Having trouble with mips program

so i have resolved a few issues since my last post, but my string encryption program is still giving me some trouble, if anyone could help, it would be very much appreciated.

here is what i have so far (with some extraneous code ommited)
the line where MARS detects the exception is marked with a frown emoticon since that will make it easy to find the line. I am getting an arithmatic overflow runtime error. Also its hard to make everything allign on here the way i need it to but i hope it is easy enough to read.



# logic:
#
# 1.) get the input file name from the user
# 2.) open the input stream using the file name
# 3.) read in characters from the file until an end line character is encountered
# 4.) close the file
# 5.) send the string to the encryptoin subprogram
# 6.) store the result from encryption subprogram
# 7.) get the output file name from user
# 8.) open the output file to write.
# 9.) append an end line character to the end of the output buffer
# 10.) determine the lenght of the output buffer
# 11.) write to the output file using the buffer address and length
# 12.) close the output file stream
# 13.) EOP
#
# register assignment:
# $s1: value for x
# $s2: value for b
#
#
#
############################################################

.text
.globl main
main:

li $v0, 4 # prompt user for file name
la $a0, fileInPrompt
syscall

li $v0, 8 # input a string
la $a0, fileIn # address of file name
li $a1, 21 # max of 21 chars for file name
syscall

la $a0, fileIn # echo file name
li $v0, 4
syscall

li $a0, 1
li $v0, 1
syscall

jal nameCleanIn # send the file name to name clean procedure to append proper terminating character

jal fileRead # send the file to the file read program to begin reading the file

li $v0, 4 # new line carrage return
la $a0, endl
syscall

li $v0, 4
la $a0, inform # to inform the user how their string will be encrypted
syscall

li $v0, 4 # prompt the user for a value for X
la $a0, promptX
syscall

li $v0, 5 # input an integer value
syscall
move $s1, $a0 # move the value of $a0 to $t0

li $v0, 4 # prompt the user for a value for B
la $a0, promptB
syscall

li $v0, 5
syscall
move $s2, $a0

jal Encrypt

la $a0, result
li $v0, 4
syscall

EOP:
li $v0 10
syscall

############################ name clean for input ###############################
# cleans the file name input by the user and replaces the \n terminating
# character with a null terminating character so it can be properly
# opened
# yes i know this procedure is very innefficient but my first concernt right now is to just make it work
# Logic
# 1) initiate a loop counter
# 2) initiate a max size
# 3) End the loop when counter reaches max
# 4) load a character from the string
# 5) if the end line character is not reached, repeat the loop
# 6) if it is found, replace it with the null terminator
# 7) loop repeat, increment counter
# 8) return to calling program,, same process for out file
######################################################################
nameCleanIn:
li $t0, 0 # loop counter
li $t1, 21 # loop end at max file name length
clean:
beq $t0, $t1, endClean # end of loop
lb $t3, fileIn($t0) # load a byte from the string
bne $t3, 0x0a, nextLoop # if the \n terminating char is not found, repeat the loop
sb $zero, fileIn($t0) # terminating char is found, replace it with a zero terminator
nextLoop:
addi $t0, $t0, 1 # increment the counter
j clean # repeat the loop

endClean:
jr $ra
#clean ouput file
nameCleanOut:
li $t0, 0 # loop counter
li $t1, 21 # loop end at max file name length
clean2:
beq $t0, $t1, endClean2 # end of loop
lb $t3, fileOut($t0) # load a byte from the string
bne $t3, 0x0a, nextLoop2 # if the \n terminating char is not found, repeat the loop
sb $zero, fileOut($t0) # terminating char is found, replace it with a zero terminator
nextLoop2:
addi $t0, $t0, 1 # increment the counter
j clean2 # repeat the loop

endClean2:
jr $ra
#####################################################################

######################### file read #################################
# opens the file, reads it to the buffer, and closes it
######################################################################

fileRead:
li $v0, 13 # system call for open file
la $a0, fileIn # input file name
li $a1, 0 # flag for reading
li $a2, 0 # mode is ignored
syscall # open the input file(file descriptor is returned)
bltz $v0, errorOpen # negative number shows an open error. jump to error
move $s0, $v0 # save the file desscriptor


li $v0, 14 # system call to read from file
move $a0, $s0 # move the file descriptor into $a0
la $a1, stringBuff # a1 contains address of buffer
li $a2, 1000 # max read of 1000 chars
syscall # read from file
bltz $v0, errorRead # negative return shows a read error. jumpt to error

li $v0, 16 # system call to close file
move $a0, $s0 # move descriptor back into a0
syscall # close the file
bltz $v0, errorClose # negative number shows error closing file jump to error

la $a0, stringBuff # echo the file contents
li $v0, 4
syscall

jr $ra # return to procedure call
########################## Exceptions
errorOpen:
la $a0, error1 # show error opening file
li $v0, 4
syscall

j EOP # end the program

errorRead:
la $a0, error2 # show error reading ifle
li $v0, 4
syscall

j EOP # end the program

errorClose:
la $a0, error3 # show error closing the file
li $v0, 4
syscall

j EOP # end the program

errorWrite:
move $a0, $v0 # show error writing backto the file
li $v0, 1
syscall
la $a0, error4
li $v0, 4
syscall

j EOP
#############################################################################


####################################### Encrypt ############################
# this procedure will take the first string apart one character(byte)
# at a time. the result will then be written to a file for decrypt to decode
# when it is loaded
# $t0: value for x
# $t1: value for b
# $t3: current char (a)
# $t4: result char (y)
# $t5: address of the input buffer string
# $t6: address of the result string
# $t7: temporary value

##############################################################################
Encrypt:
la $t5, stringBuff # load the address of the string to t1
la $t6, result

nextChar:
lb $t3, ($t5) # load a byte off the string
beqz $t5, endEncrypt # a end on null terminator
mul $t7, $t3, $s1 # y = ax
add $t4, $t7, $s2 # y = ax + b ARITHMATIC OVERFLOW EXCEPTION OCCURS HERE
rem $t4, $t7, 127 # find the remainder for encryption to make sure that the byte to be stored will be an ascii value
sb $t4, ($t6) # store the character back
add $t6, $t6, 1 # next position in result
add $t5, $t5, 1 # next position in buffer

add $t5, $t5, 1 # add next byte

j nextChar # repeat the loop


endEncrypt:
jr $ra


.data
promptX: .asciiz "Give a value for x: "
promptB: .asciiz "Give a value for b: "
inform: .asciiz "Your string will be encrypted with this formula; y = ax + b\n where y is the resulting character, and a is the starting character"
fileInPrompt: .asciiz "Input your file name: "
fileOutPrompt: .asciiz "Input the file name for output: "
fileIn: .space 22
fileOut: .space 22
error1: .asciiz "\nERROR OPENING FILE\n"
error2: .asciiz "\nERROR READING FILE\n"
error3: .asciiz "\nERROR CLOSING FILE\n"
error4: .asciiz "\nERROR WRITING FILE\n"
stringBuff: .space 1000
endl: .asciiz "\n"
result: .space 1000