how does the processor read memory?

Question

I\'m trying to re-implement malloc and I need to understand the purpose of the alignment. As I understand it, if the memory is aligned, the code will be executed faster because

Answer 1

modern processors and memory are built to optimize memory access as much as possible. One the current way of accessing memory is to address it not byte by byte but by an address of a bigger block, e.g. by an 8 byte blocks. You do not need 3 lower bits of the address this way. To access a certain byte within the block the processs needs to get the block at the aligned address, then shift and mask the byte. So, it gets slower.

When fields in the struct are not aligned, there is a risk of slowing down the access to them. Therefore, it is better to align them.

But the aslignment requirements are based on the underlying platform. For systems which support word access (32 bit), 4-byte alignment is ok, otherwise 8-byte can be used or some other. The compiler (and libc) knows the requirements.

So, in your example char, short, char, the short will start with an odd byte position if not padded. To access it, the system might eed to read the 64 bit word for the struct, then shift it 1 byte right and then mask 2 bytes in order to provide you with this byte.