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The Zilog Z80 is an 8-bit microprocessor designed and manufactured by Zilog from 1976 onwards. It was widely used both in desktop and embedded computer designs, and is one of the most popular CPUs of all time. Although Zilog made several attempts to move off the Z80 onto more powerful 16-bit (Zilog Z800, Zilog Z8000) and 32-bit (Zilog Z80000) platforms, other companies were offering CPUs in this performance range years earlier, and the Zilog chips never caught on.
The Z80 is an enhanced (and fully binary-compatible) version of the Intel 8080. The Z80 was set in motion at the end of 1974, when Federico Faggin left Intel, after working on the 8080, to found Zilog with Ralph Ungermann. By July 1976, Zilog had the Z80 on the market. It was designed to be binary compatible with the Intel 8080 so that code for the latter could run unmodified on it, notably the CP/M operating system. Masatoshi Shima, the principal logic and transistor level-designer of the 4004 and the 8080, designed most of the microarchitecture as well as the gate and transistor levels of the Z80.
The Z80 offered five real improvements over the 8080:
The Z80 quickly took over from the 8080 in the market, and became the most popular 8-bit CPU of all time - indeed, if one takes the absolute size of the market into account, the most successful CPU ever. Later versions increased in speed from the early models' 1 MHz up to as much as 20 MHz.
Perhaps key to the success of the Z80 was the built-in DRAM controller and memory refresh register (R), which allowed systems to be built with fewer support chips. Competitor MOS Technology, Inc, maker of the famous 6502 processor, later included this very useful feature in its second generation color video chip VIC-II.