判断MIPS汇编代码对表达式f=-g-A[4]的实现是否正确

I'm working on an assignment and want to verify if my implementation of the expression f = -g - A[4] is correct. Here's the mapping:

• f corresponds to register $s0
• g corresponds to register $s1
• Base address of array A is in register $s6

My implementation code:

subu $t1, $zero, $s1 // negate g
addi $t2, $zero, 4 // index for A
add $t3, $t2, $s6 // put A[4] in $t3
subu $s0, $t1, $t3 // f = -g -A[4]

Let's break down the issues with your current code and fix it step by step:

Key Problems in Your Implementation #

• Incorrect array element addressing: MIPS uses byte-addressable memory. For an array of integers (each taking 4 bytes, the standard for MIPS), the address of A[4] isn't just base_address + 4 — it's base_address + 4 * 4 (since each index step skips 4 bytes). Your code calculates $s6 +4, which points to the second byte of A[0], not the start of A[4].
• Missing load instruction: Your code treats the address calculation result as the value of A[4], but you need to load the actual value from memory using the lw (load word) instruction to get the content stored at that address.

Corrected MIPS Code #

subu $t1, $zero, $s1   # Compute -g and store in $t1
addi $t2, $zero, 16    # Calculate offset for A[4]: 4 elements * 4 bytes = 16 bytes
add $t3, $s6, $t2      # Compute the memory address of A[4]
lw $t3, 0($t3)         # Load the value of A[4] from memory into $t3
subu $s0, $t1, $t3     # Compute f = -g - A[4] (equivalent to $t1 - $t3)

Alternatively, you can combine the address calculation and load into one line for brevity:

subu $t1, $zero, $s1   # Compute -g
lw $t2, 16($s6)        # Directly load A[4] using an offset of 16 from base address $s6
subu $s0, $t1, $t2     # Compute f = -g - A[4]

Quick Verification #

Let's cross-check against your target expression:

• subu $t1, $zero, $s1 correctly calculates -g (since 0 - g = -g).
• The offset 16 ensures we're pointing to the 5th element of the integer array A.
• subu $s0, $t1, $t2 computes -g - A[4] exactly, matching your desired expression.

内容的提问来源于stack exchange，提问作者randochick