1 ## compute the factorial of 5, and return the result in the exit code
  2 #
  3 # To run (from the subx directory):
  4 #   $ subx translate apps/factorial.subx -o apps/factorial
  5 #   $ subx run apps/factorial
  6 # Expected result:
  7 #   $ echo $?
  8 #   120
  9 #
 10 # You can also run the automated test suite:
 11 #   $ subx run apps/factorial test
 12 # Expected output:
 13 #   ........
 14 # Every '.' indicates a passing test. Failing tests get a 'F'.
 15 # When running tests the exit status doesn't mean anything. Yet.
 16 
 17 == code
 18 # instruction                     effective address                                                   operand     displacement    immediate
 19 # op          subop               mod             rm32          base        index         scale       r32
 20 # 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
 21 
 22 # main:
 23   # prolog
 24   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 25   # if (argc > 1)
 26   81          7/subop/compare     1/mod/*+disp8   4/rm32/SIB    5/base/EBP  4/index/none  .           .           0/disp8         1/imm32           # compare *EBP
 27   7e/jump-if-lesser-or-equal  $run_main/disp8
 28   # and if (argv[1] == "test")
 29     # push args
 30   68/push  "test"/imm32
 31   ff          6/subop/push        1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           0x8/disp8       .                 # push *(EBP+8)
 32     # call
 33   e8/call  kernel_string_equal/disp32
 34     # discard args
 35   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
 36     # check result
 37   3d/compare-EAX  1/imm32
 38   75/jump-if-not-equal  $run_main/disp8
 39   # then
 40   e8/call  run_tests/disp32
 41   eb/jump  $main_exit/disp8
 42   # else EAX <- factorial(5)
 43 $run_main:
 44     # push arg
 45   68/push  5/imm32
 46     # EAX <- call
 47   e8/call  factorial/disp32
 48     # discard arg
 49   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
 50 $main_exit:
 51   # exit(EAX)
 52   89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
 53   b8/copy                         .               .             .           .             .           .           .               1/imm32           # copy to EAX
 54   cd/syscall  0x80/imm8
 55 
 56 # factorial(n)
 57 factorial:
 58   # prolog
 59   55/push-EBP
 60   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 61   53/push-EBX
 62   # initialize EAX to 1 (base case)
 63   b8/copy                         .               .             .           .             .           .           .               1/imm32           # copy to EAX
 64   # if (n <= 1) jump exit
 65   81          7/subop/compare     1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           8/disp8         1/imm32           # compare *(EBP+8)
 66   7e/jump-if-<=  $factorial:exit/disp8
 67   # EBX: n-1
 68   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none              3/r32/EBX   8/disp8         .                 # copy *(EBP+8) to EBX
 69   81          5/subop/subtract    3/mod/direct    3/rm32/EBX    .           .             .           .           .               1/imm32           # subtract from EBX
 70   # EAX: factorial(n-1)
 71   53/push-EBX
 72   e8/call                         .               .             .           .             .           .           factorial/disp32
 73   # discard arg
 74   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
 75   # return n * factorial(n-1)
 76   f7          4/subop/multiply    1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none                          8/disp8         .                 # multiply *(EBP+8) into EAX
 77   # TODO: check for overflow
 78 $factorial:exit:
 79   # epilog
 80   5b/pop-to-EBX
 81   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
 82   5d/pop-to-EBP
 83   c3/return
 84 
 85 test_factorial:
 86   # factorial(5)
 87     # push arg
 88   68/push  5/imm32
 89     # call
 90   e8/call  factorial/disp32
 91     # discard arg
 92   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
 93   # check_ints_equal(EAX, 120, failure message)
 94     # push args
 95   68/push  "F - test_factorial"/imm32
 96   68/push  0x78/imm32/expected-120
 97   50/push-EAX
 98     # call
 99   e8/call  check_ints_equal/disp32
100     # discard args
101   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
102   # end
103   c3/return
104 
105 # vim:ft=subx:nowrap:so=0