# Toy lisp interpreter
#
# To run:
#   $ ./ntranslate 0*.subx apps/subx-common.subx apps/mulisp.subx
#   $ ./a.elf
#   42
#   => 42
#   ^D
#   $

== code

Entry:  # run tests if necessary, a REPL if not
    # . prolog
    89/<- %ebp 4/r32/esp
    # initialize heap
    (new-segment Heap-size Heap)
    # if (argc <= 1) goto run-main
    81 7/subop/compare *ebp 1/imm32
    7e/jump-if-lesser-or-equal $run-main/disp8
    # if (argv[1] != "test")) goto run-main
    (kernel-string-equal? *(ebp+8) "test")  # => eax
    3d/compare-eax-and 0/imm32
    74/jump-if-equal $run-main/disp8
    #
    (run-tests)
    # syscall(exit, *Num-test-failures)
    8b/-> *Num-test-failures 3/r32/ebx
    eb/jump $main:end/disp8
$run-main:
    (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 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 $(address _)
#
# 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: $(address array byte)
#     data contains an (address array byte) containing the string in utf-8
#   - symbol: cell{ tag: 4/SYMBOL, data: $(address array byte) 0 }
#     data contains an (address 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: $(address cell) $(address cell)  }
#     data contains pointers to car and cdr
#   - array: cell{ tag: 6/ARRAY, data: $tag $(address array data)
#     data contains a pointer to an array of 8-byte data fields and the common
#     tag for them all

repl:  # in : (address buffered-file), out : (address buffered-file) -> <void>
    # . prolog
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    50/push-eax
$repl:loop:
    (lisp-read Stdin)  # => eax : (address cell)
    # if (eax == 0) break
    3d/compare-eax-and 0/imm32
    74/jump-if-equal $repl:end/disp8
    #
    (lisp-eval %eax)  # => eax : (address cell)
    (lisp-print Stdout %eax)
    # loop
    eb/jump $repl:loop/disp8
$repl:end:
    # . restore registers
    58/pop-to-eax
    # . epilog
    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
lisp-read:  # in : (address buffered-file) -> eax : (address cell)
    # . prolog
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    # var s/ecx : (address stream) = new-stream(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
    #
    (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-not-equal $lisp-read:loop/disp8
    b8/copy-to-eax 0/imm32/eof
    eb/jump $lisp-read:end/disp8
$lisp-read:loop:
    # return s
    89/<- %eax 1/r32/ecx
$lisp-read:end:
    # . reclaim locals
    81 0/subop/add %esp 0x20c/imm32
    # . restore registers
    # . epilog
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

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

lisp-print:  # out : (address buffered-file), x : (address cell)
    # . prolog
    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
    # . epilog
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

== data

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