subx/examples/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 (from the subx directory):
#   $ subx translate examples/ex9.subx -o examples/ex9
#   $ subx run examples/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

    # . prolog
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # . . call ascii-difference(argv[1], argv[2])
    # push argv[2]
    ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0xc/disp8       .                 # push *(EBP+12)
    # push argv[1]
    ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0x8/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
    b8/copy-to-EAX  1/imm32/exit
    cd/syscall  0x80/imm8

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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