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A '''register''' is a special small [[memory]] unit; usually within a [[Central Processing Unit|CPU]], i.e. not part of the machine's [[main memory]], although [[device controller]]s usually include registers too. For the ones which are part of the CPU, access to them is very fast - usually faster than main memory.

The first computers did not include many registers in the CPU, since each register needs separate [[hardware]], and that was a scarce resource early on. Many machines only had a single register available for doing computations with, the '''accumulator'''; these machines were typically a [[load-store architecture]].

Later on, hardware was not so much of an issue, but computers still tend to have somewhat limited numbers of programmer-visible registers, because they are named by fields in [[instruction]]s, and [[bit]]s in the instruction [[word]] are (and always will be) a limited resource. ([[Register renaming]] was developed to allow more registers to be used, without being able to explicitly name them.)

==See also==

* [[Stack Pointer]]
* [[Program Counter]]

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[[Category:Hardware Basics]]