path: root/apps/mulisp.subx

# Toy lisp interpreter. Incomplete.
#
# To run:
#   $ ./translate_subx init.linux 0*.subx apps/mulisp.subx
#   $ ./a.elf
#   42
#   => 42
#   ^D
#   $

== code

Entry:  # run tests if necessary, a REPL if not
    # . prologue
    89/<- %ebp 4/r32/esp
    # initialize heap
    (new-segment *Heap-size Heap)
    {
      # if (argc <= 1) break
      81 7/subop/compare *ebp 1/imm32
      7e/jump-if-<= break/disp8
      # if (argv[1] != "test")) break
      (kernel-string-equal? *(ebp+8) "test")  # => eax
      3d/compare-eax-and 0/imm32/false
      74/jump-if-= break/disp8
      #
      (run-tests)
      # syscall(exit, *Num-test-failures)
      8b/-> *Num-test-failures 3/r32/ebx
      eb/jump $main:end/disp8
    }
    (repl Stdin Stdout)
    # syscall(exit, 0)
    bb/copy-to-ebx 0/imm32
$main:end:
    b8/copy-to-eax 1/imm32/exit
    cd/syscall 0x80/imm8

# Data structures
#
# Lisp is dynamically typed. Values always carry around knowledge of their
# type.
#
# There's several types of types in the description below, so we need a
# glossary and notational convention to disambiguate:
#   lisp type: what Lisp code can see. Looks how you type it at the prompt.
#     nil num char string symbol pair array
#   type tag: the numeric code for a lisp type. All caps.
#     NIL NUM CHAR STRING SYMBOL PAIR ARRAY
#   memory type: a type specifying memory layout at the SubX level. Starts
#   with a '$'.
#     $int $array $(addr _)
#
# Lisp values are represented in memory by the _cell_ data structure. A cell
# is 12 bytes long:
#   tag: $int (4 bytes; we're not concerned about wasting space)
#   data: 8 bytes whose contents and meaning depend on tag
#
# What values of the different Lisp types look like in memory:
#   - nil: cell{ tag: 0/NIL, data: 0 0 }
#   - num: cell{ tag: 1/NUM, data: $int 0 }
#     data contains the number
#   - char: cell{ tag: 2/CHAR, data: $int 0 }
#     data contains the utf-8 code of the character (no compound glyphs, no
#     modifiers, etc., etc.)
#   - string: cell{ tag: 3/STRING, data: $(addr stream byte)
#     data contains an (addr array byte) containing the string in utf-8
#   - symbol: cell{ tag: 4/SYMBOL, data: $(addr array byte) 0 }
#     data contains an (addr array byte) containing the name of the symbol in utf-8
#     alternatively, data could contain an index into the table of interned symbols
#   - pair: cell{ tag: 5/PAIR, data: $(addr cell) $(addr cell)  }
#     data contains pointers to car and cdr
#   - array: cell{ tag: 6/ARRAY, data: $tag $(addr stream data)
#     data contains a pointer to an array of 8-byte data fields and the common
#     tag for them all

repl:  # in: (addr buffered-file), out: (addr buffered-file)
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    50/push-eax
    {
      (lisp-read Stdin)  # => eax: (handle cell)
      # if (eax == 0) break
      3d/compare-eax-and 0/imm32
      74/jump-if-= break/disp8
      #
      (lisp-eval %eax)  # => eax: (handle cell)
      (lisp-print Stdout %eax)
      eb/jump loop/disp8
    }
$repl:end:
    # . restore registers
    58/pop-to-eax
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

# numbers start with a digit and are always in hex
# characters start with a backslash
# pairs start with '('
# arrays start with '['
# symbols start with anything else but quote, backquote, unquote or splice
# only one s-expression per line
lisp-read:  # in: (addr buffered-file) -> eax: (handle cell)
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    51/push-ecx
    # var s/ecx: (stream byte 512)
    81 5/subop/subtract %esp 0x200/imm32
    68/push 0x200/imm32/size
    68/push 0/imm32/read
    68/push 0/imm32/write
    89/<- %ecx 4/r32/esp
    {
      # read line into s
      (clear-stream %ecx)
      (read-line-buffered *(ebp+8) %ecx)
      # if (s->write == 0) return null
      {
        81 7/subop/compare *ecx 0/imm32
        75/jump-if-!= break/disp8
        b8/copy-to-eax 0/imm32/eof
        eb/jump $lisp-read:end/disp8
      }
      # ...
#?       eb/jump loop/disp8
    }
    # return s
    89/<- %eax 1/r32/ecx
$lisp-read:end:
    # . reclaim locals
    81 0/subop/add %esp 0x20c/imm32
    # . restore registers
    59/pop-to-ecx
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

# lisp-read:  in: (addr buffered-file) -> (handle cell)
#   token tmp = next-mulisp-token(in)
#   if is-int(tmp) return cell(tmp)
#   if is-string(tmp) return cell(tmp)
#   if is-pair(tmp) ...
#   if is-array(tmp) ...

next-mulisp-token:  # in: (addr buffered-file), line: (addr stream byte), result: (addr slice)
    # pseudocode:
    #   if (line->read >= line->write)
    #     read-line-buffered(in, line)
    #     recurse
    #   if (line->data[line->read] == ' ')
    #     skip-chars-matching-whitespace(line)
    #     recurse
    #   if (line->data[line->read] == '#')
    #     read-line-buffered(in, line)
    #     recurse
    #   eax = line->data[line->read]
    #   if (eax == '"')
    #     result->start = &line->data[line->read]
    #     skip-string(in)
    #     result->end = &line->data[line->read]
    #     return
    #   if (is-digit(eax))
    #     result->start = &line->data[line->read]
    #     skip-hex-int(in)
    #     result->end = &line->data[line->read]
    #     return
    #   if (eax in '(' ')' '[' ']')
    #     result->start = &line->data[line->read]
    #     ++line->read
    #     result->en = &line->data[line->read]
    #     return
    #   else
    #     result->start = &line->data[line->read]
    #     skip-lisp-word(line)
    #     result->en = &line->data[line->read]
    #     return
    #
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
$next-mulisp-token:end:
    # . reclaim locals
    # . restore registers
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

new-int-cell:  # in: (addr slice) -> eax: (handle cell)

new-string-cell:  # in: (addr slice) -> eax: (handle cell)

lisp-eval:  # in: (addr cell) -> eax: (handle cell)
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    8b/-> *(ebp+8) 0/r32/eax
$lisp-eval:end:
    # . restore registers
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

lisp-print:  # out: (addr buffered-file), x: (addr cell)
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    # write(x)
    (write-buffered Stdout "=> ")
    (write-stream-data Stdout *(ebp+0xc))
    (flush Stdout)
$lisp-print:end:
    # . restore registers
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

== data

Nil:
  0/imm32/tag
  0/imm32/data