1 # Example showing arg order on the stack. 2 # 3 # Show difference between ascii codes of first letter of first arg and first 4 # letter of second arg. 5 # 6 # To run: 7 # $ ./subx translate init.linux examples/ex9.subx -o examples/ex9 8 # $ ./subx run examples/ex9 z x 9 # Expected result: 10 # $ echo $? 11 # 2 12 # 13 # At the start of a SubX program: 14 # argc: *esp 15 # argv[0]: *(esp+4) 16 # argv[1]: *(esp+8) 17 # ... 18 # Locals start from esp-4 downwards. 19 20 == code 21 # instruction effective address register displacement immediate 22 # . op subop mod rm32 base index scale r32 23 # . 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 24 25 Entry: 26 # . prologue 27 89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp 28 # ascii-difference(argv[1], argv[2]) 29 # . . push argv[2] 30 ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12) 31 # . . push argv[1] 32 ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8) 33 # . . call 34 e8/call ascii-difference/disp32 35 # . . discard args 36 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp 37 # exit(eax) 38 89/copy 3/mod/direct 3/rm32/ebx . . . 0/r32/eax . . # copy eax to ebx 39 e8/call syscall_exit/disp32 40 41 ascii-difference: # (s1, s2) : null-terminated ascii strings 42 # a = first letter of s1 (ecx) 43 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none 0/r32/eax 4/disp8 . # copy *(esp+4) to eax 44 8b/copy 0/mod/indirect 0/rm32/eax . . . 0/r32/eax . . # copy *eax to eax 45 # b = first letter of s2 (edx) 46 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none 1/r32/ecx 8/disp8 # copy *(esp+8) to ecx 47 8b/copy 0/mod/indirect 1/rm32/ecx . . . 1/r32/ecx . . # copy *ecx to ecx 48 # a-b 49 29/subtract 3/mod/direct 0/rm32/eax . . . 1/r32/ecx . . # subtract ecx from eax 50 c3/return 51 52 # . . vim:nowrap:textwidth=0