## compute the factorial of 5, and return the result in the exit code
#
# To run:
#   $ subx translate ex7.subx ex7
#   $ subx run ex7
# Expected result:
#   $ echo $?
#   120

== 0x08048054  # code segment, after leaving room for ELF header
# 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:
  # prepare to make a call
  55/push                         .               .             .           .             .           .           .               .                 # push EBP
  89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
  # factorial(5)
  68/push                         .               .             .           .             .           .           .               5/imm32           # push 5
  e8/call                         .               .             .           .             .           .           factorial/disp32
  # discard arg
  5a/pop                          .               .             .           .             .           .           .               .                 # pop into EDX
  # clean up after call
  89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
  5d/pop                          .               .             .           .             .           .           .               .                 # pop to EBP

  # exit(EAX)
  89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
  b8/copy                         .               .             .           .             .           .           .               1/imm32           # copy 1 to EAX
  cd/syscall                      .               .             .           .             .           .           .               0x80/imm8         # int 80h

# factorial(n)
factorial:
  # initialize n
  8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              2/r32/EDX   4/disp8         .                 # copy *(ESP+4) to EDX
  # initialize EAX to 1 (base case)
  b8/copy                         .               .             .           .             .           .           .               1/imm32           # copy 1 to EAX
  # if (n <= 1) jump exit
  81          7/subop/compare     3/mod/direct    2/rm32/EDX    .           .             .           .           .               1/imm32           # compare EDX with 1
  7e/jump-if-<=                   .               .             .           .             .           .           $exit/disp8                       # jump if <= to $exit
  # EBX: n-1
  89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           2/r32/EDX   .               .                 # copy EDX to EBX
  81          5/subop/subtract    3/mod/direct    3/rm32/EBX    .           .             .           .           .               1/imm32           # subtract 1 from EBX
  # prepare call
  55/push                         .               .             .           .             .           .           .               .                 # push EBP
  89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
  # EAX: factorial(n-1)
  53/push                         .               .             .           .             .           .           .               .                 # push EBX
  e8/call                         .               .             .           .             .           .           factorial/disp32
  # discard arg
  5e/pop                          .               .             .           .             .           .           .               .                 # pop into ESI
  # clean up after call
  89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
  5d/pop                          .               .             .           .             .           .           .               .                 # pop to EBP
  # refresh n
  8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              2/r32/EDX   4/disp8         .                 # copy *(ESP+4) to EDX
  # return n * factorial(n-1)
  0f af/multiply                  3/mod/direct    2/rm32/EDX    .           .             .           0/r32/EAX   .               .                 # multiply EDX (n) into EAX (factorial(n-1))
  # TODO: check for overflow
$exit:
  c3/return

# vim:ft=subx:nowrap:so=0