Primary Storage

Primary storage is directly connected to the central processing unit of the computer. It must be present for the CPU to function correctly, just as in a biological analogy the lungs must be present (for oxygen storage) for the heart to function (to pump and oxygenate the blood). As shown in the diagram, primary storage typically consists of three kinds of storage:

Processor registers are internal to the central processing unit Registers contain information that the arithmetic and logic unit needs to carry out the current instruction. They are technically the fastest of all forms of computer storage, being switching transistors integrated on the CPU's silicon chip, and functioning as electronic "flip-flops".

Cache memory is a special type of internal memory used by many central processing units to increase their performance or "throughput". Some of the information in the main memory is duplicated in the cache memory, which is slightly slower but of much greater capacity than the processor registers, and faster but much smaller than main memory. Multi-level cache memory is also commonly used—"primary cache" being smallest, fastest and closest to the processing device; "secondary cache" being larger and slower, but still faster and much smaller than main memory.

Main memory contains the programs that are currently being run and the data the programs are operating on. In modern computers, the main memory is the electronic solid-state random access memory. It is directly connected to the CPU via a "memory bus" (shown in the diagram) and a "data bus". The arithmetic and logic unit can very quickly transfer information between a processor register and locations in main storage, also known as a "memory addresses". The memory bus is also called an address bus or front side bus and both busses are high-speed digital "superhighways". Access methods and speed are two of the fundamental technical differences between memory and mass storage devices. (Note that all memory sizes and storage capacities shown in the diagram will inevitably be exceeded with advances in technology over time.)