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## compute the factorial of 5, and return the result in the exit code
#
# To run (from the subx directory):
# $ subx translate apps/factorial.subx -o apps/factorial
# $ subx run apps/factorial
# Expected result:
# $ echo $?
# 120
#
# You can also run the automated test suite:
# $ subx run apps/factorial test
# Expected output:
# ........
# Every '.' indicates a passing test. Failing tests get a 'F'.
# When running tests the exit status doesn't mean anything. Yet.
== 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:
# prolog
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
# if (argc > 1)
81 7/subop/compare 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 0/disp8 1/imm32 # compare *EBP
7e/jump-if-lesser-or-equal $run_main/disp8
# and if (argv[1] == "test")
# push args
68/push "test"/imm32
ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 0x8/disp8 . # push *(EBP+8)
# call
e8/call kernel_string_equal/disp32
# discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check result
3d/compare-EAX 1/imm32
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
# call
e8/call factorial/disp32
# discard arg
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add 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 to EAX
cd/syscall 0x80/imm8
# factorial(n)
factorial:
# prolog
55/push-EBP
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
53/push-EBX
# initialize EAX to 1 (base case)
b8/copy . . . . . . . 1/imm32 # copy to EAX
# if (n <= 1) jump exit
81 7/subop/compare 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 8/disp8 1/imm32 # compare *(EBP+8)
7e/jump-if-<= $factorial:exit/disp8
# EBX: n-1
8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none 3/r32/EBX 8/disp8 . # copy *(EBP+8) to EBX
81 5/subop/subtract 3/mod/direct 3/rm32/EBX . . . . . 1/imm32 # subtract from EBX
# EAX = factorial(n-1)
# push args
53/push-EBX
# call
e8/call . . . . . . factorial/disp32
# discard arg
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
# return n * factorial(n-1)
f7 4/subop/multiply 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none 8/disp8 . # multiply *(EBP+8) into EAX
# TODO: check for overflow
$factorial:exit:
# epilog
5b/pop-to-EBX
89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
5d/pop-to-EBP
c3/return
test_factorial:
# factorial(5)
# push arg
68/push 5/imm32
# call
e8/call factorial/disp32
# discard arg
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
# check_ints_equal(EAX, 120, failure message)
# push args
68/push "F - test_factorial"/imm32
68/push 0x78/imm32/expected-120
50/push-EAX
# call
e8/call check_ints_equal/disp32
# discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
# end
c3/return
# vim:nowrap:textwidth=0
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