Z80

The Zilog Z80 is an 8-bit microprocessor introduced by Zilog in 1976. It was designed by Federico Faggin and colleagues as a successor to the Intel 8080, intended to be software-compatible while expanding performance and features.

The Z80 uses an 8-bit data bus and a 16-bit register set, including A and F, B C, D E, and H L, plus the program counter (PC) and stack pointer (SP). It offers alternate 8-bit/16-bit register pairs and two 16-bit index registers, IX and IY, for more flexible addressing. It can address up to 64 kilobytes of memory. It includes a dedicated refresh mechanism using the R register to support dynamic RAM.

In addition to a large base of 8080-compatible instructions, the Z80 adds many operations and prefixed instruction sets (CB for bit manipulations, ED for additional operations, and DD/FD prefixes to use IX or IY as base registers). It supports the I/O space separate from memory and has three interrupt modes (IM 0, IM 1, IM 2) for different interrupt handling.

The Z80 achieved enduring popularity in the late 1970s and 1980s, powering CP/M machines and a wide range of home computers. Notable users included Sinclair's ZX80, ZX81, and ZX Spectrum; the Amstrad CPC family; and many MSX machines. The processor's combination of compatibility with 8080 software and a rich feature set helped it dominate early microcomputer markets alongside the 6502.

Zilog later extended the family with higher-clock-rate variants such as Z80A and Z80B and compatible successors (notably the Z180 in later years). The Z80 has had a lasting legacy in microprocessor design and remains a staple in teaching and retrocomputing, with emulators and modern derivatives continuing to support its instruction set.