Computer Organization HW1

MIPS assembly code: (copied from textbook)
( n corresponds to the argument register $a0 )
fact:	addi	$sp,$sp,-8	# adjust stack for 2 items
	sw	$ra,4($sp)	# save the return address
	sw	$a0,0($sp)	# save the argument n
	slti	$t0,$a0,1	# test for n < 1
	beq	$t0,$zero,L1	# if n >= 1, go to L1.
	addi	$v0,$zero,1	# return 1
	addi	$sp,$sp,8	# delete 0 and address
	jr	$ra	# return to after jal of L1
L1:	addi	$a0,$a0,-1	# n >= 1: argument gets (n - 1)
	jal	fact	# call fact with (n - 1)
	lw	$a0,0($sp)	# return from jal:restore argument n
	lw	$ra,4($sp)	# restore the return address
	addi	$sp,$sp,8	# adjust stack pointer to pop 2 items
	mul	$v0,$a0,$v0	# return n * fact(n-1)
	jr	$ra	# return to the caller
trace: n = 3
fact:	addi	$sp,$sp,-8	# adjust stack for 2 items
	sw	$ra,4($sp)	# save the return address in stack
	sw	$a0,0($sp)	# save the argument n = 3 in stack
	slti	$t0,$a0,1	# test if n < 1
	beq	$t0,$zero,L1	# if n >= 1, go to L1. Now n = 3, so go to L1
L1:	addi	$a0,$a0,-1	# n = 3 - 1 = 2
	jal	fact	# call fact with n = 2
fact:	addi	$sp,$sp,-8	# adjust stack for 2 items
	sw	$ra,4($sp)	# save the return address in stack
	sw	$a0,0($sp)	# save the argument n = 2 in stack
	slti	$t0,$a0,1	# test if n < 1
	beq	$t0,$zero,L1	# if n >= 1, go to L1. Now n = 2, so go to L1
L1:	addi	$a0,$a0,-1	# n = 2 - 1 = 1
	jal	fact	# call fact with n = 1
fact:	addi	$sp,$sp,-8	# adjust stack for 2 items
	sw	$ra,4($sp)	# save the return address in stack
	sw	$a0,0($sp)	# save the argument n = 1 in stack
	slti	$t0,$a0,1	# test if n < 1
	beq	$t0,$zero,L1	# if n >= 1, go to L1. Now n = 1, so go to L1
L1:	addi	$a0,$a0,-1	# n = 1 - 1 = 0
	jal	fact	# call fact with n = 0
fact:	addi	$sp,$sp,-8	# adjust stack for 2 items
	sw	$ra,4($sp)	# save the return address in stack
	sw	$a0,0($sp)	# save the argument n = 0 in stack
	slti	$t0,$a0,1	# test if n < 1
	beq	$t0,$zero,L1	# if n >= 1, goto L1. n = 0,so goto next line
	addi	$v0,$zero,1	# return 1
	addi	$sp,$sp,8	# pop 2 items(0 and return address) of stack
	jr	$ra	# return to the instruction after jal
	lw	$a0,0($sp)	# returned from jal. restore argument 1
	lw	$ra,4($sp)	# restore the return address
	addi	$sp,$sp,8	# pop 2 items(1 and return address) of stack
	mul	$v0,$a0,$v0	# return n*fact(n-1) = 1*fact(0) = 1 * 1 = 1
	jr	$ra	# return to the instruction after jal
	lw	$a0,0($sp)	# returned from jal. restore argument 2
	lw	$ra,4($sp)	# restore the return address
	addi	$sp,$sp,8	# pop 2 items(2 and return address) of stack
	mul	$v0,$a0,$v0	# return n*fact(n-1) = 2*fact(1) = 2 * 1 = 2
	jr	$ra	# return to the instruction after jal
	lw	$a0,0($sp)	# returned from jal. restore argument 3
	lw	$ra,4($sp)	# restore the return address
	addi	$sp,$sp,8	# pop 2 items(3 and return address) of stack
	mul	$v0,$a0,$v0	# return n*fact(n-1) = 3*fact(2) = 3 * 2 = 6
	jr	$ra	# return to the caller