History of General Purpose CPUs - 1970s: Large Scale Integration

1970s: Large Scale Integration

In the 1960s, the Apollo guidance computer and Minuteman missile made the integrated circuit economical and practical.

Around 1971, the first calculator and clock chips began to show that very small computers might be possible. The first microprocessor was the Intel 4004, designed in 1971 for a calculator company (Busicom), and produced by Intel. In 1972, Intel introduced a microprocessor having a different architecture: the 8008. The 8008 is the direct ancestor of the current Core i7, even now maintaining code compatibility (every instruction of the 8008's instruction set has a direct equivalent in the Intel Core i7's much larger instruction set, although the opcode values are different).

By the mid-1970s, the use of integrated circuits in computers was commonplace. The whole decade consists of upheavals caused by the shrinking price of transistors.

It became possible to put an entire CPU on a single printed circuit board. The result was that minicomputers, usually with 16-bit words, and 4k to 64K of memory, came to be commonplace.

CISCs were believed to be the most powerful types of computers, because their microcode was small and could be stored in very high-speed memory. The CISC architecture also addressed the "semantic gap" as it was perceived at the time. This was a defined distance between the machine language, and the higher level language people used to program a machine. It was felt that compilers could do a better job with a richer instruction set.

Custom CISCs were commonly constructed using "bit slice" computer logic such as the AMD 2900 chips, with custom microcode. A bit slice component is a piece of an ALU, register file or microsequencer. Most bit-slice integrated circuits were 4-bits wide.

By the early 1970s, the PDP-11 was developed, arguably the most advanced small computer of its day. Almost immediately, wider-word CISCs were introduced, the 32-bit VAX and 36-bit PDP-10.

Also, to control a cruise missile, Intel developed a more-capable version of its 8008 microprocessor, the 8080.

IBM continued to make large, fast computers. However the definition of large and fast now meant more than a megabyte of RAM, clock speeds near one megahertz, and tens of megabytes of disk drives.

IBM's System 370 was a version of the 360 tweaked to run virtual computing environments. The virtual computer was developed in order to reduce the possibility of an unrecoverable software failure.

The Burroughs B5000/B6000/B7000 series reached its largest market share. It was a stack computer whose OS was programmed in a dialect of Algol.

All these different developments competed for market share.