Work on the ZX81 hardware began in September 1979, even before the launch of the ZX80, but it was the development of the uncommitted logic array, or ULA, which allowed the machine to go into production. The ULA, produced by Ferranti for Sinclair, reduced the chip count and brought the retail cost of the machine, in kit-form, down to £49.95.
As with the ZX80, Nine Tiles was called upon to provide the new BASIC, but this time there was 8K to play with. Steve Vickers, who had joined Nine Tiles in January 1980, added the floating-point arithmetic, including trigonometric and other functions. "As far as Clive was concerned, it wasn't a question of what the machine ought to be able to do, but more what could be crammed into the machine given the component budget he'd set his mind on," said Vickers in an interview on July 23, 1985. "The only firm brief for the '81 was that the '80's math package must be improved." The ROM was almost complete by the end of autumn 1980, but support still had to be added for the ZX Printer. Somewhere between this time and the launch, a bug crept in which caused the square root of 0.25 to be 1.3591409. Nine Tiles quickly fixed the bug, but Sinclair was somewhat tardy in making this version available to people who had already bought the machine.
The Sinclair ZX81 replaced the ZX80 in February 1981. This new machine now provided SLOW and FAST mode, which meant the ZX81 was capable of executing a program and writing to the screen at the same time (SLOW mode). The ZX80 only had FAST mode which could not do this. The 4K ROM included 30 new functions plus some new instructions to control a printer.
Because of better componentisation, the ZX81 was able to be sold for £30 less than the outgoing ZX80.
The ZX81 was well received and became a massive success, spawning a series of clones, both illegal and licensed by Timex, which was manufacturing the UK models for Sinclair at its Dundee plant. Inspired by the public reaction to the ZX81, and annoyed at not winning the contract to design a computer for the British Broadcasting Corporation, Sinclair decided the market needed a budget colour computer.
The ZX80 and ZX81 hardware had been the primarily the work of one man; Jim Westwood, but he had been moved to the flat-screen television department, so the hardware design job on the machine which became the ZX Spectrum, was given to Richard Altwasser, while at Nine Tiles, Vickers was again asked to provide the BASIC for the Spectrum.
The US version of the ZX81 was marketed by Timex-Sinclair, called the TS1000. This varied only slightly, with 2K RAM instead of 1K, a shielded case, TV output on channels 3 or 4, and the screen was black on light blue background, instead of a white background.
15 May 2016New 1K Hi-Res ZX81 games
Johan "Dr Beep" Koelman's been in touch, informing us of a bunch of new 1K Hi-resolution ZX81 games he's been working on.
15 February 2016Smallest FIDE-rules Chess game for ZX81
Just released for ZX81 enthusiasts is a new chess program - one that is able to play full FIDE chess with the base 1K hardware!
26 October 2015ZXzine Issue 2 out now
Issue 2 of Tim Swenson's new e-zine, ZXzine, is now available for download.
This page was last updated on 22nd April 2014.