## 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

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