apps/ex9.subx


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# Example showing arg order on the stack.
#
# Show difference between ascii codes of first letter of first arg and first
# letter of second arg.
#
# To run:
#   $ ./bootstrap translate init.linux apps/ex9.subx -o apps/ex9
#   $ ./bootstrap run apps/ex9 z x
# Expected result:
#   $ echo $?
#   2
#
# At the start of a SubX program:
#   argc: *esp
#   argv[0]: *(esp+4)
#   argv[1]: *(esp+8)
#   ...
# Locals start from esp-4 downwards.

== code
#   instruction                     effective address                                                   register    displacement    immediate
# . op          subop               mod             rm32          base        index         scale       r32
# . 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes

Entry:
    # . prologue
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # ascii-difference(argv[1], argv[2])
    # . . push argv[2]
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
    # . . push argv[1]
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  ascii-difference/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # exit(eax)
    89/copy                         3/mod/direct    3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to ebx
    e8/call  syscall_exit/disp32

ascii-difference:  # (s1, s2): null-terminated ascii strings
    # a = first letter of s1 (ecx)
    8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           0/r32/eax   4/disp8         .                 # copy *(esp+4) to eax
    8b/copy                         0/mod/indirect  0/rm32/eax    .           .             .           0/r32/eax   .               .                 # copy *eax to eax
    # b = first letter of s2 (edx)
    8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           1/r32/ecx   8/disp8                           # copy *(esp+8) to ecx
    8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           1/r32/ecx   .               .                 # copy *ecx to ecx
    # a-b
    29/subtract                     3/mod/direct    0/rm32/eax    .           .             .           1/r32/ecx   .               .                 # subtract ecx from eax
    c3/return

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