#
# bad_mult.asm
# Jeff Ondich, 1 April 2014
#
# MIPS has multiplication instructions built in, so there's no
# need to implement multiplication ourselves. That said, learning
# how subroutines are structured is worth writing something simple,
# and a bad implementation of multiplication fits the bill.
#
# Pay particular attention to the roles of the a-registers,
# v-registers, and t-registers, as well as the instructions
# jal and jr.
#
# Also:
# -- What happens if factor1 is negative?
# -- What happens if factor2 is negative?
# -- What happens if factor1 * factor2 >= 2^32 ?
#


.data
f1: .word 5
f2: .word 6

.text
# The main program
main:
      # Load factors into registers a0 and a1
      la   $a0, f1
      lw   $a0, 0($a0)
      la   $a1, f2
      lw   $a1, 0($a1)

      # Invoke the multiplication subroutine
      jal  mult

      # Move the product into a0 and print
      add  $a0, $zero, $v0
      li   $v0, 1
      syscall

      # Exit
      li   $v0, 10
      syscall

# The multiplication subroutine
mult: li   $t0, 0
      mult $a0, $a1
      mfhi $t6
      mflo $t7
      
loop: beqz $a0, end
      add  $t0, $t0, $a1
      addi $a0, $a0, -1
      b    loop
end:  add  $v0, $zero, $t0
      jr   $ra