## long mult -- multiplies two 32-bit integers
## Register usage: 
##  This program accomplishes the same thing as mul or mult
##  But this shows the inner workings.

## $t1 ­ mask 
## $t2 ­ multiplier (and right_hand_sum). 
## $t3 ­ left_hand_sum. 
## $t4 ­ multiplicand. 
## $t5 - clear.
## $t6 - rightmost bit of left_hand_sum.
## $t7 - rightmost bit of right_hand_sum.
## $t8 - counter

## $v0 ­ syscall parameter / return values. 
## $a0 ­ syscall parameters. 
## $a1 ­ syscall parameters. 

.text 
.globl __start
__start: 

## read the two integers
	li $v0, 5 		# load multiplicand. 
	syscall 
	move $t4, $v0

	li $v0, 5 		# load multiplier. 
	syscall 
	move $t2, $v0

	lw $t1, mask
	lw $t5, clear	
	
	li $t8, 32
continue:	
	
	beqz $t8, end


	and $t7, $t1, $t2
	beqz $t7, shift
	add $t3, $t3, $t4

shift:	and $t6, $t3, $t1	# store rightmost bit og l_sum in $t6
	and $t2, $t2, $t5	# clear rightmost bit of r_sum
	or $t2, $t2, $t6	# move stored right bit of l_sum to rightmost position of r_sum
	ror $t2, $t2, 1		# rotate r_sum
	srl $t3, $t3, 1		# shift l_sum right
				# Use sra for twos complement numbers
	addiu $t8, $t8, -1
	b continue

	
end:	sw $t2, r_sum
	sw $t3, l_sum

	li $v0, 10
	syscall
.data 
	r_sum: .word 0
	l_sum: .word 0

	mask:  .word 0x00000001
	clear: .word 0xFFFFFFFE