Why is memory alignment required? [duplicate]

Question

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Purpose of memory alignment

I read some articles on net about memory alignment and could understand that from properly aligned memory (take 2-byte alignment) we can fetch data fastly in one go.

But if we have memory like a single hardware piece, then given an address, why cannot we read 2-byte directly from that position. like:

I thought over it. I think that if the memory is in odd-even banks kind of then the theory would apply.

What am i missing ?

Answer 1

Your pictures describe how we (humans) visualize computer memory.

In reality, think about memory as huge matrix of bits. Each matrix column has a "reader" attached that can read/write any bit from this column. Each matrix row has a "selector", which can select the specific bit that the reader will read/write.

Therefore, this reader can read the whole selected matrix row at once. Length of this row (number of matrix columns) define how much data can be read at once. For instance, if you have 64 columns, your memory controller can read 8 bytes at once (it usually can do more than that though).

As long as you keep your data aligned, you will need less of these memory accesses. Even if you need to read just two bits, but they are located on different rows, you will need two accesses to memory instead of one.

Also, there's a whole aspect of writing, which is a different problem.

Just as you can read the whole row, you also can write the whole row. If your data isn't aligned, when you write something that is not a full row, you will need to do read-modify-write (read the old content of the row, modify the relevant part and write the new content).