp.value :pval] +output pinteger :pval +end + +to pinteger :expr [:pval noimmediate :expr] +local "type +make "type exp.type :pval +if memberp :type [integer boolean char] [output exp.value :pval] +(throw "error sentence exp.type :pval [isn't ordinal]) +end + +to pchar :expr [:pval noimmediate :expr] +if equalp exp.type :pval "char [output exp.value :pval] +(throw "error sentence exp.type :pval [not character value]) +end + +to pboolean :expr [:pval noimmediate :expr] +if equalp exp.type :pval "boolean [output exp.value :pval] +(throw "error sentence exp.type :pval [not true or false]) +end + +to parrayassign :id +localmake "right token +if equalp first :right "' ~ + [pstringassign :type (butlast butfirst :right) stop] +localmake "rid getid :right +if not equalp (id.type :id) (id.type :rid) ~ + [(throw "error (sentence "arrays :name "and :right [unequal types]))] +localmake "size arraysize id.type :id +localmake "ltarget memsetup (id.pointer :id) (id.varp :id) 0 +localmake "rtarget memsetup (id.pointer :rid) (id.varp :rid) 0 +copyarray +end + +to pstringassign :type :string +if not equalp first :type "char [stringlose] +if not emptyp butfirst last :type [stringlose] +if not equalp (last first last :type) (count :string) [stringlose] +localmake "ltarget memsetup (id.pointer :id) (id.varp :id) 0 +pstringassign1 newregister (first :ltarget) (last :ltarget) :string +regfree last :ltarget +end + +to pstringassign1 :tempreg :offset :reg :string +if emptyp :string [regfree :tempreg stop] +code (list "addi :tempreg reg.zero ascii first :string) +code (list "store :tempreg (memaddr :offset :reg)) +pstringassign1 :tempreg :offset+1 :reg (butfirst :string) +end + +to stringlose +(throw "error sentence :name [not string array or wrong size]) +end + +;; Multiple array indices to linear index computation + +to setindex :parseflag +ifelse listp :type ~ + [if :parseflag + [mustbe "|[| make "index commalist [pexpr] mustbe "|]| ] + make "index lindex last :type :index + make "type first :type] ~ + [make "index 0] +end + +to lindex :bounds :index +output lindex1 (offset pinteger noimmediate first :index + first first :bounds) ~ + butfirst :bounds butfirst :index +end + +to lindex1 :sofar :bounds :index +if emptyp :bounds [output :sofar] +output lindex1 (nextindex :sofar + last first :bounds + pinteger noimmediate first :index + first first :bounds) ~ + butfirst :bounds butfirst :index +end + +to nextindex :old :factor :new :offset +code (list "muli :old :old :factor) +localmake "newreg offset :new :offset +code (list "add :old :old :newreg) +regfree :newreg +output :old +end + +to offset :indexreg :lowbound +if not equalp :lowbound 0 [code (list "subi :indexreg :indexreg :lowbound)] +output :indexreg +end + +;; Memory interface: load and store instructions + +to codeload :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "rload :reg targetaddr) +regfree last :target +end + +to codestore :reg :pointer :varflag :index +localmake "target memsetup :pointer :varflag :index +code (list "store :reg targetaddr) +regfree last :target +end + +to targetaddr +output memaddr (first :target) (last :target) +end + +to memaddr :offset :index +output (word :offset "\( :index "\)) +end + +to memsetup :pointer :varflag :index +localmake "depth first :pointer +localmake "offset last :pointer +local "newreg +ifelse equalp :depth 0 ~ + [make "newreg reg.globalptr] ~ + [ifelse equalp :depth :lexical.depth + [make "newreg reg.frameptr] + [make "newreg newregister + code (list "rload :newreg + (memaddr frame.outerframe reg.frameptr)) + repeat (:lexical.depth - :depth) - 1 + [code (list "rload :newreg + (memaddr frame.outerframe :newreg))]]] +if :varflag ~ + [ifelse :newreg = reg.frameptr + [make "newreg newregister + code (list "rload :newreg (memaddr :offset reg.frameptr))] + [code (list "rload :newreg (memaddr :offset :newreg))] + make "offset 0] +if not equalp :index 0 ~ + [code (list "add :index :index :newreg) + regfree :newreg + make "newreg :index] +output list :offset :newreg +end + +to copyarray +localmake "looptag gensym +localmake "sizereg newregister +code (list "addi :sizereg reg.zero :size) +code :looptag +localmake "tempreg newregister +code (list "rload :tempreg (memaddr (first :rtarget) (last :rtarget))) +code (list "store :tempreg (memaddr (first :ltarget) (last :ltarget))) +code (list "addi (last :rtarget) (last :rtarget) 1) +code (list "addi (last :ltarget) (last :ltarget) 1) +code (list "subi :sizereg :sizereg 1) +code (list "gtr :tempreg :sizereg reg.zero) +code (list "jumpt :tempreg (word "" :looptag)) +regfree :sizereg +regfree :tempreg +regfree last :ltarget +regfree last :rtarget +end + +;; Expressions + +to pexpr +local [opstack datastack parenlevel] +make "opstack [[popen 1 0]] +make "datastack [] +make "parenlevel 0 +output pexpr1 +end + +to pexpr1 +local [token op] +make "token token +while [equalp :token "|(|] [popen make "token token] +make "op pgetunary :token +if not emptyp :op [output pexprop :op] +push "datastack pdata :token +make "token token +while [and (:parenlevel > 0) (equalp :token "|)| )] ~ + [pclose make "token token] +make "op pgetbinary :token +if not emptyp :op [output pexprop :op] +make "peektoken :token +pclose +if not emptyp :opstack [(throw "error [too many operators])] +if not emptyp butfirst :datastack [(throw "error [too many operands])] +output pop "datastack +end + +to pexprop :op +while [(op.prec :op) < (1 + op.prec first :opstack)] [ppopop] +push "opstack :op +output pexpr1 +end + +to popen +push "opstack [popen 1 0] +make "parenlevel :parenlevel + 1 +end + +to pclose +while [(op.prec first :opstack) > 0] [ppopop] +ignore pop "opstack +make "parenlevel :parenlevel - 1 +end + +to pgetunary :token +output gprop :token "unary +end + +to pgetbinary :token +output gprop :token "binary +end + +to ppopop +local [op function args left right type reg] +make "op pop "opstack +make "function op.instr :op +if equalp :function "plus [stop] +make "args op.nargs :op +make "right pop "datastack +make "left (ifelse equalp :args 2 [pop "datastack] [[[] []]]) +make "type pnewtype :op exp.type :left exp.type :right +if equalp exp.mode :left "immediate ~ + [localmake "leftreg newregister + code (list "addi :leftreg reg.zero exp.value :left) + make "left (list exp.type :left "register :leftreg)] +ifelse equalp exp.mode :left "register ~ + [make "reg exp.value :left] ~ + [ifelse equalp exp.mode :right "register + [make "reg exp.value :right] + [make "reg newregister]] +if equalp :function "minus ~ + [make "left (list exp.type :right "register reg.zero) + make "function "sub + make "args 2] +if equalp exp.mode :right "immediate ~ + [make "function word :function "i] +ifelse equalp :args 2 ~ + [code (list :function :reg exp.value :left exp.value :right)] ~ + [code (list :function :reg exp.value :right)] +if not equalp :reg exp.value :left [regfree exp.value :left] +if (and (equalp exp.mode :right "register) + (not equalp :reg exp.value :right)) ~ + [regfree exp.value :right] +push "datastack (list :type "register :reg) +end + +to pnewtype :op :ltype :rtype +local "type +make "type op.types :op +if emptyp :ltype [make "ltype :rtype] +if not emptyp last :type [pchecktype last :type :ltype :rtype] +if and (equalp :ltype "real) (equalp :rtype "integer) [make "rtype "real] +if and (equalp :ltype "integer) (equalp :rtype "real) [make "ltype "real] +if not equalp :ltype :rtype [(throw "error [type clash])] +if emptyp last :type ~ + [if not memberp :rtype [integer real] + [(throw "error [nonarithmetic type])]] +if emptyp first :type [output :rtype] +output first :type +end + +to pchecktype :want :left :right +if not equalp :want :left [(throw "error (sentence :left "isn't :want))] +if not equalp :want :right [(throw "error (sentence :right "isn't :want))] +end + +;; Expression elements + +to pdata :token +if equalp :token "true [output [boolean immediate 1]] +if equalp :token "false [output [boolean immediate 0]] +if equalp first :token "' [output pchardata :token] +if numberp :token [output (list numtype :token "immediate :token)] +localmake "id getid :token +if emptyp :id [(throw "error sentence [undefined symbol] :token)] +localmake "type id.type :id +if equalp :type "function [output pfuncall :token] +local "index +setindex "true +localmake "reg newregister +codeload :reg (id.pointer :id) (id.varp :id) :index +output (list :type "register :reg) +end + +to pchardata :token +if not equalp count :token 3 ~ + [(throw "error sentence :token [not single character])] +output (list "char "immediate ascii first