MIPS help

I've been working on a program which reads in a sequence of numbers entered by the user. It's read in as a string and then converted to numbers. Then I am suppose to get the sum of those numbers and report it back. I got most done and I know I'm close (hope!) but I am stuck at trying to get the number to sum up correctly. This is what I have so far:


##########################################################################

# CS 282 Winter 2008 Lab 2  -- Using Pointers

# Author: Pat Leiker

# Modified 19 Feb. 2008 6:15 pm

#

##########################################################################

#

# Functional Description

# 

# Prompt user to enter a list of integers

# Accepts any list of nonnegative integers entered by the user

# Echo the user input

# Print the sum of the integers

#

#########################################################################

#

#     Cross reference

#

#     $s0: Location in the string

#	  $s1: Summed value

#	  $s2: Current character value

#	  $s3: Temporary Value (for multiplying)

#	  $t1: ' ' character- ASCII 32

#	  $t2: current character pointer (c*)

#	  $t3: next character pointer (c+1)*

#     

###########################################################################


            .data

Prompt1:    .asciiz       "\nPlease enter a list of integers."

newline:	.asciiz		  "\n"

Buffer:		.space		  400


            .globl        main

            .text

main:                                   

		li	  	$v0, 4				# syscall for Print String

		la	  	$a0, Prompt1  		# load address of prompt into $a0

		syscall						# print the prompt


		li	  	$v0, 8		    	# syscall for Read String

		la 	  	$a0, Buffer

		li	  	$a1, 400

		syscall						# read the value of a into $v0

		la    	$s0, Buffer     	# $s0 <= Buffer


		li    	$v0, 4				# output entered string

		la	  	$a0, Buffer

		syscall


		li 		$v0, 4				# output a newline

		la 		$a0, newline

		syscall


		li 	  	$t1, 32         	# set $t1 to ' '

		move 	$s1, $zero			# set value to 0

		move	$s3, $zero			# set temp value to 0


while:

	    lb      $t2, 0($s0)       	# set p*

  		beq  	$t2, $zero, exit	# exit if p* is null

		move	$s2, $zero			# reset current character value

		addi	$s2, $t2, -48		# set current character value


		addi	$s4, $s0, 1			

		lb		$t3, 0($s4)			# set (p+1)*

		

		bne		$t3, $t1, multiply	# Go to multiply if the next character is not whitespace


		add		$s3, $s3, $t2		# temp value = temp value + character value


		add		$s1, $s1, $s3		# value = value + temp value	


		addi	$s0, $s0, 1			# increment pointer p*

		move	$s3, $zero			# reset temp value

		j		while				# go to beginning of while loop



multiply:

		

		sll		$t4, $s3, 3			# x = value multiplied by eight

		sll		$t5, $s3, 1			# y = value multiplied by two

		add		$s3, $t4, $t5		# value = x + y = value * 10


		add		$s3, $s3, $t2		# add the current character to the multiplied value


		addi	$s0, $s0, 1			# increment pointer p*


		j		while				# go to beginning of while loop



exit:

		li 		$v0, 1				# output the value for the sum

		move 	$a0, $s1 

		syscall


		li       $v0, 10           	# terminate run

        syscall                     # return to operating system

# note : the teacher said we are not allowed to use mutiplication yet because he hasn't taught us that yet so that is the cause for the shifting.

Any help is appreciated thanks!