Zilog Z80

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.

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:
  1. an enhanced instruction set including the new indexing registers IX/IY and instructions for them, many new bit-manipulating and shifting operations, and enhanced capabilities for processing interrupts;
  2. two instances of each of the 16-bit registers AF (accumulator+flags), BC, DE, and HL, which could be quickly switched between to speed up response to interrupts or other context-switching;
  3. a limited ability for SIMD (Single Instruction, Multiple Data) with instructions to perform copy, compare, input, and output over contiguous blocks of memory;
  4. a built-in DRAM refresh address counter that would otherwise have to be provided by external circuitry;
  5. a much lower price.

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. 


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