https://github.com/akkartik/mu/blob/master/subx/examples/ex9.subx

     1 # Example showing arg order on the stack.
     2 #
     3 # Show difference between ascii codes of first letter of first arg and first
     4 # letter of second arg.
     5 #
     6 # To run (from the subx directory):
     7 #   $ ./subx translate examples/ex9.subx -o examples/ex9
     8 #   $ ./subx run examples/ex9 z x
     9 # Expected result:
    10 #   $ echo $?
    11 #   2
    12 #
    13 # At the start of a SubX program:
    14 #   argc: *ESP
    15 #   argv[0]: *(ESP+4)
    16 #   argv[1]: *(ESP+8)
    17 #   ...
    18 # Locals start from ESP-4 downwards.
    19
    20 == code
    21 #   instruction                     effective address                                                   register    displacement    immediate
    22 # . op          subop               mod             rm32          base        index         scale       r32
    23 # . 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
    24
    25     # . prolog
    26     89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    27     # ascii-difference(argv[1], argv[2])
    28     # . . push argv[2]
    29     ff          6/subop/push        1/mod/*+disp8   5/rm32/EBP    .           .             .           .           0xc/disp8       .                 # push *(EBP+12)
    30     # . . push argv[1]
    31     ff          6/subop/push        1/mod/*+disp8   5/rm32/EBP    .           .             .           .           8/disp8         .                 # push *(EBP+8)
    32     # . . call
    33     e8/call  ascii-difference/disp32
    34     # . . discard args
    35     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    36     # syscall(exit, EAX)
    37     89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
    38     b8/copy-to-EAX  1/imm32/exit
    39     cd/syscall  0x80/imm8
    40
    41 ascii-difference:  # (s1, s2) : null-terminated ascii strings
    42     # a = first letter of s1 (ECX)
    43     8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              0/r32/EAX   4/disp8         .                 # copy *(ESP+4) to EAX
    44     8b/copy                         0/mod/indirect  0/rm32/EAX    .           .             .           0/r32/EAX   .               .                 # copy *EAX to EAX
    45     # b = first letter of s2 (EDX)
    46     8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              1/r32/ECX   8/disp8                           # copy *(ESP+8) to ECX
    47     8b/copy                         0/mod/indirect  1/rm32/ECX    .           .             .           1/r32/ECX   .               .                 # copy *ECX to ECX
    48     # a-b
    49     29/subtract                     3/mod/direct    0/rm32/EAX    .           .             .           1/r32/ECX   .               .                 # subtract ECX from EAX
    50     c3/return
    51
    52 # . . vim:nowrap:textwidth=0