# Mu - subx/examples/ex9.subx

     1 ## Example showing arg order on the stack.
     2 # Show difference between ascii codes of first letter of first arg and first
     3 # letter of second arg.
     4 #
     5 # To run (from the subx directory):
     6 #   $ subx translate examples/ex9.subx -o examples/ex9
     7 #   $ subx run examples/ex9 z x
     8 # Expected result:
     9 #   $ echo $?
    10 #   2
    11 #
    12 # At the start of a SubX program:
    13 #   argc: *ESP
    14 #   argv[0]: *(ESP+4)
    15 #   argv[1]: *(ESP+8)
    16 #   ...
    17 # Locals start from ESP-4 downwards.
    18
    19 == code
    20 # instruction                     effective address                                                   operand     displacement    immediate
    21 # op          subop               mod             rm32          base        index         scale       r32
    22 # 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
    23   # prolog
    24   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    25   # call ascii-difference(argv[1], argv[2])
    26     # push argv[2]
    27   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0xc/disp8       .                 # push *(EBP+12)
    28     # push argv[1]
    29   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0x8/disp8       .                 # push *(EBP+8)
    30     # call
    31   e8/call  ascii-difference/disp32
    32     # discard args
    33   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    34   # exit(EAX)
    35   89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
    36   b8/copy-to-EAX  1/imm32/exit
    37   cd/syscall  0x80/imm8
    38
    39 ascii-difference:  # (s1, s2) : null-terminated ascii strings
    40   # a = first letter of s1 (ECX)
    41   8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              0/r32/EAX   4/disp8         .                 # copy *(ESP+4) to EAX
    42   8b/copy                         0/mod/indirect  0/rm32/EAX    .           .             .           0/r32/EAX   .               .                 # copy *EAX to EAX
    43   # b = first letter of s2 (EDX)
    44   8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              1/r32/ECX   8/disp8                           # copy *(ESP+8) to ECX
    45   8b/copy                         0/mod/indirect  1/rm32/ECX    .           .             .           1/r32/ECX   .               .                 # copy *ECX to ECX
    46   # a-b
    47   29/subtract                     3/mod/direct    0/rm32/EAX    .           .             .           1/r32/ECX   .               .                 # subtract ECX from EAX
    48   c3/return
    49
    50 # vim:nowrap:textwidth=0