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The CPU contains a special set of memory cells known as registers that can be both readable and writable more rapidly than the main memory area. There are typically between two and one hundred registers based upon the type of CPU. Registers are used for the most frequently needed data items to avoid having to access main memory every time when data is needed. Computer main memory comes in two principal varieties: random access memory or RAM and read-only memory or ROM.

RAM can be read and written to anytime the CPU commands it, but ROM is pre-loaded with data and software that never changes, so the CPU can only read from it. In general, the contents of RAM are erased when the power to the computer is turned off while ROM retains its data indefinitely. Generally computers with this sort of cache are designed to move frequently needed data into the cache automatically, often without the need for any intervention on the programmer's part.