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 (from the subx directory): 6 # $ subx translate examples/ex9.subx -o examples/ex9 7 # $ subx run examples/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 # prolog 24 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 25 # call ascii-difference(argv[1], argv[2]) 26 # push argv[2] 27 ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 0xc/disp8 . # push *(EBP+12) 28 # push argv[1] 29 ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 0x8/disp8 . # push *(EBP+8) 30 # call 31 e8/call ascii-difference/disp32 32 # discard args 33 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 34 # exit(EAX) 35 89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX 36 b8/copy-to-EAX 1/imm32/exit 37 cd/syscall 0x80/imm8 38 39 ascii-difference: # (s1, s2) : null-terminated ascii strings 40 # a = first letter of s1 (ECX) 41 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 0/r32/EAX 4/disp8 . # copy *(ESP+4) to EAX 42 8b/copy 0/mod/indirect 0/rm32/EAX . . . 0/r32/EAX . . # copy *EAX to EAX 43 # b = first letter of s2 (EDX) 44 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 1/r32/ECX 8/disp8 # copy *(ESP+8) to ECX 45 8b/copy 0/mod/indirect 1/rm32/ECX . . . 1/r32/ECX . . # copy *ECX to ECX 46 # a-b 47 29/subtract 3/mod/direct 0/rm32/EAX . . . 1/r32/ECX . . # subtract ECX from EAX 48 c3/return 49 50 # vim:nowrap:textwidth=0