The Intel 8080 & Zilog Z-80

Dan Knight - September 2001

Although the Intel 8080 never ran DOS, it is the direct predecessor of the 8086 and 8088 CPUs used in the first IBM PC. The 2 MHz 8080 was released on April 1, 1974 and formed the core of the first personal computers, the MITS Altair and the IMSAI 8080.

The 8080 was an 8-bit CPU containing 8,000 transistors and capable of addressing a then-impressive 64 KB of memory (it's predecessor, the 8008, could only address 16 KB). The 8080 used 6 micron traces. The Zilog Z-80, designed by former Intel employees, was essentially an 8080 clone with 80 additional instructions. It came out in July 1976. The Z-80 ran at speeds from 2.5-10 MHz.

Intel also introduced a slightly improved version of the 8080, the 8085.

The standard operating system of the 8080, Z-80, and 8085 was CP/M, although several computers using these chips also had proprietary operating systems, such as Tandy's TRS-DOS.

While 8-bit processors seemed impressive in the late 1970s, by the early 1980s it was becoming evident that their days were numbered. Newer CPUs with 16-bit designs were able to address vastly increased amounts of memory, something made necessary by large spreadsheets, large word processing documents, and large databases.

Further Reading

• Great Microprocessors of the Past and Present, Intel 8080, John Bayko
• Great Microprocessors of the Past and Present, Zilog Z-80, John Bayko

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