A May 1976 advertisement for the Zilog Z-80 8-bit microprocessor
The Z80 came about when physicist Federico Faggin left Intel at the end of 1974 to found Zilog with
Ralph Ungermann. At Fairchild Semiconductor, and later at Intel, Faggin had been working on fundamental transistor and semiconductor manufacturing technology. He also developed the basic design methodology used for memories and microprocessors at Intel and led the work on the Intel 4004, the 8080 and several other ICs. Masatoshi Shima, the principal logic and transistor level-designer of the 4004 and the 8080 under Faggin's supervision, joined the Zilog team.
According to the designers, the primary targets for the Z80 CPU (and its optional support and peripheral ICs[ii]) were products like intelligent terminals, high end printers and advanced cash registers as well as telecom equipment, industrial robots and other kinds of automation equipment.
By March 1976, Zilog had developed the Z80 as well as an accompanying assembler based development system for its customers, and by July 1976, this was formally launched onto the market. (Some of the Z80 support and peripheral ICs were under development at this point, and many of them were launched during the following year.)
Early Z80s were manufactured by Synertek and Mostek, before Zilog had its own manufacturing factory ready, in late 1976. These companies were chosen because they could do the ion implantation needed to create the depletion-mode MOSFETs that the Z80 design used as load transistors in order to cope with a single 5 Volt power supply.[iii]
Faggin designed the instruction set to be binary compatible with the Intel 8080 so that most 8080 code, notably the CP/M operating system and Intel's PL/M compiler for 8080 (as well as its generated code), would run unmodified on the new Z80 CPU. Masatoshi Shima designed most of the microarchitecture as well as the gate and transistor levels of the Z80 CPU, assisted by a small number of engineers and layout people. CEO Federico Faggin was actually heavily involved in the chip layout work, together with two dedicated layout people. According to Faggin, he worked 80 hours a week in order to meet the tight schedule given by the financial investors.
The Z80 offered many improvements over the 8080:
- An enhanced instruction set including single-bit addressing, shifts/rotates on memory and registers other than the accumulator, rotate instructions for BCD number strings in memory, program looping, program counter relative jumps, block copy, block input/output (I/O), and byte search instructions. The Z80 incorporated an overflow flag and had better support for signed 8- and 16-bit arithmetics.[iv]
- New IX and IY index registers with instructions for direct base+offset addressing
- A better interrupt system
- A more automatic and general vectorized interrupt system, mode 2, primarily intended for Zilog's line of counter/timers, DMA and communications controllers, as well as a fixed vector interrupt system, mode 1, for simple systems with minimal hardware (with mode 0 being the 8080-compatible mode).
- A non maskable interrupt (NMI) which can be used to respond to power down situations or other high priority events (and allowing a minimalistic Z80 system to easily implement a two-level interrupt scheme in mode 1).
- Two separate register files, which could be quickly switched, to speed up response to interrupts such as fast asynchronous event handlers or a multitasking dispatcher. Although they were not intended as extra registers for general code, they were nevertheless used that way in some applications.[v]
- Less hardware required for power supply, clock generation and interface to memory and I/O
- Single 5-volt power supply (the 8080 needed -5 V/+5 V/+12 V).
- Single-phase 5 V clock (the 8080 needed a high-amplitude (9 to 12 volt) non-overlapping two-phase clock).
- A built-in DRAM refresh mechanism that would otherwise have to be provided by external circuitry.[vi]
- Non-multiplexed buses (the 8080 had state-signals multiplexed onto the data bus).
- A special reset function which clears only the program counter so that a single Z80 CPU could be used in a development system such as an in-circuit emulator.
The Z80 took over from the 8080 and its offspring, the 8085, in the processor market, and became one of the most popular 8-bit CPUs. Zilog was later by a few years than Intel (with its 80C85) to produce a low-power CMOS CPU, and this resulted in the Intel chip first appearing in use with battery-powered portable computers, such as Tandy/Radio Shack's TRS-80 Model 100 laptop from April 1983. In following years, however, the CMOS Z80 would dominate this market.
Some organisations, such as BT, remained loyal to the 8085 for embedded applications owing to their familiarity with it, and to its on-chip support for a serial interface and multi-level interrupt architecture. Perhaps a key to the initial success of the Z80 was the built-in DRAM refresh, and other features which allowed systems to be built with fewer support chips (Z80 embedded systems typically use static RAM and hence do not need this refresh).
For the original NMOS design, the specified upper clock frequency limit increased successively from the introductory 2.5 MHz, via the well known 4 MHz (Z80A), up to 6 (Z80B) and 8 MHz (Z80H). The NMOS version has been produced as a 10 MHz part since the late 1980s. CMOS versions were developed with specified upper frequency limits ranging from 4 MHz up to 20 MHz for the version sold today. The CMOS versions allowed low-power sleep with internal state retained, having no lower frequency limit.[vii] The fully compatible derivatives HD64180/Z180 and eZ80 are currently specified for up to 33 and 50 MHz respectively.