I'm trying to wrap my head around why memory alignment is so important. I've read that when data isn't aligned with the operation word size, it can lead to a lot of extra work for the CPU, like multiple requests and shifts to grab the right values and put them into registers. I know we should avoid this for performance reasons, but I'm curious—why can't we access data at any arbitrary address?
I've been pondering a few things:
1. If we request a value of 1 byte, 2 bytes, or 4 bytes, it seems like the least significant bits should consistently end up in the same spot from the hardware perspective, which would simplify the circuitry needed.
2. Since our data bus is 4 bytes wide, we always request 4 bytes regardless of whether we're asking for 1 or 2 bytes. This means those smaller values would also be placed at the least significant pins.
3. This leads to the idea that we can operate on any address, adjusting our requests according to the byte size. But if that's the case, where's the issue? Am I missing something? Is there a specific engineering problem that prevents this in circuits?
Could it be related to the structure of DRAM? I'm wondering if we're limited to addressing the granularity of memory bank rows. However, that seems inefficient since accessing a single byte can land us in the middle of a row, meaning we'll have to shift the data anyway. Why is it said that accessing one byte has no performance implications?
2 Answers
You’ve nailed the second point! Alignment requirements stem from how memory works. Your first point might be slightly off since the CPU can select bits from any part of the data bus, not just the rightmost bits. So, to clarify your third point: physical memory is organized in 32-bit (4-byte) slots. When you access data within a single 32-bit memory slot, it only takes one operation. If the data spans two slots, you need two operations, which is why alignment is key.
Exactly! So, it sounds like you understand the crux—memory is laid out in granular slots, which is why we need alignment. If you request a 1-byte piece that’s in the middle of a 4-byte slot, the CPU can grab it, but if the value crosses into another slot, that complicates things. That's the core of alignment problems. While memory can technically be accessed at a byte level, the internal structure makes it inefficient to do so when you're crossing into different slots. This is why it's encouraged to keep data aligned.
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