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                         .               .             .           .             .           .           .               1/imm32/exit      # copy to EAX
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