1 ## Example showing arg order on the stack. 2 # Show difference between ascii codes of first letter of first arg and first 3 # letter of second arg. 4 # 5 # To run: 6 # $ subx translate ex9.subx ex9 7 # $ subx run ex9 z x 8 # Expected result: 9 # $ echo $? 10 # 2 11 # 12 # At the start of a SubX program: 13 # argc: *ESP 14 # argv[0]: *(ESP+4) 15 # argv[1]: *(ESP+8) 16 # ... 17 # Locals start from ESP-4 downwards. 18 19 == code 20 # instruction effective address operand displacement immediate 21 # op subop mod rm32 base index scale r32 22 # 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 23 # s1 = argv[1] (EAX) 24 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 0/r32/EAX 8/disp8 . # copy *(ESP+8) to EAX 25 # s2 = argv[2] (EBX) 26 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 3/r32/EBX 0xc/disp8 . # copy *(ESP+12) to EBX 27 # call string_equal(s1, s2) 28 # push args 29 50/push-EAX 30 53/push-EBX 31 # call 32 e8/call ascii_difference/disp32 33 # discard args 34 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add 8 to ESP 35 # exit(EAX) 36 89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX 37 b8/copy . . . . . . . 1/imm32/exit # copy 1 to EAX 38 cd/syscall 0x80/imm8 39 40 ascii_difference: # (s1, s2) : null-terminated ascii strings 41 # a = first letter of s1 (ECX) 42 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 0/r32/EAX 8/disp8 . # copy *(ESP+8) to EAX 43 8b/copy 0/mod/indirect 0/rm32/EAX . . . 0/r32/EAX . . # copy *EAX to EAX 44 # b = first letter of s2 (EDX) 45 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 1/r32/ECX 4/disp8 # copy *(ESP+4) to ECX 46 8b/copy 0/mod/indirect 1/rm32/ECX . . . 1/r32/ECX . . # copy *ECX to ECX 47 # a-b 48 29/subtract 3/mod/direct 0/rm32/EAX . . . 1/r32/ECX . . # subtract ECX from EAX 49 c3/return