https://github.com/akkartik/mu/blob/main/linux/factorial3.subx

     1 ## compute the factorial of 5, and return the result in the exit code
     2 #
     3 # Uses syntax sugar for:
     4 #   rm32 operands
     5 #   function calls
     6 #
     7 # To run:
     8 #   $ ./translate_subx init.linux [01]*.subx apps/factorial.subx -o apps/factorial
     9 #   $ bootstrap/bootstrap run factorial
    10 # Expected result:
    11 #   $ echo $?
    12 #   120
    13 #
    14 # You can also run the automated test suite:
    15 #   $ bootstrap/bootstrap run factorial test
    16 # Expected output:
    17 #   ........
    18 # Every '.' indicates a passing test. Failing tests get a 'F'.
    19 #
    20 # Compare apps/factorial2.subx
    21
    22 == code
    23
    24 factorial:  # n: int -> _/eax: int
    25     # . prologue
    26     55/push-ebp
    27     89/<- %ebp 4/r32/esp
    28     # save registers
    29     51/push-ecx
    30     # if (n <= 1) return 1
    31     b8/copy-to-eax 1/imm32
    32     81 7/subop/compare *(ebp+8) 1/imm32
    33     7e/jump-if-<= $factorial:end/disp8
    34     # n > 1; return n * factorial(n-1)
    35     8b/-> *(ebp+8) 1/r32/ecx
    36     49/decrement-ecx
    37     (factorial %ecx)  # => eax
    38     f7 4/subop/multiply-into-eax *(ebp+8)
    39     # TODO: check for overflow
    40 $factorial:end:
    41     # restore registers
    42     59/pop-to-ecx
    43     # . epilogue
    44     89/<- %esp 5/r32/ebp
    45     5d/pop-to-ebp
    46     c3/return
    47
    48 test-factorial:
    49     (factorial 5)
    50     (check-ints-equal %eax 0x78 "F - test-factorial")
    51     c3/return
    52
    53 Entry:  # run tests if necessary, compute `factorial(5)` if not
    54     # . prologue
    55     89/<- %ebp 4/r32/esp
    56
    57     # initialize heap (needed by tests elsewhere)
    58     (new-segment *Heap-size Heap)
    59
    60     # if (argc <= 1) return factorial(5)
    61     81 7/subop/compare *ebp 1/imm32
    62     7f/jump-if-> $main:run-tests/disp8
    63     (factorial 5)  # => eax
    64     89/<- %ebx 0/r32/eax
    65     eb/jump $main:end/disp8
    66 $main:run-tests:
    67     # otherwise if first arg is "test", then return run_tests()
    68     # if (!kernel-string-equal?(argv[1], "test")) goto do-nothing
    69     (kernel-string-equal? *(ebp+8) "test")  # => eax
    70     3d/compare-eax-and 0/imm32/false
    71     74/jump-if-= $main:do-nothing/disp8
    72     #
    73     (run-tests)
    74     # exit(*Num-test-failures)
    75     8b/-> *Num-test-failures 3/r32/ebx
    76     eb/jump $main:end/disp8
    77 $main:do-nothing:
    78     bb/copy-to-ebx 0/imm32
    79 $main:end:
    80     e8/call  syscall_exit/disp32