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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 register 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 and argv[1] == "test" then return run_tests()
# . argc > 1
81 7/subop/compare 1/mod/*+disp8 5/rm32/EBP . . . . 0/disp8 1/imm32 # compare *EBP
7e/jump-if-lesser-or-equal $run-main/disp8
# . argv[1] == "test"
# . . push args
68/push "test"/imm32
ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/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
# . run-tests()
e8/call run-tests/disp32
8b/copy 0/mod/indirect 5/rm32/.disp32 . . 0/r32/EAX Num-test-failures/disp32 # copy *Num-test-failures to EAX
eb/jump $main:end/disp8 # where EAX will get copied to EBX
# - otherwise return factorial(5)
$run-main:
# . . push args
68/push 5/imm32
# . . call
e8/call factorial/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
$main:end:
# syscall(exit, EAX)
89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX
b8/copy-to-EAX 1/imm32/exit
cd/syscall 0x80/imm8
factorial: # n : int -> int/EAX
# . prolog
55/push-EBP
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
53/push-EBX
# EAX = 1 (base case)
b8/copy-to-EAX 1/imm32
# if (n <= 1) return
81 7/subop/compare 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 1/imm32 # compare *(EBP+8)
7e/jump-if-<= $factorial:end/disp8
# EBX = n-1
8b/copy 1/mod/*+disp8 5/rm32/EBP . . 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 args
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 5/rm32/EBP . . 8/disp8 . # multiply *(EBP+8) into EAX
# TODO: check for overflow
$factorial:end:
# . 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 args
68/push 5/imm32
# . . call
e8/call factorial/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
# check-ints-equal(EAX, 120, msg)
# . . 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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