Is 8-byte alignment for “double” type necessary?

Question

I understand word-alignment, which makes the cpu only need to read once when reading an integer into a register.

But is 8-byte alignment (let\'s assume 32bit system

Answer 1

Edited:

The advantage of byte alignment is to reduce the number of memory cycles to retrieve the data. For example, an 8 byte which might take a single cycle if it is aligned might now take 2 cycles since a part of it is obtained the first time and the second part in the next memory cycle.

I came across this: "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). So the CPU doesn't fetch a single byte at a time - it fetches 4 or 8 bytes starting at the requested address. Therefore, 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!)."

You might see this link for more details. http://www.ibm.com/developerworks/library/pa-dalign/