Difference between word addressable and byte addressable

Question

Can someone explain what\'s the different between Word and Byte addressable? How is it related to memory size etc.?

Answer 1

Addressability is the size of a unit of memory that has its own address. It's also the smallest chunk of memory that you can modify without affecting its neighbours.

For example: a machine where bytes are the normal 8 bits, and the word-size = 4 bytes. If it's a word-addressable machine, there's no such thing as the address of the second byte of an int. Dealing with strings (e.g. an array like char str[]) becomes inconvenient, because you still store characters packed together. Modifying just str[1] means loading the word that contains it, doing some shift/and/or operations to apply the change, then doing a word store.

Note that this is different from a machine that doesn't allow unaligned word load/stores (where the low 2 bits of a word address have to be 0). Such machines usually have a byte load/store instruction. We're talking about machines without even that.

CPU addresses might actually still include the low bits, but require them to always be zero (or ignore them). However, after checking that they're zero, the could be discarded, so the rest of the memory system only sees the word address, where two adjacent words have an address that differs by 1 (not 4). However, on a 16-bit CPU where a register can only hold 64k different addresses, you wouldn't likely do this. Each separate CPU address would refer to a different 2 bytes of memory, instead of discarding the low bit. 2B word-addressable memory would let you address 128kiB of memory, instead of just 64kiB with byte-addressable memory.

Fun fact: ARM used to use the low 2 bits of an address as a shuffle control for unaligned word loads. (But it always had byte load/store instructions.)

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See also:

• https://en.wikipedia.org/wiki/Word-addressable
• https://en.wikipedia.org/wiki/Byte_addressing

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Note that bit-addressable memory could exist, but doesn't. 8-bit bytes are nearly universally standard now. (Ancient computers sometimes had larger bytes, see the history section of wikipedia's Byte article.)