path: root/linux/apps/factorial4.subx

## compute the factorial of 5, and return the result in the exit code
#
# Uses syntax sugar for:
#   rm32 operands
#   function calls
#   control flow
#
# To run:
#   $ ./translate_subx init.linux [01]*.subx apps/factorial4.subx
#   $ ./a.elf
# Expected result:
#   $ echo $?
#   120
#
# You can also run the automated test suite:
#   $ ./a.elf test
# Expected output:
#   ........
# Every '.' indicates a passing test. Failing tests get a 'F'.
#
# Compare factorial3.subx

== code

factorial:  # n: int -> _/eax: int
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # save registers
    51/push-ecx
    # if (n <= 1) return 1
    81 7/subop/compare *(ebp+8) 1/imm32
    {
      7f/jump-if-> break/disp8
      b8/copy-to-eax 1/imm32
      eb/jump $factorial:end/disp8
    }
    # n > 1; return n * factorial(n-1)
    8b/-> *(ebp+8) 1/r32/ecx
    49/decrement-ecx
    (factorial %ecx)  # => eax
    f7 4/subop/multiply-into-eax *(ebp+8)
    # TODO: check for overflow
$factorial:end:
    # restore registers
    59/pop-to-ecx
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

test-factorial:
    (factorial 5)
    (check-ints-equal %eax 0x78 "F - test-factorial")
    c3/return

Entry:  # run tests if necessary, compute `factorial(5)` if not
    # . prologue
    89/<- %ebp 4/r32/esp

    # initialize heap (needed by tests elsewhere)
    (new-segment *Heap-size Heap)

    # if (argc <= 1) return factorial(5)
    {
      # if (argc > 1) break
      81 7/subop/compare *ebp 1/imm32
      7f/jump-if-> break/disp8
      # ebx = factorial(5)
      (factorial 5)  # => eax
      89/<- %ebx 0/r32/eax
      eb/jump $main:end/disp8
    }
    # otherwise if first arg is "test", then return run_tests()
    {
      # if (!kernel-string-equal?(argv[1], "test")) break
      (kernel-string-equal? *(ebp+8) "test")  # => eax
      3d/compare-eax-and 0/imm32/false
      74/jump-if-= break/disp8
      #
      (run-tests)
      # exit(*Num-test-failures)
      8b/-> *Num-test-failures 3/r32/ebx
      eb/jump $main:end/disp8
    }
    bb/copy-to-ebx 0/imm32
$main:end:
    e8/call  syscall_exit/disp32