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