transfers. The interference produces the zigzag stripes. The unexpanded ZX81's tiny memory presents a major challenge to programmers. Simply displaying a full screen takes up to 793 bytes, the system variables take up another 125 bytes, and the program, input buffer and stacks need more memory on top of that. Nonetheless, ingenious programmers are able to achieve a surprising amount with just 1 KB. One example is the Super Micro Chess Engine by Stefano Maragò, which includes all of the rules of chess. The ZX81 conserves its memory to a certain extent by representing entire BASIC commands as one-byte tokens, stored as individual "characters" in the upper reaches of the machine's unique (non-ASCII) character set. The edge connector or external interface at the rear of the ZX81 is an extension of the main printed circuit board. This provides a set of address, control, and data lines that can be used to communicate with external devices. Enthusiasts and a variety of third-party companies make use of this facility to create a wide range of add-ons for the ZX81. Comparisons with other computing devices The following table provides a comparison between the capabilities of the ZX81 and various other competing microcomputers that were available in June 1981, about the time that the first ZX81 orders were delivered. The prices given are as of June 1981 from Your Computer UK. N.B.: Commodore PET includes monitor and cassette recorder. The Video Genie includes a built in cassette recorder. The Compukit 101 is an Ohio Scientific SuperBoard made under licence in Britain. Compukit 101 and MicroTan are kit computers and are not supplied with a case. The Atari 400 and TRS-80 model I were officially only expandable to 16 KB, but expansion to 48 KB was available from third party suppliers. The ZX81 and the other systems referred to as having block graphics have a resolution of 256 × 192 pixels, there are 32 columns × 22 rows of text and graphics can be addressed in quarter character blocks. History Background Clive Sinclair, a former radio kit salesman, established his first company, Sinclair Radionics, in 1962. The company made its name producing a wide range of cheap electronics aimed at the hobbyist market. Its products include amplifiers, radios, multimeters and other items which were generally sold in kit form to hi-fi enthusiasts and other electronics hobbyists. The company entered a new market in 1972 when