example6_loop_with_function.asm

File contents

# Simple routine to demo a loop
# Compute the sum of N integers: 1 + 2 + 3 + ... + N
# Same result as example4, but here a function performs the
# addition operation:  int add(int num1, int num2)

# ------------------------------------------------------------------
	
	.text

	.globl	main
main:
	# Register assignments
	# $s0 = N
	# $s1 = counter (i)
	# $s2 = sum
	
	# Print msg1
	li	$v0,4		# print_string syscall code = 4
	la	$a0, msg1	
	syscall

	# Get N from user and save
	li	$v0,5		# read_int syscall code = 5
	syscall	
	move	$s0,$v0		# syscall results returned in $v0

	# Initialize registers
	li	$s1, 0		# Reg $s1 = counter (i)
	li	$s2, 0		# Reg $s2 = sum

	# Main loop body
loop:	addi	$s1, $s1, 1	# i = i + 1
	
	# Call add function
	move	$a0, $s2	# Argument 1: sum ($s2)
	move	$a1, $s1	# Argument 2: i ($s1)
	jal	add2		# Save current PC in $ra, and jump to add2
	move	$s2,$v0		# Return value saved in $v0. This is sum ($s2)
	beq	$s0, $s1, exit	# if i = N, continue
	j	loop

	# Exit routine - print msg2
exit:	li	$v0, 4		# print_string syscall code = 4
	la	$a0, msg2
	syscall

	# Print sum
	li	$v0,1		# print_string syscall code = 4
	move	$a0, $s2
	syscall

	# Print newline
	li	$v0,4		# print_string syscall code = 4
	la	$a0, lf
	syscall
	li	$v0,10		# exit
	syscall

# ------------------------------------------------------------------
	
	# FUNCTION: int add(int num1, int num2)
	# Arguments are stored in $a0 and $a1
	# Return value is stored in $v0
	# Return address is stored in $ra (put there by jal instruction)
	# Typical function operation is:
	#  1.) Store registers on the stack that we will overwrite
	#  2.) Run the function
	#  3.) Save the return value
	#  4.) Restore registers from the stack
	#  5.) Return (jump) to previous location
	# Note: This function is longer than it needs to be,
	# in order to demonstrate the usual 5 step function process...
	
add2:	# Store registers on the stack that we will overwrite (just $s0)
	addi $sp,$sp, -4	# Adjust stack pointer
	sw $s0,0($sp)		# Save $s0 on the stack

	# Run the function
	add $s0,$a0,$a1		# Sum = sum + i

	# Save the return value in $v0
	move $v0,$s0

	# Restore overwritten registers from the stack
	lw $s0,0($sp)
	addi $sp,$sp,4		# Adjust stack pointer
	
	# Return from function
	jr $ra			# Jump to addr stored in $ra
	
# ------------------------------------------------------------------
	
	# Start .data segment (data!)
	.data
msg1:	.asciiz	"Number of integers (N)?  "
msg2:	.asciiz	"Sum = "
lf:     .asciiz	"\n"