我有一个任务，建立一个计算器在mips，我需要从用户那里得到符号，并使用这个操作数进行计算。我把操作数保存在数据中，我想让他们比较。我不知道我做错了什么，但这是行不通的。

.data

IfStart: .asciiz "Start Calculator? (0: no, 1: yes):"
msg: .asciiz "Enter operation symbol (+, -, *) or ‘!’ to quit:"
mulb: .asciiz "*"
addb: .asciiz "+"
subb: .asciiz "-"
stopsign: .asciiz "!"
newline: .asciiz "\n"
pick1: .asciiz "Enter first number (A):"
pick2: .asciiz "Enter second number (B):"
badnumber: .asciiz "Bad input\n"
bye: .asciiz "Good bye!"
one: .word 1
result: .asciiz "The result is:"
buffer: .space 20

.text

main:

li $v0,4 
la $a0,IfStart
syscall

li $v0,5 #get number from user
syscall 

move $t0,$v0

lw $t1, one

beq $t1,$t0,if1
beq $zero,$t0,if0

li $v0,4 
la $a0,badnumber
syscall

li $v0,4 
la $a0,bye
syscall

li $v0,10
syscall

if1: 
jal Calculator

if0:
li $v0,4 
la $a0,bye
syscall

li $v0,10
syscall


Calculator:

li $v0,4 #print first message
la $a0,msg
syscall

li $v0,8 #take in input
la $a0, buffer #load byte space into address
li $a1, 20 # allot the byte space for string
move $t2,$a0 #save string t2
syscall

li $v0,5 #Read int A
syscall 

move $t0,$v0 #t0 is A

li $v0,5 #Read int B
syscall 

move $t1,$v0 #t1 is B

lw $t3, mulb #mul sign
lw $t6, subb #sub sign
lw $t4, stopsign
lw $t5, addb #add sign



beq $t3,$a0,stop
beq $t4,$a0,multp
beq $t5,$a0,adds
beq $t6,$a0,subbs

multp:
jal Multiply
j print

adds:
jal Sum
j print

subbs:
jal Substract

print:
li $v0,4 #print the result is:
la $a0,result
syscall

#print result
li $v0, 1
move $a0, $t3
syscall

li $v0,4 #print new line
la $a0,newline
syscall

jal Calculator
stop:
li $v0,10
syscall

在执行下列其中一个数据列之后：

lw $t4, stopsign  
lw $t5, addb #add sign

李世民总是不停地写信给我这个错误：
Runtime exception at 0x004000b8: fetch address not aligned on word boundary 0x1001005a
但类似的行：

lw $t3, mulb #mul sign  
lw $t6, subb #sub sign

完全没有问题。是字符做的问题（！/+）吗？
谢谢你的帮助！