The eight-bit Z80 is dead. Long live the 16-bit Z80!

Feature The Z80 has a long series of successor models – some compatible and some not. There are multiple options for hobbyist computer builders.

Zilog has announced that it will end production of the original, classic DIP-format, pin-through-hole eight-bit Z80 processor. Although there will be stock in the channel for a while, all the same this is bad news for many hobbyist computer builders… but all is not lost.

The company is still trading 50 years after it was founded. Remarkably, Zilog spent about a decade owned by Exxon. Now, it's owned by the inventor of car blade fuses, Littelfuse, which appeared on the Reg once a couple of years ago when it was slapped for its board makeup.

Even though it rose to fame with a better CPU that was compatible with Intel's 8080, Zilog failed to capitalize on that until it was too late

The Z80 started off as an improved version of the Intel 8080, thanks to which it could run Digital Research's CP/M, which significantly helped the Z80's early success. The memory-resident kernel of CP/M 2.2 is only about 3.5kB in size, and its combination of tiny size and simplicity plus its wealth of software – plus its open source status – are reasons it's a hobbyist favorite today.

What Captain Zilog did next

Zilog's initial efforts to follow on from its big hit were inspired by the Z80 – but, unfortunately for their sales, not compatible with it. The 16-bit Z8000 came out in 1979, designed by the late Dr Bernard Peuto. Before Commodore acquired Hi-Toro to get the Lorraine computer, which became the Amiga, it had an in-house 16-bit workstation, the Z8000-based CBM 900, running an early version of the Mark Williams Company's Unix-like OS Coherent.

The Z8000 was followed in 1986 by a fully 32-bit successor with a fast, fully-pipelined design, the Z80000, a contemporary of Intel's 80386. Although these chips still have admirers, they were unsuccessful – as the Chip Letter blog summarizes in Captain Zilog Crushed!

ZILOG Z8000-CPU - Click to enlarge

Zilog had been working on a compatible 16-bit successor to the Z80, the NMOS-based Z800, which was described in its databooks back in 1983. This was originally planned for release in 1985, but the company was distracted and focused on its higher-end offerings instead.

Enter the compatibles

Zilog was beaten to market in 1985 by Hitachi with the HD64180 [PDF], an enhanced, faster Z80: still an eight-bit part, but with an onboard memory-management unit that could handle up to 1MB of RAM.

Hitachi enjoyed some success with this device: it was used in several CP/M machines, such as the MicroMint SB-180, and as a faster second processor in a handful of MSX computers, such as the Victor HC95 and its single-floppy sibling the HC-90. The chip was also sold as the XLRer go-faster upgrade for the TRS-80 Model 4.

In the end, Hitachi licenced its design back to Zilog as the Z61480, which in 1988 formed the basis of the Z180 [PDF] – and it's still listed [PDF] on the company's site today.

In the meantime, though, Zilog had spotted its mistake, and re-engineered the Z800 into the CMOS-based Z280 which it introduced in 1987. Its manual [PDF] says this had a three-stage pipeline, could access up to 16MB of RAM, and run at up to 25MHz.

Another design derived from the Z800 was ASCII Corporation's R800, designed for the final generation of MSX computers, the MSX Turbo-R. Like the earlier Victor machines, these had two, switchable CPUs: a plain old Z80, plus the partly-16-bit R800, running at 7.159MHz – just twice the Z80's speed, but at four times as many instructions per clock, significantly faster.

These 16-bit designs was followed in 1994 by the 32-bit Z380 [PDF]. Launched the same year as Motorola's $263, 50MHz 68060, the $12 Z280 ran at a modest 18MHz – firmly aimed at the budget market.

Zilog's own last offering in the family was the eZ80 [PDF], launched in 2001. This dropped much of the complexity of the Z280 and Z380, such as multiplexed buses and external cache memory, along with software compatibility – but 24-bit registers meant it was able to directly address up to 16MB of RAM.

The eZ80 is still available to buy as new today – the Z180 is marked end-of-life by Mouser, and the Z280 was phased out years ago – and of course, the Z80 is a simple chip by modern standards and is easy to implement on an FPGA, with several examples out there such as Goran Devic's cycle-accurate one on OpenCores.

Even though it rose to fame with a better CPU that was compatible with Intel's 8080, Zilog failed to capitalize on that until it was too late. When Intel put out its next-gen CPUs, the 8088 and 8086, it was NEC that cashed in with improved versions, its V20 and V30.

Of all the many Z80-based home computers in the 1980s, only one made the leap to a 16-bit version, and then only partially: the last-ever generation of MSX computers. In 1983, the original MSX 1 machines were decent but unremarkable eight-bit boxes. By 1985, the MSX 2 (1985) were very nice eight-bitters indeed, with bank-switching for up to 4MB RAM, and a primitive GPU for good-by-Z80-standards graphics. Most models came with a floppy drives and 128kB RAM. By 1988, the MSX 2+ range were gorgeous. Some could handle about 6MHz speeds, and the GPU had at least 128 kB VRAM, so they had serious video capabilities for eight-bit machines – e.g. 12 to 19 thousand colour graphics. But in 1990, the two Panasonic MSX Turbo R computers were remarkable. Effectively a roughly 30MHz 16-bit CPU, 96kB ROM, 256kB RAM (some battery-backed), a GPU with its own 128kB of VRAM, and stereo sound via multiple sound chips plus MIDI.

Saying that, most of the 6502 computers didn't adopt its 16-bit successor, the WDC 65C816, either: the Apple IIgs, and a rarely-seen successor to the BBC Micro, the Acorn Communicator, plus some accelerators for the C64. The chip is still on sale too, including pre-installed on a developer board.

Apple, Atari, Commodore and Sinclair all moved to the Motorola 68000 family instead, which meant dropping backwards compatibility. That – and the Communicator – are in part why Acorn went its own way with the ARM, which was fast enough to do so – the Acorn ARM machines came equipped with an program that emulated a BBC Micro's 6502 second processor – that is, in an expansion box on the BBC's Tube interface, with no I/O of its own. If your code was clean enough to run on that, you could run it on RISC OS out of the box.

RC2014 Pro - Click to enlarge

And now?

The end of the Z80 is bad news for the RC2014 and other homebrew Z80 machines, but it's not Ragnarök. There is room to continue to explore what the Z80 architecture could have done, if Zilog embraced its early success and tried to build directly upon it from the start. To inspire you, here are some existing home-built Z180 computers, including the SC-111 CPU board for the RC2014 DIY Z80 computer.

There's a Z280 board for the RC2014 as well, the Z280RC with plans on Github, while the ZZ80MB3 is a standalone machine with RC2014 expansion slots. Firmware and OSes for these machines is available thanks to RomWBW.

The most impressive off-the-shelf eZ80 machine we've seen is the Byte Attic Agon series. These are available off-the-shelf in several versions, such as the €50 (£43, $53) AgonLight2 from Olimex, and there's copious info on its own wiki. ®

Bootnote

Yes, Captain Zilog was real. Well, as real as any superhero, anyway: he was part of a Zilog marketing campaign from the end of the 1970s.