# MIPS assembly program to calculate base ^ power recursively
#
# Purpose of registers:
# $s0 = base
# $s1 = exponent
# $s2 = result
# $s3 = counter variable 

	.text
main:
	li $s0, 3		# Let's compute 3^4 = 81.
	li $s1, 4
	li $s2, 1		# initially, set answer to 1.
	move $s3, $s1		# Set the counter = exponent.

	jal power

	li $v0, 1		# Now, print the answer
	move $a0, $s2	
	syscall

	li $v0, 10		# end of program
	syscall

#--------------------------------------------------------------------
power:
	sw $ra, 0($sp)		# save return address on system stack
	addi $sp, $sp, -4

if:
	blt $s3, 2, else	# if counter < 2, time to multiply
	addi $s3, $s3, -1	# otherwise, decrement counter and
	jal power		#   call power recursively

else:
	mul $s2, $s2, $s0	# multiply one power at a time

	addi $sp, $sp, 4	# Restore return address
	lw $ra, 0($sp)
	jr $ra			# return

#---------------------------------------------------------------------
# At the point where take the base case and begin to multiply,
#
#  PC = 0x00400064
# $ra = 0x00400060
# $sp = 0x7fffefec
#
# memory(0x7fffefec) = 0x00000000
# memory(0x7fffeff0) = 0x00400060 = address of "mul" instruction
# memory(0x7fffeff4) = 0x00400060
# memory(0x7fffeff8) = 0x00400060
# memory(0x7fffeffc) = 0x00400034 = address of "li $v0, 1"