MIPS程序运行时地址异常问题求助

Hey there, let's break down why you're hitting that "Bad address" exception and fix it step by step:

Core Issue: Insufficient Stack Allocation #

The biggest problem is in your loop_over_strings function—you only allocate 4 bytes of stack space with addi $sp, $sp, -4, but then try to store three different registers ($ra at 0($sp), $a0 at 4($sp), $a1 at 8($sp)). Accessing 4($sp) and 8($sp) is out of bounds for the 4-byte stack you reserved, which corrupts the stack structure and leads to invalid memory accesses when you try to load values back later.

Other Problems to Fix #

• Invalid String Syntax: Your .asciiz line uses HTML-escaped quotes (") instead of regular double quotes ("). QtSpim won't parse this correctly, so your string address will be wrong.
• Missing ascii Function: You're jumping to ascii with jalr $a1, but there's no implementation for this function in your code. Without it, the jump will land at an invalid address and trigger another exception.
• Redundant .text Section: You have two .text declarations, which is unnecessary and can cause confusion for the assembler.

Fixed Code #

Here's the corrected version that should run without exceptions in QtSpim:

.data
STR_str: .asciiz "Cow, Bird, Beer"  # Fixed quote syntax
.text

# String traversal function with proper stack handling
.globl loop_over_strings
loop_over_strings:
    addi $sp, $sp, -12  # Allocate 12 bytes for 3 registers ($ra, $a0, $a1)
    sw $ra, 0($sp)
    sw $a1, 8($sp)      # Now safely within allocated stack space

loop:
    sw $a0, 4($sp)
    lb $t0, 0($a0)      # Load current character
    beqz $t0, end_loop  # Exit if we hit the null terminator

    jalr $a1            # Call the ascii function

    lw $a0, 4($sp)      # Restore $a0
    lw $a1, 8($sp)      # Restore $a1
    addi $a0, $a0, 1    # Move to next character
    j loop

end_loop:
    lw $ra, 0($sp)      # Restore return address
    addi $sp, $sp, 12   # Deallocate stack space
    jr $ra

# Main function
.globl main
main:
    addi $sp, $sp, -4
    sw $ra, 0($sp)

    la $a0, STR_str     # Load string address into $a0
    la $a1, ascii       # Load ascii function address into $a1
    jal loop_over_strings

    lw $ra, 0($sp)
    addi $sp, $sp, 4
    jr $ra

# Example implementation of the ascii function
# This prints each character from the string
ascii:
    li $v0, 11          # Syscall code for printing a character
    move $a0, $t0       # Pass the current character to the syscall
    syscall
    jr $ra

Key Takeaways #

• Always calculate exactly how much stack space you need: each register takes 4 bytes, so multiply the number of registers you're saving by 4.
• Match stack allocation and deallocation: if you subtract 12 from $sp at the start, add 12 back before returning.
• Make sure all functions you reference (like ascii) are implemented—undefined functions lead to invalid jumps.

You can use QtSpim's step-through debugging to watch the $sp register and confirm that all stack accesses are within the allocated space now.

内容的提问来源于stack exchange，提问作者John Smith