What a microprocessor is
Every computer has a processor, the part that carries out instructions. In the machines of the 1950s and 1960s, the processor was a large assembly of parts: first vacuum tubes, then transistors, and later circuit boards filled with many small chips. A microprocessor places the main functions of a processor onto a single piece of silicon. The idea was possible only because of earlier progress in integrated circuits, which pack many transistors onto one chip.
A microprocessor is not a complete computer by itself. It needs memory to hold programs and data, plus ways to connect to a keyboard, display or storage. But because the most complex part sits on one chip, the rest can be built around it far more simply and cheaply than before. For a plain-language description of what such a unit does, see the explanation of the central processing unit.
Early chips of the 1970s
Intel is widely credited with producing one of the first commercially available microprocessors, the 4004, which appeared in 1971. It was a 4-bit design developed for a line of calculators, a detail that shows how the technology began as a practical component rather than as a computer for the public. The following year Intel released an 8-bit chip, the 8008, and in 1974 a more capable 8-bit chip, the 8080. Other companies soon produced their own designs, including Motorola’s 6800 and the MOS Technology 6502 in the mid 1970s, and the Zilog Z80 slightly later.
As with many “firsts”, the history is more tangled than a single date suggests. Other teams worked on similar ideas around the same time, and there have been disputes about credit and about which design deserves the label. A safe summary is that single-chip processors became available in the early 1970s and improved rapidly over the rest of the decade.
From component to computer
The effect on small computers was direct. Hobbyists and tiny companies could now buy a processor chip and build a computer around it. The machines described under microcomputer kits depended on this fact. Without an affordable processor, the Altair, the Apple I and the ready-made machines of 1977 would have been far more difficult and expensive to make.
Cost is the heart of the story. A processor that once required a cabinet of electronics could now be a part purchased from a catalog, and prices tended to fall as manufacturing improved. That change turned computing from something only institutions could afford into something small businesses, schools and individuals could consider.
It also meant that the design of a computer became easier to copy. Because companies could buy the same chips and assemble them with standard memory, differences between machines were increasingly in software, case design and convenience rather than in the underlying processor. This helps explain why many companies entered the market quickly in the late 1970s.
Beyond the personal computer
The impact of the microprocessor went well beyond the machines we call personal computers. Cheap processors began appearing inside many other products: calculators, cash registers, industrial controllers, game machines, appliances and vehicles. Many of these devices are never thought of as computers by their owners, yet they contain processors and run programs.
Seen this way, the microprocessor spread computing in two directions at once. One was visible, in the form of desk machines with keyboards. The other was hidden, in the form of control logic embedded in everyday equipment. Even a popular item such as a home video game system relies on this kind of chip. The later growth of mobile phones and the Internet of Things also depends on the same basic idea, a small and inexpensive processor with enough capability to be useful.
Improvement over time
Processors changed rapidly after their introduction. Word sizes grew from 4 and 8 bits to 16 and 32 bits and later to 64 bits, which in general terms allowed processors to handle larger numbers and more memory at once. Chips were manufactured with ever more transistors. This pattern is often connected to an observation known as Moore’s Law, though that rule is frequently misunderstood, and a separate article on the transistor doubling rule sets out what it does and does not say.
The microprocessors used in the first IBM PC and its successors came from the same lineage that began in the early 1970s, so the family of chips running most desktop machines can be traced back through several generations. Even so, the newest processors share only the broad idea with their ancestors; the internal designs are very different, and it would be a mistake to treat the early chips as simply smaller versions of modern ones.
What the microprocessor did not do
It is easy to give one component all the credit. A few cautions are in order.
- A processor alone does not create a useful computer. Memory, storage, displays, software and interfaces mattered as much to ordinary users.
- The microprocessor did not make personal computing inevitable. Companies still had to decide to sell such machines, and customers had to find uses for them.
- Earlier ideas, including the stored-program design and the transistor, were prerequisites. The single-chip processor built on decades of work by many engineers.
- Smaller and cheaper did not mean easier at first. Early microprocessor-based machines demanded technical skill, and ease of use came years later through software.
The microprocessor is best seen as an enabling technology. By lowering the cost and size of the central part of a computer, it made possible a wave of experiments in small computing, and some of those experiments grew into a mass market. What people did with the machines depended on much else, from programming languages to the willingness of buyers to try something new.