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 came about when Federico Faggin left Intel after working on the 8080, and by July 1976 Zilog had the Z80 on the market. It was designed to be binary compatible with the Intel 8080 so that most 8080 code could run unmodified on it, notably the CP/M operating system.
The Z80 offered five real improvements over the 8080:
- an enhanced instruction set including new IX and IY index registers and instructions for them
- two instances of each register which could be quickly switched between, to speed up response to interrupts
- a limited ability for SIMD (Single Instruction, Multiple Data) with block move and copy instructions. ¹
- a built-in DRAM refresh address counter that would otherwise have to be provided by external circuitry
- 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. 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, 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.
By the early 1980s it was used in a host of home computer designs including the MSX, Radio-Shack TRS-80, Sinclair ZX80² & ZX81² and ZX Spectrum. It also featured in the great number of fairly anonymous business-oriented CP/M machines (plus the non-anonymous Osborne 1) that dominated the market of the time in the way that Windows-based machines do today. In the mid-1980s the Z80 was used in Tatung's Einstein and the Amstrad CPC and PCW home/office computer ranges as well as forming the CPU basis for the MSX computer standard.
Such was the popularity of the Z80 and CP/M that the Commodore 128 featured a Z80 processor alongside its MOS Technology 8502 processor for compatibility. Other 6502 based computers already on the market such as the BBC Micro, Apple II and the 6510 based Commodore 64 can make use of the Z80 with an external unit or a plug-in card or cartridge.
Notable later uses of the processor include several Texas Instruments (TI) graphing calculators (like the TI-85 and TI-83), and SEGA's Master System and Game Gear video game consoles. Both the SNK Neo-Geo and SEGA Megadrive/Genesis consoles use it as an audio coprocessor. Nintendo's Game Boy and Game Boy Color handheld game systems used a Z80 clone manufactured by Sharp Corporation, which had a slightly different instruction set. The Zilog Z80 has also become a popular embedded microprocessor and microcontroller core, where it remains in widespread use today.
In East Germany, an unlicenced clone of the Z80, known as the U880, was manufactured. It was very popular and was used in Robotron's and VEB Mikroelektronik Mühlhausen's computer systems (e.g. the KC85-series) and also in many self-made computer systems (ex. COMP JU+TER). In Romania, several Z80 clones were manufactured: HC85, HC90, HC91, HC2000 (by the Felix Computers Factory, based in Bucharest) and TimS (by the Timisoara Technical University). The HC85 and TimS clones were the most popular models.
A functionally equivalent CPU core (T80 & TV80) is available for free under a BSD style license as VHDL  (http://www.opencores.org/projects.cgi/web/t80/overview) and Verilog  (http://www.opencores.org/projects.cgi/web/tv80/overview) source. The VHDL version, once synthesized, can be clocked up to 35 Mhz on a Xilinx Spartan II FPGA. Software emulation of the Z80 instruction set on modern PCs runs faster than the original Z80 CPU ran and is used today with the MAME video game emulator's execution of 1980's vintage video games.
See also: List of home computers by category
- These were considered very powerful at the time: modern 3DNow! and SSE instructions work on highly advanced versions of the same basic principle.
- The Sinclair ZX80 and -81 were equipped with the Z80 clone NEC 780C.
- Mostek Z80 Processor Technical Manual (http://www.mcu51.com/download/digitpdf/CPU/z80/z80-mostek.pdf)
- Sinclair Nostalgia Products (http://www.interface1.net/zx)