examples/ex10.subx


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pre { line-height: 125%; }
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.highlight .# String comparison: return 1 iff the two args passed in at the commandline are equal.
#
# To run:
#   $ ./subx translate examples/ex10.subx -o examples/ex10
#   $ ./subx run examples/ex10 abc abd
# Expected result:
#   $ echo $?
#   0  # false

== code 0x09000000
#   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:  # return argv-equal(argv[1], argv[2])
#       At the start of a SubX program:
#         argc: *esp
#         argv[0]: *(esp+4)
#         argv[1]: *(esp+8)
#         ...
    # . prolog
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # argv-equal(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 argv-equal/disp32
    # syscall(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

# compare two null-terminated ascii strings
# reason for the name: the only place we should have null-terminated ascii strings is from commandline args
argv-equal:  # (s1, s2) : null-terminated ascii strings -> eax : boolean
    # initialize s1 (ecx) and s2 (edx)
    8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           1/r32/ecx   4/disp8         .                 # copy *(esp+4) to ecx
    8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           2/r32/edx   8/disp8         .                 # copy *(esp+8) to edx
$argv-equal:loop:
    # c1/eax, c2/ebx = *s1, *s2
    b8/copy-to-eax  0/imm32
    8a/copy-byte                    0/mod/indirect  1/rm32/ecx    .