The Zilog Z80 is an 8-bit microprocessor introduced in 1976 by Zilog, the company Federico Faggin co-founded after leading the design of the Intel 4004 and 8080. Faggin designed the Z80 to be binary-compatible with the Intel 8080 — it could run existing 8080 machine code unchanged — while adding a richer instruction set, more registers, and integrated features that made it cheaper and easier to build systems around. That combination of compatibility and improvement made the Z80 enormously successful.
The Zilog Z80 CPU User Manual documents the architecture: in addition to the 8080-compatible register set, the Z80 added a second, alternate set of general-purpose registers, two 16-bit index registers (IX and IY), a refresh register for dynamic RAM, and new instructions including bit manipulation, block move and search, and relative jumps. Crucially, the Z80 needed only a single +5V supply and included on-chip logic to refresh DRAM, removing support chips that the 8080 required and lowering the cost of a complete system.
The Z80 became one of the defining CPUs of the late-1970s and 1980s. It ran CP/M, the dominant pre-MS-DOS business operating system, and powered a huge range of home computers including the Sinclair ZX Spectrum, the Amstrad CPC, the TRS-80, and the MSX line, as well as arcade machines and the Game Boy’s custom variant. Its blend of capability and low cost made it a default choice for both personal computers and embedded designs.
Long after it ceased to be a mainstream PC processor, the Z80 lived on in embedded systems, calculators (the TI-83/84 graphing-calculator line used a Z80 for decades), and industrial controllers. Its longevity — the part remained in production into the 2020s — and its place in the 8-bit home-computer boom make it one of the most historically important microprocessors, a direct descendant of the Intel line that its own creator had earlier built.