1 # Example reading commandline arguments: compute length of first arg.
 2 #
 3 # To run:
 4 #   $ ./subx translate init.linux examples/ex8.subx -o examples/ex8
 5 #   $ ./subx run examples/ex8 abc de fghi
 6 # Expected result:
 7 #   $ echo $?
 8 #   3  # length of 'abc'
 9 #
10 # At the start of a SubX program:
11 #   argc: *esp
12 #   argv[0]: *(esp+4)
13 #   argv[1]: *(esp+8)
14 #   ...
15 # Locals start from esp-4 downwards.
16 
17 == code
18 #   instruction                     effective address                                                   register    displacement    immediate
19 # . op          subop               mod             rm32          base        index         scale       r32
20 # . 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
21 
22 Entry:
23     # . prologue
24     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
25     # eax = ascii-length(argv[1])
26     # . . push args
27     ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
28     # . . call
29     e8/call  ascii-length/disp32
30     # . . discard args
31     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
32 
33     # exit(eax)
34     89/copy                         3/mod/direct    3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to ebx
35     e8/call  syscall_exit/disp32
36 
37 ascii-length:  # s : (addr array byte) -> n/eax
38     # edx = s
39     8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           2/r32/edx   4/disp8         .                 # copy *(esp+4) to edx
40     # var result/eax = 0
41     b8/copy-to-eax  0/imm32
42 $ascii-length:loop:
43     # var c/ecx = *s
44     8a/copy-byte                    0/mod/*         2/rm32/edx    .           .             .           1/r32/CL    .               .                 # copy byte at *edx to CL
45     # if (c == '\0') break
46     81          7/subop/compare     3/mod/direct    1/rm32/ecx    .           .             .           .           .               0/imm32           # compare ecx
47     74/jump-if-=  $ascii-length:end/disp8
48     # ++s
49     42/increment-edx
50     # ++result
51     40/increment-eax
52     # loop
53     eb/jump  $ascii-length:loop/disp8
54 $ascii-length:end:
55     # return eax
56     c3/return
57 
58 == data
59 
60 # . . vim:nowrap:textwidth=0