diff options
Diffstat (limited to 'subx/apps/factorial.subx')
| -rw-r--r-- | subx/apps/factorial.subx | 64 |
1 files changed, 64 insertions, 0 deletions
diff --git a/subx/apps/factorial.subx b/subx/apps/factorial.subx new file mode 100644 index 00000000..de9953bf --- /dev/null +++ b/subx/apps/factorial.subx @@ -0,0 +1,64 @@ +## 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 + +== 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 $factorial:exit + # EBX: n-1 + 89/copy 3/mod/direct 3/rm32/EBX . . . 2/r32/EDX . . # copy EDX to EBX + 81 5/subop/subtract 3/mod/direct 3/rm32/EBX . . . . . 1/imm32 # subtract 1 from EBX + # prepare call + 55/push . . . . . . . . # push EBP + 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP + # EAX: factorial(n-1) + 53/push . . . . . . . . # push EBX + e8/call . . . . . . factorial/disp32 + # discard arg + 5e/pop . . . . . . . . # pop into ESI + # clean up after call + 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP + 5d/pop . . . . . . . . # pop to EBP + # refresh n + 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 2/r32/EDX 4/disp8 . # copy *(ESP+4) to EDX + # return n * factorial(n-1) + 0f af/multiply 3/mod/direct 2/rm32/EDX . . . 0/r32/EAX . . # multiply EDX (n) into EAX (factorial(n-1)) + # TODO: check for overflow +$factorial:exit: + c3/return + +# vim:ft=subx:nowrap:so=0 |