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 0x09000000
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     # . prolog
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     b8/copy-to-EAX  1/imm32/exit
36     cd/syscall  0x80/imm8
37 
38 ascii-length:  # s : (address array byte) -> n/EAX
39     # EDX = s
40     8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none  .           2/r32/EDX   4/disp8         .                 # copy *(ESP+4) to EDX
41     # var result/EAX = 0
42     b8/copy-to-EAX  0/imm32
43 $ascii-length:loop:
44     # var c/ECX = *s
45     8a/copy-byte                    0/mod/*         2/rm32/EDX    .           .             .           1/r32/CL    .               .                 # copy byte at *EDX to CL
46     # if (c == '\0') break
47     81          7/subop/compare     3/mod/direct    1/rm32/ECX    .           .             .           .           .               0/imm32           # compare ECX
48     74/jump-if-equal  $ascii-length:end/disp8
49     # ++s
50     42/increment-EDX
51     # ++result
52     40/increment-EAX
53     # loop
54     eb/jump  $ascii-length:loop/disp8
55 $ascii-length:end:
56     # return EAX
57     c3/return
58 
59 == data 0x0a000000
60 
61 # . . vim:nowrap:textwidth=0