## compute the factorial of 5, and return the result in the exit code
#
# To run:
#   $ ./subx translate init.linux 0*.subx 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'.

== 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

Entry:  # run tests if necessary, compute `factorial(5)` if not
    # . prologue
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp

    # initialize heap
    # . Heap = new-segment(Heap-size)
    # . . push args
    68/push  Heap/imm32
    ff          6/subop/push        0/mod/indirect  5/rm32/.disp32            .             .           .           Heap-size/disp32                  # push *Heap-size
    # . . call
    e8/call  new-segment/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp

    # - if argc > 1 and argv[1] == "test", then return run_tests()
    # if (argc <= 1) goto run-main
    81          7/subop/compare     1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0/disp8         1/imm32           # compare *ebp
    7e/jump-if-<=  $run-main/disp8
    # if (!kernel-string-equal?(argv[1], "test")) goto run-main
    # . eax = kernel-string-equal?(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
    # . if (eax == false) goto run-main
    3d/compare-eax-and  0/imm32/false
    74/jump-if-=  $run-main/disp8
    # run-tests()
    e8/call  run-tests/disp32
    # syscall(exit, *Num-test-failures)
    8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           3/r32/ebx   Num-test-failures/disp32          # copy *Num-test-failures to ebx
    eb/jump  $main:end/disp8
$run-main:
    # - otherwise return factorial(5)
    # eax = 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
    # syscall(exit, eax)
    89/copy                         3/mod/direct    3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to ebx
$main:end:
    b8/copy-to-eax  1/imm32/exit
    cd/syscall  0x80/imm8

factorial:  # n : int -> int/eax
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    53/push-ebx
    # if (n <= 1) return 1
    b8/copy-to-eax  1/imm32
    81          7/subop/compare     1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         1/imm32           # compare *(ebp+8)
    7e/jump-if-<=  $factorial:end/disp8
    # var ebx : int = n-1
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .                         3/r32/ebx   8/disp8         .                 # copy *(ebp+8) to ebx
    4b/decrement-ebx
    # var eax : int = 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:
    # . epilogue
    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