ZX Spectrum

An issue 2 1982 ZX Spectrum
Developer	Sinclair Research
Type	Home computer
Generation	8-bit
Release date	UK: 23 April 1982 (23 April 1982)[1] US: 1983 (1983) EU: 1985 (1985)
Introductory price	UK: £125 (16 KB) (equivalent to £557 in 2023) / £175 (48 KB) (equivalent to £780 in 2023) ,[2] US: $200, ESP: Pta44,250
Discontinued	1992[3]
Units sold	5 million[4]
Media	Compact Cassette, ZX Microdrive, 3-inch floppy disk on Spectrum +3
Operating system	Sinclair BASIC
CPU	Z80A (or equivalent) @ 3.5 MHz
Memory	16 KB / 48 KB / 128 KB (IEC: KiB)
Display	PAL RF modulator out, 256 × 192, 15 colours
Graphics	ULA
Sound	Beeper, AY-3-8912 (128K models)
Predecessor	ZX81
Successor	QL, Sinclair PC200

The ZX Spectrum (UK: /zɛd ɛks/) is an 8-bit home computer developed and marketed by Sinclair Research. Considered one of the most influential computers ever made, it is also one of the best-selling British computers ever, with over five million units sold. It was released in the United Kingdom on 23 April 1982, and around the world in the following years, most notably in Europe, the United States, and Eastern Bloc countries.

The machine was designed by English entrepreneur and inventor Sir Clive Sinclair and his small team in Cambridge, and was manufactured in Dundee, Scotland by Timex Corporation.^[5] It was made to be small, simple, and most importantly inexpensive, with as few components as possible. The addendum "Spectrum" was chosen to highlight the machine's colour display, which differed from the black-and-white display of its predecessor, the ZX81. Rick Dickinson designed its distinctive case, rainbow motif, and rubber keyboard. Video output is transmitted to a television set rather than a dedicated monitor, while application software is loaded and saved onto compact audio cassettes.

The ZX Spectrum was initially distributed by mail order, but after severe backlogs it was sold through High Street chains in the United Kingdom. It was released in the US as the Timex Sinclair 2068 in 1983, and in some parts of Europe as the Timex Computer 2048. Ultimately the Spectrum was released as seven different models, ranging from the entry level with 16 KB RAM released in 1982 to the ZX Spectrum +3 with 128 KB RAM and built-in floppy disk drive in 1987. Throughout its life, the machine primarily competed with the Commodore 64, BBC Micro, Dragon 32, and the Amstrad CPC range. Over 24,000 different software products were released for the ZX Spectrum.^[1]

The Spectrum played a pivotal role in the early history of personal computing and video gaming, leaving an enduring legacy that influenced generations. Its introduction led to a boom in companies producing software and hardware, the effects of which are still seen. It was among the first home computers aimed at a mainstream audience, with some crediting it as responsible for launching the British information technology industry. The Spectrum is one of the best-selling British computers of all time, retaining the title of Britain's top-selling computer until the Amstrad PCW surpassed it in the 1990s.^[6][7][8] It was discontinued in 1992.

The ZX Spectrum was conceived and designed by engineers at Sinclair Research, founded by English entrepreneur and inventor Clive Sinclair, who was well known for his eccentricity and pioneering ethic.^[9] On 25 July 1961, three years after passing his A-levels, he founded Sinclair Radionics Ltd as a vehicle to advertise his inventions and buy components.^[10] In 1972, Sinclair had competed with Texas Instruments to produce the world's first pocket calculator, the Sinclair Executive.^[11] By the mid 1970s, Sinclair Radionics was producing handheld electronic calculators, miniature televisions, and the ill-fated digital Black Watch wristwatch.^[12] Due to financial losses, Sinclair sought investors from the National Enterprise Board (NEB), who had bought a 43% interest in the company and streamlined his product line. Sinclair's relationship with the NEB had worsened, however, and by 1979 it opted to break up Sinclair Radionics entirely,^[13] selling off its television division to Binatone and its calculator division to ESL Bristol.^[14]

After incurring a £7 million investment loss, Sinclair was given a golden handshake and an estimated £10,000 severance package.^[11][15] He had a former employee, Christopher Curry, establish a "corporate lifeboat" company named Science of Cambridge Ltd, in July 1977, called such as they were located near the University of Cambridge.^[16] By this time inexpensive microprocessors had started appearing on the market, which prompted Sinclair to start producing the MK14, a computer teaching kit which sold well at a very low price.^[17] Encouraged by this success, Sinclair renamed his company to Sinclair Research, and started looking to manufacture personal computers. Keeping the cost low was essential for Sinclair to avoid his products from becoming outpriced by American or Japanese equivalents as had happened to several of the previous Sinclair Radionics products.^[11] On 29 January 1980, the ZX80 home computer was launched to immediate popularity; notable for being one of the first computers available in the United Kingdom for less than £100.^[18][19] The company conducted no market research whatsoever prior to the launch of the ZX80; according to Sinclair, he "simply had a hunch" that the public was sufficiently interested to make such a project feasible and went ahead with ordering 100,000 sets of parts so that he could launch at high volume.^[20]

