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## 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 $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
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