Games - Sinclair ZX81

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	Games - Sinclair ZX81

The Sinclair ZX81 home computer, released by Sinclair Research in 1981, was the follow up to the company's ZX80. The case was black, with a membrane keyboard; 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 magnetic audio tapes. It was very significant historically because it was the first home computer under $100 USD (in kit form), and therefore was a big volume seller. A US-only version was produced by Timex as the "Timex Sinclair 1000".

General description

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 a custom chip and now had only four chips: a Z80A microprocessor, a custom logic chip (Ferranti ULA) or ASIC, a 4118 1Kx8 bit RAM chip and a 2364 8Kx8 bit ROM chip. 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. However blocky graphics with a resolution of 64 by 48 pixels were possible by the use of the PLOT command, which ingeniously selected among 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 followed by the code for a new line, the rest of the line being automatically assumed to be spaces. Using this knowledge, it was common to write programs that kept to the top left of the screen to save memory. As another 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 ($100 in the US). 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 spark printer (although it was sometimes misleadingly called a "thermal printer") in which a wire point sparked the dot pattern into 4-inch-wide silvery-grey aluminised paper, accompanied by a distinct odor of ozone.

Even so, there were many games and applications that run in the minimalistic 1 K, including a basic game of Chess. It was not that hard to get to know, understand, and control the computer completely, something almost impossible today.

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).

DK'tronics sold a case and keyboard which, with considerable skill, could be used to replace the membrane keyboard and black "doorstop" case.

In the ZX80 and ZX81, the video output was generated by the Z80 chip. In the ZX80, when a program ran the screen blanked until the program paused again for input. An improvement of the ZX81 over the ZX80 was that the ZX81 had two modes of operation. The ZX81 could run in FAST mode like the ZX80, blanking while programs ran, or in SLOW mode (approximately 1/4 as fast) in which the video was maintained since programs ran only while the TV's electron gun moved from the bottom to the screen back to the top. Since a FOR-NEXT loop from 1 to 1000 took 19 seconds, it was common to run the machine in FAST all the time, even when editing a program, causing the TV to flash every time a key was pressed into the editor.

Another trait of the ZX81 was that it echoed the signal from the tape recorder to the screen whilst loading and saving programs using cassettes, causing the TV to display zigzagging patterns.

The ZX81 did not have the ability to make sound, but by clever coding it was possible to modulate the interference that the processor caused on the TV and create a VERY simple musical keyboard.

The ZX81 did not use ASCII but had its own character set. Character code 0 was space, codes 1-10 were used for blocky graphics, codes 11-63 corresponded to punctuation, numbers and upper case characters. Character codes 128-191 were reverse video versions of the first 64 characters. Other codes represented BASIC keywords and control codes such as NEWLINE. There were no lower case characters.

[ Visit the complete Wikipedia entry for Sinclair ZX81 ]

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