The central processing unit (CPU) is the component of a computer that actually processes the data. CPU designs vary a great deal, but for our purposes we may treat all CPUs as having simply an arithmetic-logic unit, a control unit, and several registers. In recent designs, a memory management unit is also often found.

In early days the various parts of a CPU were located in separate physical devices, so the computer as a whole had to be quite bulky. Since Intel came out with the 4004 microprocessor in the 1970s, however, the entire CPU has been located on a single silicon chip. Any chip that holds a complete CPU is called a microprocessor; indeed, today the words "CPU" and "microprocessor" are virtually synonymous. The complexity of chips has increased tremendously in the last 30 years. For instance, Intel's 4004 contained only about 29,000 transistors, while...