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                                                   operand     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   # prolog
22   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
23   # call ascii-length(argv[1])
24     # push args
25   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0x8/disp8       .                 # push *(EBP+8)
26     # call
27   e8/call  ascii-length/disp32
28     # discard args
29   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
30 
31   # exit(EAX)
32   89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
33   b8/copy-to-EAX  1/imm32/exit
34   cd/syscall  0x80/imm8
35 
36 ascii-length:  # (s)
37   # initialize s (EDX)
38   8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none  .           2/r32/EDX   4/disp8         .                 # copy *(ESP+4) to EDX
39   # var result = 0 (EAX)
40   b8/copy-to-EAX  0/imm32
41 $ascii-length-loop:
42   # var c = *s (ECX)
43   8a/copy                         0/mod/*         2/rm32/EDX    .           .             .           1/r32/ECX   .               .                 # copy byte at *EDX to lower byte of ECX
44   # if c == '\0' break
45   81          7/subop/compare     3/mod/direct    1/rm32/ECX    .           .             .           .           .               0/imm32           # compare ECX
46   74/jump-if-equal  $ascii-length-ret/disp8
47   # ++s
48   81          0/subop/add         3/mod/direct    2/rm32/EDX    .           .             .           .           .               1/imm32           # add to EDX
49   # ++result
50   40/inc-EAX
51   # loop
52   eb/jump  $ascii-length-loop/disp8
53 $ascii-length-ret:
54   # return (result in EAX)
55   c3/return
56 
57 # vim:nowrap:textwidth=0