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 return run-tests()
 40   e8/call  run-tests/disp32
 41   8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           0/r32/EAX   Num-test-failures/disp32          # copy *Num-test-failures to EAX
 42   eb/jump  $main:end/disp8  # where EAX will get copied to EBX
 43   # else EAX = factorial(5)
 44 $run-main:
 45     # push args
 46   68/push  5/imm32
 47     # call
 48   e8/call  factorial/disp32
 49     # discard args
 50   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
 51 $main:end:
 52   # exit(EAX)
 53   89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
 54   b8/copy-to-EAX  1/imm32
 55   cd/syscall  0x80/imm8
 56 
 57 # factorial(n)
 58 factorial:
 59   # prolog
 60   55/push-EBP
 61   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 62   53/push-EBX
 63   # initialize EAX to 1 (base case)
 64   b8/copy-to-EAX  1/imm32
 65   # if (n <= 1) jump exit
 66   81          7/subop/compare     1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           .           8/disp8         1/imm32           # compare *(EBP+8)
 67   7e/jump-if-<=  $factorial:end/disp8
 68   # EBX = n-1
 69   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none              3/r32/EBX   8/disp8         .                 # copy *(EBP+8) to EBX
 70   81          5/subop/subtract    3/mod/direct    3/rm32/EBX    .           .             .           .           .               1/imm32           # subtract from EBX
 71   # EAX = factorial(n-1)
 72     # push args
 73   53/push-EBX
 74     # call
 75   e8/call  factorial/disp32
 76     # discard args
 77   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
 78   # return n * factorial(n-1)
 79   f7          4/subop/multiply    1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none                          8/disp8         .                 # multiply *(EBP+8) into EAX
 80   # TODO: check for overflow
 81 $factorial:end:
 82   # epilog
 83   5b/pop-to-EBX
 84   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
 85   5d/pop-to-EBP
 86   c3/return
 87 
 88 test-factorial:
 89   # factorial(5)
 90     # push args
 91   68/push  5/imm32
 92     # call
 93   e8/call  factorial/disp32
 94     # discard args
 95   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
 96   # check-ints-equal(EAX, 120, failure message)
 97     # push args
 98   68/push  "F - test-factorial"/imm32
 99   68/push  0x78/imm32/expected-120
100   50/push-EAX
101     # call
102   e8/call  check-ints-equal/disp32
103     # discard args
104   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
105   # end
106   c3/return
107 
108 # vim:nowrap:textwidth=0