butfirst :token) +end + +to numtype :number +if memberp ". :number [output "real] +if memberp "e :number [output "real] +output "integer +end + +to pfuncall :pname +localmake "id getid :pname +localmake "lname id.lname :id +if namep (word "need :lname) [make (word "need :lname) "true] +localmake "vartypes thing :lname +localmake "returntype first :vartypes +make "vartypes butfirst :vartypes +pproccall1 framesize.fun +localmake "reg newregister +code (list "add :reg reg.retval reg.zero) +output (list :returntype "register :reg) +end + +;; Parsing assistance + +to code :stuff +if emptyp :stuff [stop] +push :codeinto :stuff +end + +to commalist :test [:sofar []] +local [result token] +make "result run :test +if emptyp :result [output :sofar] +ifbe ", [output (commalist :test (lput :result :sofar))] +output lput :result :sofar +end + +.macro ifbe :wanted :action +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output [] +end + +.macro ifbeelse :wanted :action :else +localmake "token token +if equalp :token :wanted [output :action] +make "peektoken :token +output :else +end + +to mustbe :wanted +localmake "token token +if equalp :token :wanted [stop] +(throw "error (sentence "expected :wanted "got :token)) +end + +to newregister +for [i reg.firstfree :numregs-1] ~ + [if not item :i :regsused [setitem :i :regsused "true output :i]] +(throw "error [not enough registers available]) +end + +to regfree :reg +setitem :reg :regsused "false +end + +to reservedp :word +output memberp :word [and array begin case const div do downto else end ~ + file for forward function goto if in label mod nil ~ + not of packed procedure program record repeat set ~ + then to type until var while with] +end + +;; Lexical analysis + +to token +local [token char] +if namep "peektoken [make "token :peektoken + ern "peektoken output :token] +make "char getchar +if equalp :char "|{| [skipcomment output token] +if equalp :char char 32 [output token] +if equalp :char char 13 [output token] +if equalp :char char 10 [output token] +if equalp :char "' [output string "'] +if memberp :char [+ - * / = ( , ) |[| |]| |;|] [output :char] +if equalp :char "|<| [output twochar "|<| [= >]] +if equalp :char "|>| [output twochar "|>| [=]] +if equalp :char ". [output twochar ". [.]] +if equalp :char ": [output twochar ": [=]] +if numberp :char [output number :char] +if letterp ascii :char [output token1 lowercase :char] +(throw "error sentence [unrecognized character:] :char) +end + +to skipcomment +if equalp getchar "|}| [stop] +skipcomment +end + +to string :string +local "char +make "char getchar +if not equalp :char "' [output string word :string :char] +make "char getchar +if equalp :char "' [output string word :string :char] +make "peekchar :char +output word :string "' +end + +to twochar :old :ok +localmake "char getchar +if memberp :char :ok [output word :old :char] +make "peekchar :char +output :old +end + +to number :num +local "char +make "char getchar +if equalp :char ". ~ + [make "char getchar ~ + ifelse equalp :char ". ~ + [make "peektoken ".. output :num] ~ + [make "peekchar :char output number word :num ".]] +if equalp :char "e [output number word :num twochar "e [+ -]] +if numberp :char [output number word :num :char] +make "peekchar :char +output :num +end + +to token1 :token +local "char +make "char getchar +if or letterp ascii :char numberp :char ~ + [output token1 word :token lowercase :char] +make "peekchar :char +output :token +end + +to letterp :code +if and (:code > 64) (:code < 91) [output "true] +output and (:code > 96) (:code < 123) +end + +to getchar +local "char +if namep "peekchar [make "char :peekchar ern "peekchar output :char] +if eofp [output char 1] +output rc1 +end + +to rc1 +local "result +make "result readchar +type :result +output :result +end + +;; Data abstraction: ID List + +to newlname :word +if memberp :word :namesused [output gensym] +if namep word "% :word [output gensym] +push "namesused :word +output word "% :word +end + +to lname :word +local "result +make "result getid :word +if not emptyp :result [output item 3 :result] +(throw "error sentence [unrecognized identifier] :word) +end + +to gettype :word +local "result +make "result getid :word +if not emptyp :result [output item 2 :result] +(throw "error sentence [unrecognized identifier] :word) +end + +to getid :word [:list :idlist] +if emptyp :list [output []] +if equalp :word first first :list [output first :list] +output (getid :word butfirst :list) +end + +to id.type :id +output item 2 :id +end + +to id.pointer :id +output item 3 :id +end + +to id.lname :id +output item 3 :id +end + +to id.varp :id +output item 4 :id +end + +to id.frame :id +output item 4 :id +end + +;; Data abstraction: Frame slots + +to frame.retaddr +output 0 +end + +to frame.save.newfp +output 1 +end + +to frame.outerframe +output 2 +end + +to frame.prevframe +output 3 +end + +to frame.regsave +output 4 +end + +to framesize.proc +output 4+:numregs +end + +to frame.retval +output 4+:numregs +end + +to framesize.fun +output 5+:numregs +end + +;; Data abstraction: Operators + +to op.instr :op +output first :op +end + +to op.nargs :op +output first bf :op +end + +to op.types :op +output item 3 :op +end + +to op.prec :op +output last :op +end + +;; Data abstraction: Expressions + +to exp.type :exp +output first :exp +end + +to exp.mode :exp +output first butfirst :exp +end + +to exp.value :exp +output last :exp +end + +;; Data abstraction: Registers + +to reg.zero +output 0 +end + +to reg.retaddr +output 1 +end + +to reg.stackptr +output 2 +end + +to reg.globalptr +output 3 +end + +to reg.frameptr +output 4 +end + +to reg.newfp +output 5 +end + +to reg.retval +output 6 +end + +to reg.firstfree +output 7 +end + +;; Runtime (machine simulation) + +to prun :progname +localmake "prog thing word "% :progname +localmake "regs (array :numregs 0) +local filter "wordp :prog +foreach :prog [if wordp ? [make ? ?rest]] +localmake "memory (array :memsize 0) +setitem 0 :regs 0 +if not procedurep "add [runsetup] +prun1 :prog +end + +to prun1 :pc +if emptyp :pc [stop] +if listp first :pc [run first :pc] +prun1 butfirst :pc +end + +to rload :reg :offset :index +setitem :reg :regs (item (item :index :regs)+:offset :memory) +end + +to store :reg :offset :index +setitem (item :index :regs)+:offset :memory (item :reg :regs) +end + +to runsetup +foreach [[add sum] [sub difference] [mul product] [quo quotient] + [div [int quotient]] [rem remainder] [land product] + [lor [tobool lessp 0 sum]] [eql [tobool equalp]] + [neq [tobool not equalp]] [less [tobool lessp]] + [gtr [tobool greaterp]] [leq [tobool not greaterp]] + [geq [tobool not lessp]]] ~ + [define first ? + `[[dest src1 src2] + [setitem :dest :regs ,@[last ?] (item :src1 :regs) + (item :src2 :regs)]] + define word first ? "i + `[[dest src1 immed] + [setitem :dest :regs ,@[last ?] (item :src1 :regs) + :immed]]] +foreach [[lnot [difference 1]] [sint int] [sround round] [srandom random]] ~ + [define first ? + `[[dest src] + [setitem :dest :regs ,@[last ?] (item :src :regs)]] + define word first ? "i + `[[dest immed] + [setitem :dest :regs ,@[last ?] :immed]]] +end + +to tobool :tf +output ifelse :tf [1] [0] +end + +to jump :label +make "pc fput :label thing :label +end + +to jumpt :reg :label +if (item :reg :regs)=1 [jump :label] +end + +to jumpf :reg :label +if (item :reg :regs)=0 [jump :label] +end + +to jr :reg +make "pc item :reg :regs +end + +to jal :reg :label +setitem :reg :regs :pc +jump :label +end + +to putch :width :reg +spaces :width 1 +type char (item :reg :regs) +end + +to putstr :width :string +spaces :width (count first :string) +type :string +end + +to puttf :width :bool +spaces :width 1 +type ifelse (item :bool :regs)=0 ["F] ["T] +end + +to putint :width :reg +localmake "num (item :reg :regs) +spaces :width count :num +type :num +end + +to putreal :width :reg +putint :width :reg +end + +to spaces :width :count +if :width > :count [repeat :width - :count [type "| |]] +end + +to newline +print [] +end + +to exit +make "pc [exit] +end +</PRE> + +<P><A HREF="../v3-toc2.html">(back to Table of Contents)</A> +<P><A HREF="https://people.eecs.berkeley.edu/~bh/v3ch4/v3ch4.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../v3ch6/v3ch6.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> diff --git a/js/games/nluqo.github.io/~bh/v3ch5/vdash.gif b/js/games/nluqo.github.io/~bh/v3ch5/vdash.gif new file mode 100644 index 0000000..ff063aa --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v3ch5/vdash.gif Binary files differ |
