Printing the x char in a string (MIPS)

问题:

My program is supposed to do the following: -Getting continiously an integer from the user (x), -printing the character at the x position in the string. -The program exits when the user inputs 0.

.text           
.globl __start  
__start:
    li $s3,20 #string length

start:      li $v0,5 
    syscall
    move $s0,$a0 #integer now in $a0
    beq $s0,$zero,exit

    li $s1,0 #counter is 0
    la $s2,str #address of string now is $s2

loop:lbu $t1,0($s2) #choosing char of string
    addi $s1,1 #increment counter by 1
    addi $s2,1 #next char
    beq $s1,$s0,print  #is the char at the position we entered?
    j loop

print:      lbu $a0,0($t1) #<------------#
    li $v0,11
    syscall
    j start

exit:       li $v0,10
    syscall     



.data
str: .asciiz "abcdefghijklmnopqrst"

I keep getting: "Exception occured at PC=0x00400034" and "Bad address in data stack read: 0x..." exactly when i try to run the line i marked.

回答1:

$t1 does not contain a valid address at the point where you do lbu $a0,0($t1). What you've got in $t1 there is the last character read from the string before you exited your loop loop.

I really don't see what the point of the loop is. You say that you have a string and and integer X, and you want to print the character at offset X in the string. So just read that character and you're done:

la $a1,string
addu $a1,$a1,$s0   # $a1 = &str[x].  assumes x is in $s0
lbu $a0,($a1)      # read the character
li $v0,11
syscall            # and print it

回答2:

.data
String: .space 1000
StringSize: .word  250
Msg: .asciiz "String length is: "

.text       
.globl main 
main:   
lw $a1, StringSize
la $a0, String                    # a0 points to the string
li $v0, 8
syscall

add $t2, $a0, $zero               # t2 points to the string
add $t1, $zero, $zero             # t1 holds the count
addi $t3, $zero, 10

LoopString: lb $t0, 0($t2)        # get a byte from string
beq $t0, $zero,  EndLoopString    # zero means end of string
beq $t0, $t3, Pointer             # remove newline (linefeed)
addi $t1,$t1, 1                   # increment count

Pointer:    addi $t2,$t2, 1       # move pointer one character
        j LoopString              # go round the loop again
EndLoopString:

la $a0, Msg                       # system call to print
add, $v0, $zero, 4                # out a message
syscall

add $a0, $t1, $zero               # system call to print
add, $v0, $zero, 1                # out the length worked out
syscall     

add, $v0, $zero, 10
syscall                           # good byeee :) ...