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– £ in ). | discontinued = 1984 | unitssold = More than 1.5 million〔Sinclair Research website〕 | unitsshipped = | media = | os = Sinclair BASIC〔 | power = 9V DC〔 | cpu = Z80 at 3.25 MHz〔 | storage = External cassette tape recorder at 250 baud〔 | memory = 1 KB (64 KB max. 56 KB usable)〔 | display = Monochrome display on UHF television〔 | graphics = 24 lines x 32 characters or 64 x 48 pixels graphics mode〔 | sound = | input = | controllers = | camera = | touchpad = | connectivity = | service = | dimensions = deep by high〔 | weight = 〔 | topgame = | compatibility = | predecessor = ZX80 | successor = ZX Spectrum | related = Timex Sinclair 1000, Timex Sinclair 1500 | website = }} The ZX81 is a home computer produced by Sinclair Research and manufactured in Scotland by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair's ZX80 and was designed to be a low-cost introduction to home computing for the general public. It was hugely successful and more than 1.5 million units were sold before it was eventually discontinued. The ZX81 found commercial success in many other countries, notably the United States, where it was initially sold as the ZX-81. Timex manufactured and distributed it under licence and enjoyed a substantial but brief boom in sales. Timex later produced its own versions of the ZX81 for the US market – the Timex Sinclair 1000 and Timex Sinclair 1500. Unauthorised clones of the ZX81 were produced in a number of countries. The ZX81 was designed to be small, simple, and above all cheap, using as few components as possible to keep the cost down. Video output was to a television set rather than a dedicated monitor. Programs and data were loaded and saved onto audio tape cassettes. It had only four silicon chips on board and a mere 1 KB of memory. The machine had no power switch or any moving parts (with the exception of a VHF TV channel selector switch present on early "ZX81 USA" models and the Timex-Sinclair 1000), and used a pressure-sensitive membrane keyboard for manual input. The ZX81's limitations prompted the emergence of a flourishing market in third-party peripherals to improve its capabilities. Such limitations, however, achieved Sinclair's objective of keeping the cost of the machine as low as possible. Its distinctive design brought its designer, Rick Dickinson, a Design Council award. The ZX81 could be bought by mail order in kit form or pre-assembled. In what was then a major innovation, it was the first cheap mass-market home computer that could be bought from high street stores, led by W.H. Smith and soon many other retailers. The ZX81 marked the first time that computing in Britain became an activity for the general public, rather than the preserve of businesspeople and electronics hobbyists. It inspired the creation of a huge community of enthusiasts, some of whom founded their own businesses producing software and hardware for the ZX81. Many went on to play a major role in the British computer industry in later years. The ZX81's commercial success made Sinclair Research one of Britain's leading computer manufacturers and earned a fortune and an eventual knighthood for the company's founder, Sir Clive Sinclair. ==Features== The ZX81 came with 1 KB of on-board memory that could officially be expanded externally to 16 KB. Its single circuit board was housed inside a wedge-shaped plastic case measuring deep by high. The memory was provided by either a single 4118 (1024 bit × 8) or two 2114 (1024 bit × 4) RAM chips. There were only three other chips on board: a 3.5 MHz Z80A 8-bit microprocessor from NEC, an uncommitted logic array (ULA) chip from Ferranti and an 8 KB ROM providing a simple BASIC interpreter. The entire machine weighed just .〔''ZX81 Operating Supplement'' (1982)〕 Early versions of the external RAM cartridge contained 15 KB of memory using an assortment of memory chips, while later versions contained 16 KB of chips but the lowest addressed kilobyte was disabled. The front part of the case is occupied by an integrated 40-key membrane keyboard displaying 20 graphic and 54 inverse video characters.〔 Each key has up to five functions, accessed via the SHIFT and FUNCTION keys or depending on context. For example, the P key combined the letter 'P', the " character and the BASIC commands PRINT and TAB. The ZX81 uses a standard QWERTY keyboard layout. The keyboard is mechanically very simple, consisting of 40 pressure-pad switches and eight diodes under a plastic overlay, connected in a matrix of eight rows and five columns. The ZX81's primary input/output is delivered via four sockets on the left side of the case. The machine uses an ordinary UHF television set to deliver a monochrome picture via a built-in RF modulator. It can display 24 lines of 32 characters each, and by using the selection of 2x2 block character graphics from the machine's character set offered an effective 64 × 44 pixel graphics mode, also directly addressable via BASIC using the PLOT and UNPLOT commands, leaving 2 lines free at the bottom. Two 3.5 mm jacks connect the ZX81 to the EAR (output) and MIC (input) sockets of an audio cassette recorder, enabling data to be saved or loaded at a rate of 50 baud. This provides a somewhat temperamental storage medium for the machine, which has no built-in storage capabilities. The ZX81 requires 420 mA of power at 7–11 V DC, delivered via a custom 9 V Sinclair DC power supply.〔 The ULA chip, described by the ZX81 manual as the "dogsbody" of the system, has a number of key functions that competing computers shared between multiple chips and integrated circuits. These comprise: * Synchronising the screen display; * Generating a 6.5 MHz clock, from which a 3.25 MHz clock is derived for the processor; * Outputting an audio signal to a cassette recorder in SAVE mode; * Processing the incoming cassette audio signal in LOAD mode; * Sensing keystrokes; * Using memory addresses provided by the CPU to decide when ROM and RAM should be active; * Controlling general system timing. The ZX81's built-in RF modulator can output a video picture to either a UHF 625-line colour or monochrome television (used in the UK, Australia, and most western European countries). France required a slightly modified version of the machine to match the positive video modulation of SECAM sets while the USA and Canada required a different ULA chip and modulator to cope with their 525-line VHF (NTSC) television systems. Both the ZX81 and its predecessor, the ZX80, have a significant drawback in the way that they handle visual output. Neither machine has enough processing power to run at full speed and simultaneously maintain the screen display. On the ZX80, this means that the screen goes blank every time the machine carries out a computation and causes an irritating flicker whenever a shorter computation – such as processing a keystroke – takes place.〔''New Scientist'' (7 February 1980)〕 The ZX81's designers adopted an improved approach, involving the use of two modes called SLOW and FAST respectively. In SLOW mode, also called "compute and display" mode, the ZX81 concentrates on driving the display. It runs the current program for only about a quarter of the time – in effect slowing the machine down fourfold, although in practice the speed difference between FAST and SLOW modes depends on what computation is being done.〔For comparisons of processing speed in the two modes, see "ZX81 v IBM PC", ''Which Micro?'', April 1983, p. 36〕 In FAST mode processing occurs continuously but the display is abandoned to its own devices – equivalent to the ZX80's standard operating mode. Another hardware quirk produced one of the most distinctive aspects of the ZX81's screen display – during loading or saving, moving zigzag stripes appear across the screen. The same pin on the ULA is used to handle the video signal as well as the tape output, producing the stripes as an interference pattern of sorts. The ULA cannot maintain the display during SAVE and LOAD operations, as it has to operate continuously to maintain the correct baud rate for data transfers. The interference produces the zigzag stripes. The unexpanded ZX81's tiny memory presented 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 were able to achieve a surprising amount with just 1 KB. One notable example was ''1K ZX Chess'' by David Horne, which actually managed to squeeze most of the rules of chess into only 672 bytes. The ZX81 conserved 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 made use of this facility to create a wide range of add-ons for the ZX81. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ZX81」の詳細全文を読む スポンサード リンク
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