Wikipedia: ZX81

The Sinclair ZX81 home computer was the followup to the ZX80, released in 1981. The case was black, with a membrane keyboard; video output, like the ZX80 was to a television, saving and loading programs was via a cassette recorder on audio cassettes. As with the ZX80, the processor was a Zilog Z80; The system board had been redesigned with custom chips, and now had only four chips. The system ROM had grown to 8K in size, and the BASIC now supported floating point arithmetic; in the early days, Sinclair offered the ROM as an upgrade for the ZX80.

The base system as supplied (for approximately 70 UK Pounds) had 1K (yes, 1024 bytes) 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; limited, blocky graphics were possible via a set of 16 graphics characters. To conserve memory, the screen bytes were kept as minimal length strings; for example, if a screen line was only 12 characters long, it would be stored as only those 12 characters, the rest of the line being automatically assumed to be spaces. BASIC keywords were stored as 1-byte tokens; if memory grew short, the number of lines displayed on screen would be reduced.

Even with all these space saving measures, 1024 bytes did not go very far, so an expansion pack was available with 16K of RAM. This plugged onto the main circuit board, and was notorious for wobbling and losing the results of hours of programming. A printer was marketed to accompany the ZX81: this was a thermal printer in which a wire point sparked the dot pattern into 4 inch wide silvery-grey thermal paper, accompanied by a distinct odour of ozone.

An improvement of the ZX81 over the ZX80 was that the ZX81 now had two modes of operation. In the ZX80, the video output was generated by the Z80 chip, so when a program actually ran the screen blanked until the program paused again for input. The ZX81 improved on this, so that the ZX81 could run in fast mode like the ZX80, blanking while programs ran, or the slow mode (approximately 4x slower) in which the video refresh was maintained while programs ran in whatever spare machine cycles remained (hence the slow down in program speed).

The ZX81 sold in large numbers, until replaced by its greatly upgraded successor, the ZX Spectrum

The sinclair ZX81 was sold in the USA by Timex as the TS1000?