Zilog Z80


The Zilog Z80 is an 8-bit microprocessor designed by Zilog, first released in 1976; it played an important role in the evolution of early personal computing. It was designed to be software-compatible with the Intel 8080, offering a compelling alternative due to its better integration and increased performance. Along with the 8080's seven registers and flags register, the Z80 introduced an alternate register set, two 16-bit index registers, and additional instructions, including bit manipulation and block copy/search.
Originally intended for use in embedded systems like the 8080 was, the Z80's combination of compatibility, affordability, and superior performance led to widespread adoption in video game systems and home computers throughout the late 1970s and early 1980s, helping to fuel the personal computing revolution. The Z80 was used in iconic products such as the Osborne 1, Radio Shack TRS-80, ColecoVision, ZX Spectrum, Sega's Master System and the Pac-Man arcade cabinet. In the early 1990s, it was used in portable devices, including the Game Gear and the TI-81 and succeeding graphing calculators.
The Z80 was the brainchild of Federico Faggin, a key figure behind the creation of the Intel 8080. After leaving Intel in 1974, he co-founded Zilog with Ralph Ungermann. The Z80 debuted in July 1976, and its success allowed Zilog to establish its own chip factories. For initial production, Zilog licensed the Z80 to U.S.-based Synertek and Mostek, along with European second-source manufacturer, SGS. The design was also copied by various Japanese, Eastern European, and Soviet manufacturers gaining global market acceptance as major companies like NEC, Toshiba, Sharp, and Hitachi produced their own versions or compatible clones.
The Z80 continued to be used in embedded systems for many years, despite the introduction of more powerful processors; it remained in production until June 2024, 48 years after its original release. Zilog also continued to enhance the basic design of the Z80 with several successors, including the Z180, Z280, and Z380, with the latest iteration, the eZ80, introduced in 2001 and available for purchase as of 2025.

History

Early history

At Fairchild Semiconductor, and later at Intel, physicist and engineer Federico 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 Intel 8080 and several other ICs. Masatoshi Shima was the principal logic and transistor-level designer of the 4004 and the 8080 under Faggin's supervision, while Ralph Ungermann was in charge of custom integrated circuit design.
In early 1974, Intel viewed microprocessors not so much as products to be sold on their own but as a way to sell more of its main products, static RAM and ROM. A reorganization placed some of the formerly independent sections under the direction of Les Vadasz, further diluting the microprocessor's place in the company. That year, the 1973–1975 recession reached a peak, and Intel laid off several employees. All of this led to Faggin becoming restless, and he invited Ungermann out for drinks and asked if he would be interested in starting their own company. Ungermann immediately agreed, and as he had less to do at Intel, he left in August or September, followed by Faggin, whose last day at Intel was Halloween 1974. When Shima heard, he asked to come to the new company as well, but having no actual product design or money, they told him to wait.
The newly formed and unnamed company initially began designing a single-chip microcontroller called the 2001. They met with Synertek to discuss fabrication on their lines, and when Faggin began to understand the costs involved, it became clear that a low-cost product like this would not be able to compete with a design from a company with its own production lines, like Intel. They then began considering a more complex microprocessor instead, initially known as the Super 80, with the main feature being its use of a +5 V bus instead of the more common −5, +5 and 12 V used by designs like the 8080. The new design was intended to be compatible with the 8080, but add a number of the features of the Motorola 6800, including index registers and improved interrupts.

