Why is number of bits always(?) a power of two?

Question

We have 8-bit, 16-bit, 32-bit and 64-bit hardware architectures and operating systems. But not, say, 42-bit or 69-bit ones.

Why? Is it something fundamental that makes 2

Answer 1

Your memory system wants to be a byte multiple, which makes your cache want to be a byte multiple, which makes your whole system want to be a byte multiple.

Speaking as a HW designer, you generally want to design CPU's to byte boundaries of some kind, ie multiples of 8. Otherwise you either have to add a lot of awkward circuitry to a 49-bit system to make it utilize the mod-8 bits, or you end up ignoring the extra bits, in which case it was a waste, unless you needed the extra bit for instructions, which is never the case on 16 bit or wider systems.