## 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 # # You can also run an automated test (that does the exact same thing): # $ subx run apps/factorial test # Expected output: # . # Every '.' indicates a passing test. Failing tests get a 'F'. == 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: # if (argc > 1) 8b/copy 0/mod/indirect 4/rm32/sib 4/base/ESP 4/index/none . 0/r32/EAX . . # copy *ESP to EAX 3d/compare . . . . . . . 1/imm32 # compare EAX with 1 7e/jump-if-lesser-or-equal $run_main/disp8 # and if (argv[1] == "test") 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 0/r32/EAX 8/disp8 . # copy *(ESP+8) to EAX # push args 68/push Test_argv/imm32 50/push-EAX # call e8/call argv_equal/disp32 # discard args 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add 8 to ESP # check result 3d/compare . . . . . . . 1/imm32 # compare EAX with 1 75/jump-if-not-equal $run_main/disp8 # then e8/call run_tests/disp32 eb/jump $main_exit/disp8 # else EAX <- factorial(5) $run_main: # push arg 68/push . . . . . . . 5/imm32 # push 5 # EAX <- call e8/call . . . . . . factorial/disp32 # discard arg 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add 4 to ESP $main_exit: # 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 # 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 # compare two null-terminated ascii strings # reason for the name: the only place we should have null-terminated ascii strings is from commandline args argv_equal: # (s1, s2) : null-terminated ascii strings -> EAX : boolean # initialize s1 (ECX) and s2 (EDX) 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 1/r32/ECX 8/disp8 . # copy *(ESP+8) to ECX 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 2/r32/EDX 4/disp8 . # copy *(ESP+4) to EDX # while (true) $argv_loop: # c1/EAX, c2/EBX = *s1, *s2 b8/copy 0/imm32 # clear EAX 8a/copy 0/mod/indirect 1/rm32/ECX . . . 0/r32/EAX . . # copy byte at *ECX to lower byte of EAX bb/copy 0/imm32 # clear EBX 8a/copy 0/mod/indirect 2/rm32/EDX . . . 3/r32/EBX . . # copy byte at *EDX to lower byte of EBX # if (c1 == 0) break 3d/compare . . . . . . . 0/imm32 # compare EAX with 0 74/jump-if-equal $argv_break/disp8 # if (c1 != c2) return false 39/compare 3/mod/direct 0/rm32/EAX . . . 3/r32/EBX . . # compare EAX with EBX 75/jump-if-not-equal $argv_fail/disp8 # ++s1, ++s2 41/inc-ECX 42/inc-EDX # end while eb/jump $argv_loop/disp8 $argv_break: # if (c2 == 0) return true 81 7/subop/compare 3/mod/direct 3/rm32/EBX . . . . . 0/imm32 # compare EBX with 0 75/jump-if-not-equal $argv_fail/disp8 b8/copy . . . . . . . 1/imm32 # copy 1 to EAX c3/return # return false $argv_fail: b8/copy . . . . . . . 0/imm32 # copy 0 to EAX c3/return write_stderr: # s : (address array byte) -> # save registers 50/push . . . . . . . . # push EAX 51/push . . . . . . . . # push ECX 52/push . . . . . . . . # push EDX 53/push . . . . . . . . # push EBX # 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 0x14/disp8 . # copy *(ESP+20) 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 0x14/disp8 . # copy *(ESP+20) 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 # restore registers 5b/pop . . . . . . . . # pop EBX 5a/pop . . . . . . . . # pop EDX 59/pop . . . . . . . . # pop ECX 58/pop . . . . . . . . # pop EAX # end c3/return == data Test_argv: # null-terminated # data 74/t 65/e 73/s 74/t 00/null 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