View source for ZX80/81

[[File:Sinclair ZX80.jpg|thumb|right|Sinclair ZX80]]
[[File:Sinclair-ZX81.png|thumb|right|Sinclair ZX81]]
[[File:Timex Sinclair 1500.png|thumb|right|Timex Sinclair 1500]]

The Sinclair ZX80 and ZX81, introduced in 1980 and 1981, are early budget microcomputers notable for their price, fleeting popularity and diminutive quantity of components.

Both machines are [[Z80]]-based and not only task the CPU with painting the display in the traditional sense of populating a display area, but through a shim on the bus reuse the Z80's program and refresh counters as display counters. While either machine is outputting pixels its CPU is usually in what looks to it like an unrolled loop of NOPs, with the support circuits piggybacking on its fetch cycle to obtain video memory contents and its refresh cycle to track location within the current scan line.

This and the ZX80's lack of a mechanism to provide a hard, fixed resynchronisation of the CPU and the display beam explains one of its most obvious pecularities: the display is stable only unless and until the user types, since it can just repeat an exactly-counted fixed length loop. If the user types it branches into code that depends on which key they pressed, causing display generation temporarily to break up. On most TV's this was evident as the display jumping and bouncing, since it interrupts the sync feed.

The ZX81 adds both an [[NMI]] generator and a WAIT generator that can allow the processor to get back into synchronisation after doing other work. The ZX81 therefore can offer a steady display but its processor is still available for non-display processing only during the vertical blank. Hence its stable display mode is known as 'SLOW' mode, and its ZX80-esque mode is 'FAST'.

The ZX81 also marks a substantial development in the machine's underlying BASIC, making it very similar to that which would ship a year later in the [[ZX Spectrum]].

Despite originally being intended as text-mode machines, various approaches have subsequently been discovered that allow full-framebuffer display from most RAM expansion packs.

A substantial disadvantage of the design of these machines is that because the refresh cycle is commandeered for other purposes, they're built around an assumption of static RAM. As a result, each shipped with only 1kb at launch and expansions beyond 16kb were rare.

PAL/SECAM compatibility was not an issue on these machines as they could only output black-and-white video signals anyway. Maybe that fooled Lord Sinclair into neglecting SECAM on its following machine, the [[ZX Spectrum]].

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== Clones ==

Timex Sinclair was a joint venture established between Sinclair Research and Timex Corporation in an effort to gain an entry into the rapidly growing early-1980s home computer market in North America. They produced 2 modified versions of the ZX81:
*TS 1000, introduced in July 1982 and essentially a modified ZX81 with 2 KB RAM.
*TS 1500, introduced in August 1983, it was a TS 1000 with 16 KB RAM and a ZX Spectrum-like case and keyboard.

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== Links ==

*[https://youtu.be/DElx9pk2oks ZX81 advertisements]
*[https://www.timexsinclair.com/ Timex Sinclair - The Authoritative Website]
*[http://searle.x10host.com/zx80/zx80.html Grant's Sinclair ZX80 Homebuilt hardware]
*[https://oldcomputer.info/8bit/zx81/ULA/ula.htm ZX81 ULA replacement] [[Media:Esquemas ZX81 ULA Andy Rea printable.pdf|ZX81 Project]] [[Media:ZX81 Complete printable.pdf|ZX81 Complete]]
*[https://archive.org/details/TimexSinclairEphemera/Zx81AssemblyManual/mode/2up ZX81 Assembly Instructions]
*[https://8bit-museum.de/heimcomputer-2/sinclair/sinclair-scans/scans-zx81-video-display-system/ ZX81 Video Display System]
*[https://problemkaputt.de/zxdocs.htm Noca$h's ZXdocs]
*[https://youtu.be/1Jr7Q1yJOUM Documentary - The Sinclair ZX80, ZX81, and Timex Sinclair 1000] by [[The 8-Bit Guy]]
*[https://youtu.be/spxAQkTzvDQ Sinclair ZX81 (Timex 1000) Grandaddy of Computers] by [[Nostalgia Nerd]]
*[https://youtu.be/qDorGWU1k3A Sinclair ZX81 - revolutionary or scam ?] [https://youtu.be/6eTGX_sEqPM ZXPAND+ ZX81's lethal weapon] [https://youtu.be/kTw8uo-53lc TOP 20 games for the Sinclair ZX81] by [[Olipix Retrotech]]
*[https://youtu.be/Liqd7ZoBVEI ZX80 - Histoire & Jeux] [https://youtu.be/GjHgIpHPgq4 Le mystère de l'ordinateur en bois] by [[Olipix]]

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[[Category:Non CPC Computers]]