Sinclair ZX81

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Sinclair ZX81 1 Sinclair ZX81 ¹

The Sinclair ZX81, a hobbyist home computer released in the UK in 1981, photo by Evan-Amos, January 31, 2012, Wikimedia Commons ↩︎

Sinclair ZX81, (ZX81, Timex Sinclair 1000)
a low-cost home computer developed by Sinclair Research and manufactured in Scotland by Timex Corporation, launched in the UK in March 1981, and as slightly modified Timex Sinclair 1000 in the US in July 1982. The ZX81 consists of a 3.5 MHz Z80A CPU from Nippon Electric Company, an ASIC dubbed Sinclair Computer Logic, an 8 KiB ROM with the operating system, the Sinclair BASIC interpreter, and one KiB of RAM, which could be expanded externally to 16 KiB. For I/O it had a membrane keyboard, a RF modulator to output 24 lines x 32 characters or 64x48 pixel as signal for a television, and a serial port and softmodem ¹, to store and load programs and data to or from an external audio cassette recorder with 250 baud ². Jim Westwood's technical dodge using non-maskable interrupts solved the flicker problem of the ZX80 and gave the ZX81 a "multi-tasking" SLOW mode with a steady display ³, slowing down programs fourfold ⁴. Z80 machine code could be inlined at absolute addresses and called via USR ⁵ or encoded in comment lines ⁶, which could be executed after some appropriate POKE instructions.

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Chess Programs

The only chess program for the non expanded ZX81 in SLOW mode was 1K ZX Chess by David Horne. Other programs, such as Artic Computing's ZX Chess, and Chris Whittington's SuperChess, require the 16 KB memory expansion.