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Sinclair ZX81

The Sinclair ZX81 home computer, released by Sinclair Research in 1981, was the followup to the company's ZX80. The case was black, with a membrane keyboard; video output, as in the ZX80, was to a television set, and saving and loading programs was via an ordinary cassette recorder to compact audio cassettes. As with the ZX80, the processor was a NEC Zilog Z80-compatible, only this time of the slightly higher clock rate of 3.5 MHz. The system board had been redesigned with custom chips and now had only four chips. The system ROM had grown to 8 KB 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 in the UK or $100 in the US) had 1KB (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 stored 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. As a memory-saving feature, BASIC keywords were stored as 1-byte tokens; if memory grew short, the number of lines displayed on the TV screen would be reduced.

Even with all these space saving measures the little machine's memory did not go very far, so an expansion pack was available with 16K of RAM. By mid-1982, 32K and 64K expansion packs were available. These plugged onto the main circuit board (and the 16K Memopak could be "stacked" with a 16K or 32K one) and were 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 odor of ozone.

There were also an RS-232 serial interface (at ~$140) and a Centronics parallel interface (at ~$105) that would allow the ZX81 to communicate to a standard printer, as well as a full-sized external keyboard (at ~$85).

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 1/4 as fast) in which the video refresh was maintained while programs ran in whatever spare machine cycles remained (hence the slow-down in program speed).

There was a notorious bug causing some ZX81s to give the square root of 0.25 as 1.3591409 rather than 0.5. Sinclair's reputation for poor quality control was due less to the existence of the bug in some machines, and more to the time it took to react once the bug had been reported.

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

The Sinclair ZX81 was sold in the U.S. by Sinclair itself (from its facility in Nashua, New Hampshire) and also by Timex as the Timex-Sinclair TS1000.