The important characteristics of a microprocessor are the widths of its internal and external address bus and data bus (and instruction), its clock rate and its instruction set. Processors are also often classified as either RISC or CISC.
The first commercial microprocessor was the Intel 4004 which appeared in 1971. This was the CPU member of a set of four LSI integrated circuits called the MCS-4, which was originally designed for use in a calculator but was marketed as "programmable controller for logic replacement". The 4004 is referred to as a 4-bit microprocessor since it processed only 4 bits of data at a time. This very short word size is due mainly to the limitations imposed by the maximum integrated circuit density then achievable.
As integrated circuit densities increased with the rapid development of integrated circuit manufacturing technology, the power and performance of the microprocessors also increased. This is reflected in the increase in the CPU word size to 4, 8, 16, and by mid-1980s, 32 bits. The smaller microprocessors have relatively simple instruction sets, e.g., no floating point instructions, but they are nevertheless suitable as controllers for a very wide range of applications such as car engines and microwave ovens.
The Intel 4004 was followed with, among others the 4040, 8008, 8080, 8086, 80186, 80286, 80386, 486 and Pentium. Other families include the Motorola 6800 and 680x0 families, National Semiconductor 16000 and National Semiconductor 32000, SPARC, ARM, MIPS, Zilog Z8000, PowerPC and the Inmos Transputer family.
The larger, more recent microprocessors families have gradually acquired most of the features of large computers. As the microprocessor industry has matured, several families of microprocessors have evolved into de facto industrial standards with multiple manufacturers and numerous "support" chips including RAM, ROM, I/O controllers etc.
A single chip microprocessor may include other components such as memory (RAM, ROM, PROM), memory management, caches, floating-point unit, input/output ports and timers. Such devices are also known as microcontrollers.
Microcomputers have given rise to a new class of general-purpose machines called personal computers. These are small low cost computers that are designed to sit on an ordinary office desk or to be portable and fuelled the computer boom of the late 1980s. The most widespread example is the also IBM PC, based on microprocessors from Intel Corporation. Apple Computers, Inc. have also produced a range of personal computers, as have several other companies.