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:
 7 #   $ ./bootstrap translate init.linux apps/ex9.subx -o apps/ex9
 8 #   $ ./bootstrap run apps/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 Entry:
26     # . prologue
27     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
28     # ascii-difference(argv[1], argv[2])
29     # . . push argv[2]
30     ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
31     # . . push argv[1]
32     ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
33     # . . call
34     e8/call  ascii-difference/disp32
35     # . . discard args
36     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
37     # exit(eax)
38     89/copy                         3/mod/direct    3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to ebx
39     e8/call  syscall_exit/disp32
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