Sinclair ZX81

The Sinclair ZX81 refined the ZX80 into a smaller and dramatically cheaper mass-market computer. A custom ULA consolidated much of the circuitry, while an expanded 8KB ROM supplied floating-point arithmetic, additional BASIC commands and a slow display mode that remained visible while programs ran.

With only 1KB of built-in RAM, owners commonly added an external 16KB pack and used cassette tapes for storage. More than a million machines were sold, introducing a generation to programming and creating a large market for inexpensive software and peripherals.

Notable Features

• Ferranti ULA reduced the principal chip count to four.
• FAST and SLOW modes traded display stability for processing speed.
• External RAM packs greatly expanded capacity but movement could crash the computer.

Historical Significance

• The first British home computer to sell more than one million units.
• Its low cost and mail-order availability broadened access to computing far beyond established hobbyists.

Collector’s Notes

• Membrane keyboards and expansion connectors often need attention after decades.
• A period 16KB RAM pack is characteristic, although the notorious wobble can corrupt a running program.

Specifications

Technical Information
Name	Sinclair ZX81
Manufacturer	Sinclair Research Ltd
Launched	Thursday, March 5, 1981
Discontinued	3/1/1982
Launch Price	£49.95 kit; £69.95 assembled
Country	United Kingdom
CPU	Zilog Z80A or compatible at 3.25 MHz
Units Sold	More than 1.5 million
Operating System	Sinclair BASIC
I/O	RF television, cassette EAR/MIC and rear expansion bus
CPU Speed	3.25 MHz
RAM	1KB; commonly expanded to 16KB or more
ROM	8KB containing Sinclair BASIC
Display	32 x 24 monochrome characters
Graphics	64 x 48 block graphics
Sound	BASIC BEEP through television speaker
Storage	External cassette recorder at approximately 250 baud
Keyboard	Integrated 40-key membrane keyboard