Can we have a computer with just registers as memory? [closed]

Question

Registers are the fastest memories in a computer. So if we want to build a computer with just registers and not even caches is it possible? I think of even replacing the magnetic discs with registers although they are naturally volatile memories. Do we have some nonvolatile registers for that use? It would become so fast! I'm just wondering if that could be happen or not?

Answer 1

The very short answer is yes, you could in theory, but it doesn't really work in real life. Let me explain...

The reason the memory hierarchy exists is because those small and fast memory stores are very expensive per a bit (registers), while the big and slow memory stores are very cheap per a bit (hard drives).

Another reason why huge numbers of registers are highly impractical is because the instructions need to reference the memory location. When you only have a handful of registers, you can store the register (or registers) number and an opcode in a handful of bits, which means that low numbers of registers make for short and fast instructions. If you're going to have a multi-gigabyte collection of registers, you will need to be able to reference them in instructions, and these will be much longer (and therefore slower) instructions. Keep in mind that if everything was a register, some things would be much faster, but by having a smaller number of registers, certain things (i.e., most of what you do with a computer) are much faster.

Having vast numbers of registers would also add a great amount of complexity to the hardware which processes the reading and writing to registers, which would make everything slower.

Of course, while most of us think in terms of computers, there are surely simple devices which do only have registers, but they would also only have a very limited amount of memory, and aren't made for general purpose computation.

You may also be interested to my answer to Assembly: Why are we bothering with registers?