Load Store Queue地址计算与访存时间估算问题求助

Hey there, I totally get how stuck you must feel right now—scouring through slides, textbooks, and videos for a solution to this exam question, only to come up empty. Let’s walk through this LSQ (Load Store Queue) problem together, since I’ve worked through similar scenarios during my own architecture studies and exam prep.

First, let’s align on the core constraints and assumptions we’ll use (these are standard for such problems unless stated otherwise):

• We’re dealing with a processor that does not perform memory dependence prediction, so load instructions cannot speculatively initiate memory accesses. This means loads have to wait for all prior store instructions (in program order) to resolve their effective addresses (EA) before they can safely access memory.
• Address calculation is a fixed pipeline stage (typically 1 cycle, unless the question specifies otherwise)—once an instruction’s input operands are ready, it can start computing its EA immediately.
• Memory access time (both read for loads and write for stores) is also a fixed duration (usually 2 cycles in basic problems; adjust if your course uses a different standard).

Step 1: Calculate Address Computation Time #

For every ld or st instruction, address computation follows this logic:

• Start time: Equal to the given input operand ready time (you can’t compute an EA until all the values needed for that calculation are available).
• Duration: Assume 1 cycle (standard for most pipeline architectures; if your course uses a different number, substitute it here).
• End time: Input operand ready time + Address computation duration
• For your table, the "Address Computation Time" column can either list the duration (e.g., 1 cycle) or the start/end time window (e.g., Cycle 3 → 4)—follow whatever format the exam question expects.

Step 2: Calculate Data Memory Access Time #

We need to handle loads and stores differently, especially with the no-speculation constraint:

For Store (st) Instructions

Stores don’t face the same speculation restriction as loads. Their memory access can start as soon as two conditions are met:

1. The store’s address computation is complete (we have the EA).
2. The data to be stored is ready (if the question doesn’t specify a separate data ready time, assume it’s the same as the input operand ready time).

• Start time: Max(Address computation end time, Data ready time)
• Duration: Assume 2 cycles (or your course’s standard memory latency).
• End time: Start time + Memory access duration

For Load (ld) Instructions

Since no memory dependence prediction is allowed, loads must wait for all program-order prior stores to finish their address computation (to confirm there’s no address conflict that would require the load to wait for the store to complete writing). Here’s the breakdown:

1. First, get the load’s address computation end time (from Step 1).
2. Find the latest address computation end time among all stores that come before the load in program order.
3. Start time: Max(Load's address computation end time, Latest prior store's address computation end time)
   • If the load’s EA matches any prior store’s EA, you’ll need to wait for that store’s memory access to finish instead (since it’s a write-after-read dependency—you have to read the updated value).
4. Duration: Same as stores (e.g., 2 cycles).
5. End time: Start time + Memory access duration

Example Walkthrough #

Let’s use a sample table to make this concrete:

For ld2:

• Its address computation finishes at cycle 5, but the prior store (st1) doesn’t finish address computation until cycle 6. Since we can’t speculate, ld2 has to wait until cycle 6 to start memory access, hence the 6→8 window.
• If ld2’s EA matched st1’s (0x100), we’d have to wait until st1’s memory access finishes (cycle 8) before starting ld2’s read, making its window 8→10.

Key Notes to Remember #

• Always follow your course’s specific assumptions (e.g., if your professor uses 3 cycles for memory access, use that instead of 2).
• Program order matters more than operand ready time for loads—even if a load’s operands are ready earlier, it can’t jump ahead of prior stores’ address resolution.
• If the question doesn’t specify pipeline stage durations, state your assumptions clearly (examiners love that—it shows you understand the problem’s context).

内容的提问来源于stack exchange，提问作者Podo