1 ## String comparison: return 1 iff the two args passed in at the commandline are equal.
 2 #
 3 # To run (from the subx directory):
 4 #   $ subx translate examples/ex10.subx -o examples/ex10
 5 #   $ subx run examples/ex10 abc abd
 6 # Expected result:
 7 #   $ echo $?
 8 #   0  # false
 9 
10 == code
11 # instruction                     effective address                                                   operand     displacement    immediate
12 # op          subop               mod             rm32          base        index         scale       r32
13 # 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
14 
15 # main: return argv-equal(argv[1], argv[2])
16 #       At the start of a SubX program:
17 #         argc: *ESP
18 #         argv[0]: *(ESP+4)
19 #         argv[1]: *(ESP+8)
20 #         ...
21   # prolog
22   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
23   # call argv-equal(argv[1], argv[2])
24     # push argv[2]
25   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0xc/disp8       .                 # push *(EBP+12)
26     # push argv[1]
27   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0x8/disp8       .                 # push *(EBP+8)
28     # call
29   e8/call argv-equal/disp32
30   # exit(EAX)
31 $exit:
32   89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
33   b8/copy-to-EAX  1/imm32
34   cd/syscall  0x80/imm8
35 
36 # compare two null-terminated ascii strings
37 # reason for the name: the only place we should have null-terminated ascii strings is from commandline args
38 argv-equal:  # (s1, s2) : null-terminated ascii strings -> EAX : boolean
39   # initialize s1 (ECX) and s2 (EDX)
40   8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none  .           1/r32/ECX   4/disp8         .                 # copy *(ESP+4) to ECX
41   8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none  .           2/r32/EDX   8/disp8         .                 # copy *(ESP+8) to EDX
42   # while (true)
43 $argv-equal:loop:
44     # c1/EAX, c2/EBX = *s1, *s2
45   b8/copy-to-EAX  0/imm32
46   8a/copy                         0/mod/indirect  1/rm32/ECX    .           .             .           0/r32/EAX   .               .                 # copy byte at *ECX to lower byte of EAX
47   bb/copy-to-EBX  0/imm32
48   8a/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           3/r32/EBX   .               .                 # copy byte at *EDX to lower byte of EBX
49     # if (c1 == 0) break
50   3d/compare-EAX  0/imm32
51   74/jump-if-equal  $argv-equal:break/disp8
52     # if (c1 != c2) return false
53   39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX with EBX
54   75/jump-if-not-equal  $argv-equal:false/disp8
55     # ++s1, ++s2
56   41/inc-ECX
57   42/inc-EDX
58   # end while
59   eb/jump  $argv-equal:loop/disp8
60 $argv-equal:break:
61   # if (c2 == 0) return true
62   81          7/subop/compare     3/mod/direct    3/rm32/EBX    .           .             .           .           .               0/imm32           # compare EBX
63   75/jump-if-not-equal  $argv-equal:false/disp8
64 $argv-equal:success:
65   b8/copy-to-EAX  1/imm32
66   c3/return
67   # return false
68 $argv-equal:false:
69   b8/copy-to-EAX  0/imm32
70   c3/return
71 
72 # vim:nowrap:textwidth=0