## compute the factorial of 5, and return the result in the exit code # # To run: # $ subx translate ex7.subx ex7 # $ subx run ex7 # Expected result: # $ echo $? # 120 == 0x08048054 # code segment, after leaving room for ELF header # instruction effective address operand displacement immediate # op subop mod rm32 base index scale r32 # 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 # main: # prepare to make a call 55/push # push EBP 89/copy 3/mod/direct 5/rm32/EBP 4/r32/ESP # copy ESP to EBP # factorial(5) 68/push 5/imm32 # push 5 e8/call factorial/disp32 # discard arg 5a/pop # pop into EDX # clean up after call 89/copy 3/mod/direct 4/rm32/ESP 5/r32/EBP # copy EBP to ESP 5d/pop # pop to EBP # exit(EAX) 89/copy 3/mod/direct 3/rm32/EBX 0/r32/EAX # copy EAX to EBX b8/copy 1/imm32 # copy 1 to EAX cd/syscall 0x80/imm8 # int 80h # factorial(n) factorial: # initialize n 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none 2/r32/edx 4/disp8 # copy *(ESP+4) to EDX # initialize EAX to 1 (base case) b8/copy 1/imm32 # copy 1 to EAX # if (n <= 1) jump exit 81 7/subop/compare 3/mod/direct 2/rm32/EDX 1/imm32 # compare EDX with 1 7e/jump-if factorial:exit/disp8 # jump if <= to exit # EBX: n-1 89/copy 3/mod/direct 3/rm32/EBX 2/r32/EDX # copy EDX to EBX 81 5/subop/subtract 3/mod/direct 3/rm32/EBX 1/imm32 # subtract 1 from EBX # prepare call 55/push # push EBP 89/copy 3/mod/direct 5/rm32/EBP 4/r32/ESP # copy ESP to EBP # EAX: factorial(n-1) 53/push # push EBX e8/call factorial/disp32 # discard arg 5e/pop # pop into ESI # clean up after call 89/copy 3/mod/direct 4/rm32/ESP 5/r32/EBP # copy EBP to ESP 5d/pop # pop to EBP # refresh n 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none 2/r32/edx 4/disp8 # copy *(ESP+4) to EDX # return n * factorial(n-1) 0f af/multiply 3/mod/direct 2/rm32/EDX 0/r32/EAX # multiply EDX (n) into EAX (factorial(n-1)) # TODO: check for overflow factorial:exit: c3/return # vim:ft=subx:nowrap:so=0