## compute the factorial of 5, and return the result in the exit code # # To run: # $ subx translate apps/factorial.subx apps/factorial # $ subx run apps/factorial # Expected result: # $ echo $? # 120 == code # 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: e8/call run_tests/disp32 #? # 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 #? 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add 4 to ESP #? # 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 EAX to 1 (base case) b8/copy . . . . . . . 1/imm32 # copy 1 to EAX # if (n <= 1) jump exit 81 7/subop/compare 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . . 4/disp8 1/imm32 # compare *(ESP+4) with 1 7e/jump-if-<= . . . . . . $factorial:exit/disp8 # jump if <= to $factorial:exit # EBX: n-1 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 3/r32/EBX 4/disp8 . # copy *(ESP+4) 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 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add 4 to ESP # 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) f7 4/subop/multiply 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 4/disp8 . # multiply *(ESP+4) (n) into EAX (factorial(n-1)) # TODO: check for overflow $factorial:exit: c3/return test_factorial: # factorial(5) # push arg 68/push . . . . . . . 5/imm32 # push 5 # call e8/call . . . . . . factorial/disp32 # discard arg 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add 4 to ESP # if EAX == 120 3d/compare . . . . . . . 0x78/imm32/120 # compare EAX with 120 75/jump-if-unequal . . . . . . $test_factorial:else/disp8 # print('.') # push args 68/push . . . . . . . Test_passed/imm32 # call e8/call . . . . . . write_stderr/disp32 # discard arg 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add 4 to ESP # return c3/return # else: $test_factorial:else: # print('F') # push args 68/push . . . . . . . Test_failed/imm32 # call e8/call . . . . . . write_stderr/disp32 # discard arg 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add 4 to ESP # end c3/return ## helpers write_stderr: # s : (address array byte) -> # write(2/stderr, (data) s+4, (size) *s) # fd = 2 (stderr) bb/copy . . . . . . . 2/imm32 # copy 2 to EBX # x = s+4 8b/copy 1/mod/*+disp8 4/rm32/SIB 4/base/ESP 4/index/none . 1/r32/ECX 4/disp8 . # copy *(ESP+4) to ECX 81 0/subop/add 3/mod/direct 1/rm32/ECX . . . . . 4/imm32 # add 4 to ECX # size = *s 8b/copy 1/mod/*+disp8 4/rm32/SIB 4/base/ESP 4/index/none . 2/r32/EDX 4/disp8 . # copy *(ESP+4) to EDX 8b/copy 0/mod/indirect 2/rm32/EDX . . . 2/r32/EDX . . # copy *EDX to EDX # call write() b8/copy . . . . . . . 4/imm32/write # copy 1 to EAX cd/syscall . . . . . . . 0x80/imm8 # int 80h # end c3/return == data Test_passed: # size 01 00 00 00 # data 2e/dot Test_failed: # size 01 00 00 00 # data 46/F # vim:ft=subx:nowrap:so=0