What is meant by "memory is 8 bytes aligned"?

Question

While going through one project, I have seen that the memory data is "8 bytes aligned". Can anyone please explain what this means?

Answer 1

An object that is "8 bytes aligned" is stored at a memory address that is a multiple of 8.

Many CPUs will only load some data types from aligned locations; on other CPUs such access is just faster. There's also several other possible reasons for using memory alignment - without seeing the code it's hard to say why.

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Aligned access is faster because the external bus to memory is not a single byte wide - it is typically 4 or 8 bytes wide (or even wider). This means that the CPU doesn't fetch a single byte at a time - it fetches 4 or 8 bytes starting at the requested address. As a consequence of this, the 2 or 3 least significant bits of the memory address are not actually sent by the CPU - the external memory can only be read or written at addresses that are a multiple of the bus width. If you requested a byte at address "9", the CPU would actually ask the memory for the block of bytes beginning at address 8, and load the second one into your register (discarding the others).

This implies that a misaligned access can require two reads from memory: If you ask for 8 bytes beginning at address 9, the CPU must fetch the 8 bytes beginning at address 8 as well as the 8 bytes beginning at address 16, then mask out the bytes you wanted. On the other hand, if you ask for the 8 bytes beginning at address 8, then only a single fetch is needed. Some CPUs will not even perform such a misaligned load - they will simply raise an exception (or even silently load the wrong data!).

Answer 2

The memory alignment is important for performance in different ways. It has a hardware related reason. Since the 80s there is a difference in access time between the CPU and the memory. The speed of the processor is growing faster than the speed of the memory. This difference is getting bigger and bigger over time (to give an example: on the Apple II the CPU was at 1.023 MHz, the memory was at twice that frequency, 1 cycle for the CPU, 1 cycle for the video. A modern PC works at about 3GHz on the CPU, with a memory at barely 400MHz). One solution to the problem of ever slowing memory, is to access it on ever wider busses, instead of accessing 1 byte at a time, the CPU will read a 64 bit wide word from the memory. This means that even if you read 1 byte from memory, the bus will deliver a whole 64bit (8 byte word). The memory will have these 8 byte units at address 0, 8, 16, 24, 32, 40 etc. A multiple of 8. If you access, for example an 8 byte word at address 4, the hardware will have to read the word at address 0, mask the high 4 bytes of that word, then read word at address 8, mask the low part of that word, combine it with the first half and give that to the register. As you can see a quite complicated (thus slow) operation. This is the first reason one likes aligned memory access. I will give another reason in 2 hours.