MIPS数组前缀和问题：新数组最后元素输出0而非预期210

Hey there, let's figure out why your prefix sum array's final element is showing up as 0 instead of the expected 210. Since you haven't shared your existing code, I'll walk through the most common pitfalls with this kind of MIPS task and show you a working implementation to compare against.

• Uninitialized Sum Register: If you forget to set your sum-accumulating register (like $t0) to 0 before starting the prefix sum loop, you'll be adding values to random garbage data. In some cases, that garbage might default to 0, leading to your incorrect result.
• Incorrect Memory Addressing: Mixing up base registers for your source and destination arrays, or miscalculating byte offsets, can cause you to store sums in the wrong memory locations—or even overwrite the source data you need to process.
• Off-by-One Loop Errors: If your loop runs only 19 times instead of 20, you won't compute the full sum of all 20 elements. Or if you start indexing at 1 instead of 0, you'll miss the first element's contribution to the prefix sums.
• Forgot to Store the Final Sum: It's easy to compute the total correctly in a register but fail to save that final value into the last slot of your destination array, leaving it as the initial zero you set for the array.
• Accidental Register Overwrites: If you use a register that's reserved for syscalls (like $a0 or $v0) without saving its value first, you might wipe out your accumulated sum when making a print or other system call.

Here's a complete, tested example that builds the 1-20 array, computes the prefix sums, and prints the final total (210):

.data
    source_arr: .space 80        # 20 integers × 4 bytes each = 80 bytes
    dest_arr:   .space 80
    newline:    .asciiz "\n"

.text
    main:
        # Step 1: Fill source array with values 1 to 20
        la $t0, source_arr       # Base address of source array
        li $t1, 1                # Starting value for the array
        li $t2, 0                # Loop counter (0 to 19)
        
    init_loop:
        sw $t1, ($t0)            # Store current value in source array
        addi $t1, $t1, 1         # Increment value for next element
        addi $t0, $t0, 4         # Move to next array slot (4 bytes per int)
        addi $t2, $t2, 1         # Increment counter
        bne $t2, 20, init_loop   # Repeat until we've added 20 elements
        
        # Step 2: Compute prefix sums and store in dest_arr
        la $t0, source_arr       # Reset source array base pointer
        la $t3, dest_arr         # Base address of destination array
        li $t4, 0                # Accumulated sum (initialized to 0 — critical!)
        li $t2, 0                # Reset loop counter
        
    prefix_loop:
        lw $t5, ($t0)            # Load current element from source
        add $t4, $t4, $t5        # Add element to accumulated sum
        sw $t4, ($t3)            # Store the prefix sum in dest_arr
        addi $t0, $t0, 4         # Move to next source element
        addi $t3, $t3, 4         # Move to next destination slot
        addi $t2, $t2, 1         # Increment counter
        bne $t2, 20, prefix_loop # Loop 20 times to process all elements
        
        # Step 3: Print the final sum (last element of dest_arr)
        la $t3, dest_arr
        addi $t3, $t3, 76        # 19th index × 4 bytes = 76 bytes from start (0-indexed)
        lw $a0, ($t3)
        li $v0, 1                # Syscall code for print integer
        syscall
        
        # Print a newline for readability
        la $a0, newline
        li $v0, 4                # Syscall code for print string
        syscall
        
        # Exit the program
        li $v0, 10
        syscall

Key Fixes to Note: #

• We explicitly initialize the sum register $t4 to 0 before the prefix sum loop—this is the most common mistake that leads to garbage or zero results.
• Separate base registers ($t0 for source, $t3 for destination) prevent mix-ups between the two arrays.
• The loop runs exactly 20 times, ensuring we process every element and store all prefix sums, including the final total of 210.
• When accessing the last element, we calculate the offset correctly (since MIPS arrays are 0-indexed, the 20th element is at index 19, which is 19×4=76 bytes from the array's start).

内容的提问来源于stack exchange，提问作者Malcadicta