Exxon investment, detailed development begins

While still being set up, the industry newsletter Electronic News heard of them and published a story on the newly formed company. This attracted the attention of Exxon Enterprises, Exxon's high-tech investment arm. At the time, in the midst of the recession, there was little venture capital available, with a total of $10 million for the entire industry being spent in all of 1975. Someone from Exxon contacted the still-unnamed company, and arranged a meeting that eventually led to them providing an initial $500,000 funding in June 1975.
With funding being discussed and a design to be built, Masatoshi Shima joined in February 1975. Shima immediately set about producing a high-level design, adding several concepts of his own. In particular, he used his experience on NEC minicomputers to add the concept of two sets of processor registers so they could quickly respond to interrupts. Ungerman began the development of a series of related controllers and peripheral chips that would complement the design.
Through this period, Shima developed a legendary reputation for being able to convert logic concepts into physical design in realtime; while discussing a proposed feature, he would often interrupt and state how much room that would take on the chip and veto its addition if it was too large. The first pass at the design was complete by April 1975. Shima had completed a logic layout by the beginning of May. A second version of the logic design was issued on August 7 and the bus details by September 16. Tape-out was completed in November and converting the tape into a production mask required two more months.
Faggin had already started looking for a production partner. By this time, Synertek and Mostek had both set up the depletion-mode production lines that could be used to produce the design. Having talked to Synertek previously, Faggin approached them first. However, the president of Synertek demanded that the company be given a second source license, allowing them to sell the design directly. Faggin thought this would mean they could never compete even if they set up their own lines, and the agreement fell through. He then turned to Mostek, who agreed to a term of exclusivity while Zilog got their lines set up, and was eventually given the second source agreement.
After considering multiple names for the new company, and finding them so unmemorable they could not recall them even a day later, Faggin and Ungermann were kicking around ideas based on "integrated logic" when Ungermann said, "How about Zilog?" Faggin immediately agreed, stating they could say it was the "last word in integrated logic". When they met the next day and both immediately recalled it, the company had its name.

Into production

The first samples were returned from Mostek on March 9, 1976. By the end of the month, they had also completed an assembler-based development system. Some of the Z80 support and peripheral ICs were under development at this point, and some of them were launched during the following year. Among them were the Z80 CTC, Z80 DMA, Z80 DART, Z80 SIO, and Z80 PIO.
The Z80 was officially launched in July 1976. At the time, concerns existed within the semiconductor industry about the unlicensed reproduction of chip designs by foreign manufacturers. To help deter reverse engineering, the Zilog team incorporated six "traps", transistors that were subtly modified to behave differently than their appearance would suggest. According to Shima, an engineer at NEC later told him that these traps delayed their reverse engineering efforts by six months. NEC later settled a patent infringement dispute with Zilog, securing a license to produce authorized versions of the Z80 and other chips.
Following the successful launch of the Z80, Faggin and Ungermann approached Exxon for funding to establish semiconductor fabrication plant. Exxon agreed, and Zilog built its own production line, enabling the company to capture an estimated 60 to 70 percent of the total market for Z80 sales. Meanwhile, Mostek was authorized to produce a licensed version of the chip, the MK3880, providing a second source for customers, a safeguard that Intel lacked. At the time, second-source agreements were considered essential, particularly for startups like Zilog, which carried a higher risk of business failure and supply disruption.

Comparison with the 8080

Faggin designed the instruction set to be binary compatible with the 8080 so that most 8080 code, notably the CP/M operating system and Intel's PL/M compiler for 8080, 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 multiple improvements over the 8080:
  • An enhanced instruction set including:
  • * a more logical, comprehensible and readable system of assembler instruction mnemonics
  • * more flexible 16-bit data movement instructions, crucially including the stack pointer SP
  • * more flexible addressing modes for input/output to external peripheral ports
  • * single-bit addressing of all registers and memory, including bit testing
  • * shifts/rotates on memory and registers other than the accumulator
  • * subtraction supported for BCD arithmetic
  • * rotate instructions for packed BCD number strings in memory
  • * 16-bit subtraction and 8-bit negation
  • * program looping
  • * program counter relative jumps
  • * block copy, block input/output, and byte search instructions.
  • An overflow flag with better support for signed 8- and 16-bit arithmetics.
  • New IX and IY index registers with base+offset addressing mode
  • 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.
  • * A non-maskable interrupt, which can be used to respond to power-down situations or other high-priority events.
  • A complete duplicate register file, 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.
  • Less hardware required for power supply, clock generation and interface to memory and I/O
  • Single 5-volt power supply.
  • Single-phase 5-volt clock.
  • Built-in DRAM refresh, which would otherwise require external circuitry. The 8080 had been designed before DRAM was widely used, and the common SRAM did not need refresh. By the mid-1970s DRAM had largely replaced SRAM for most roles, but using DRAM with the 8080 required additional external circuitry. Implementing this internally in the Z80 reduced the complexity and cost of a complete system.
  • Non-multiplexed buses.
  • A special reset that zeroes only the program counter, so that a single Z80 CPU could be used in a development system such as an in-circuit emulator.