1 # Example reading commandline arguments: compute length of first arg.
 2 #
 3 # To run (from the subx directory):
 4 #   $ ./subx translate 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     # . prolog
23     89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
24     # EAX = ascii-length(argv[1])
25     # . . push args
26     ff          6/subop/push        1/mod/*+disp8   5/rm32/EBP    .           .             .           .           8/disp8         .                 # push *(EBP+8)
27     # . . call
28     e8/call  ascii-length/disp32
29     # . . discard args
30     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
31 
32     # exit(EAX)
33     89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
34     b8/copy-to-EAX  1/imm32/exit
35     cd/syscall  0x80/imm8
36 
37 ascii-length:  # s : (address 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                         0/mod/*         2/rm32/EDX    .           .             .           1/r32/ECX   .               .                 # copy byte at *EDX to lower byte of ECX
45     # if c == '\0' break
46     81          7/subop/compare     3/mod/direct    1/rm32/ECX    .           .             .           .           .               0/imm32           # compare ECX
47     74/jump-if-equal  $ascii-length-ret/disp8
48     # ++s
49     42/increment-EDX
50     # ++result
51     40/inc-EAX
52     # loop
53     eb/jump  $ascii-length-loop/disp8
54 $ascii-length-ret:
55     # return EAX
56     c3/return
57 
58 # . . vim:nowrap:textwidth=0