Renjith G Renjith G - 5 months ago 55
C Question

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

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

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Answer Source

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.

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!).

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