The Sinclair ZX81 was a home computer released in 1981 by Sinclair Research. It was the follow-up to the Sinclair ZX80.
The machine's distinctive appearance was the work of industrial designer Rick Dickinson. Video output, as in the ZX80, was to a television set, and saving and loading programs was via an ordinary home audio tape recorder to audio cassette. Like its predecessor it used a membrane keyboard.
Timex Corporation manufactured kits as well as assembled machines for Sinclair Research. In the United States a version with double the RAM and an NTSC television standard was marketed as the Timex Sinclair 1000.
As with the ZX80, the processor was a NEC Zilog Z80-compatible, running at a clock rate of 3.25 MHz, but the system ROM had grown to 8192 bytes in size, and the BASIC now supported floating point arithmetic. It was an adaptation of the ZX80 ROM by Steve Vickers on contract from Nine Tiles Ltd, the authors of Sinclair BASIC. The new ROM also worked in the ZX80 and Sinclair offered it as an upgrade for the older ZX80 for a while.
The base system as supplied had 1 KB (KB) of RAM. This RAM was used to hold the computer's system variables, the screen image, and any programs and data. The screen was text only, 32 characters wide by 24 high. Blocky graphics with a resolution of 64 by 48 pixels were possible by the use of the PLOT command, which selected among a set of 16 graphics characters. The ZX81 uses a resizable display-file (screen buffer) meaning that it can be expanded or shrunk depending on the amount of installed memory and the amount of free space at the moment.
The ZX81 was originally sold via mail order in kit form requiring soldering  (priced at £49.95) or assembled (£69.95 or US$100 in the US). A later deal with high street retail W.H.Smith saw the ZX81 and all accessories being sold on the high street (ZX81 was £69.99, ZX 16K RAM pack £49.99, ZX Printer £49.99)