On 5 March 1981, the ZX81 was launched worldwide to immense success with more than 1.5 million units sold,^[21] 60% of which was outside Britain.^[22] According to Ben Rosen, by pricing the ZX81 so low, the company had "opened up a completely new market among people who had never previously considered owning a computer".^[23] After its release, computing in Britain became an activity for the general public rather than the preserve of office workers and hobbyists. The ZX81's commercial success made Sinclair Research one of Britain's leading computer manufacturers, with Sinclair himself reportedly "amused and gratified" by the attention the machine received.^[24]

Development of the ZX Spectrum began in September 1981, a few months after the release of the ZX81. Sinclair resolved to make his own products obsolete before his rivals developed the products that would do so. Parts of designs from the ZX80 and ZX81 were reused to ensure a speedy and cost-effective manufacturing process. The team consisted of 20 engineers housed in a small office at 6 King's Parade, Cambridge.^[25] During early production, the machine was known as the ZX81 Colour or the ZX82 to highlight the machine's colour display, which differed from the black and white of its predecessors. The addendum "Spectrum" was added later on, to emphasise its 15-colour palette.^[26] Aside from a new crystal oscillator and extra chips to add additional kilobytes of memory,^[27] the ZX Spectrum was intended to be, as quoted by Sinclair's marketing manager, essentially a "ZX81 with colour".^[25] According to Sinclair, the team also wanted to combine the ZX81's separate random-access memory sections for audio and video into a single bank.^[28]

Chief engineer Richard Altwasser was responsible for the ZX Spectrum's hardware design. His main contribution was the design of the semi-custom uncommitted logic array (ULA) integrated circuit, which integrated, on a single chip, the essential hardware functions. Altwasser designed a graphics mode that required less than 7 kilobytes of memory and implemented it on the ULA. Vickers wrote most of the ROM code. Lengthy discussions between Altwasser and Sinclair engineers resulted in a broad agreement that the ZX Spectrum must have high-resolution graphics, 16 kilobytes of memory, an improved cassette interface, and an impressive colour palette.^[29] To achieve this, the team had to divorce the central processing unit (CPU) away from the main display to enable it to work at full efficiency – a method which contrasted with the ZX81's integrated CPU.^[29] The inclusion of colour to the display proved a major obstacle to the engineers. A Teletext-like approach was briefly considered, in which each line of text would have colour-change codes inserted into it. However, this was ruled out, as it was deemed unsuitable for high-resolution graphs or diagrams that involved multiple colour changes. Altwasser devised the idea of allocating a colour attribute to each character position on the screen. This ultimately used eight bits of memory for each character position; three bits to provide any one of eight foreground colours and three bits for the eight background colours, one bit for extra brightness and one bit for flashing. Overall, the system took up slightly less than 7 kilobytes of memory, leaving an additional 9 kilobytes to write programs — a figure that pleased the team.^[30]

Much of the firmware was written by computer scientist Steve Vickers from Nine Tiles,^[28] who compiled all control routines to produce the Sinclair BASIC interpreter, a custom variant of the general purpose BASIC programming language. Making a custom interpreter made it possible to fit all of its functionality into a very small amount of read-only memory (ROM).^[27] The development process of the software was marked by disagreements between Nine Tiles and Sinclair Research. Sinclair placed an emphasis on expediting the release of the Spectrum, primarily by minimising alterations in the software from the ZX81, which had in turn been based on the ZX80's software. The software architecture of the ZX80, however, had been tailored for a severely constrained memory system, and in Nine Tiles' opinion was unsuitable for the enhanced processing demands of the ZX Spectrum. Sinclair favoured solving this with expansion modules on the existing framework like with the ZX81, which Nine Tiles disagreed with. Ultimately, both designs were developed, but Vickers and Nine Tiles were unable to finish their version before the launch of the Spectrum and it was not used.^[31]

The distinctive case and colourful design of the ZX Spectrum was the creation of Rick Dickinson, a young British industrial designer who had been hired by Sinclair to design the ZX81. Dickinson was tasked to design a sleeker and more "marketable" appearance to the new machine, whilst ensuring all 192 BASIC functions could fit onto 40 physical keys.^[25] Early sketches from August 1981 showed the case was to be more angular and wedge-like, in similar vein to an upgraded ZX81 model. Dickinson later settled on a flatter design with a raised rear section and rounded sides in order to depict the machine as "more advanced" as opposed to a mere upgrade. In drawing up potential logos, Dickinson proposed a series of different logotypes which all featured rainbow slashes across the keyboard.^[32]

The design of the Spectrum's rubber keyboard was simplified from several hundred components to a conventional moving keyboard down to "four to five" moving parts using a new technology.^[33] The keyboard was still undergoing changes as late as February 1982; some sketches included a roundel-on-square key design which was later featured on the later Spectrum+ model.^[32] Dickinson recalled in 2007 that "everything was cost driven" and that the minimalist, Bauhaus approach to the Spectrum gave it an elegant yet "[non] revolutionary" form.^[3] The drawing board on which Dickinson designed the ZX Spectrum is now on display in the Science Museum in London.^[34][35]

The need for an improved cassette interface was apparent from the number of complaints received from ZX81 users, who encountered problems when trying to save and load programs.^[36] To increase the data transfer speed, the team significantly decreased the length of tones that represent binary data.^[36] To increase the reliability, a leading period of constant tone was introduced, which allowed the cassette recorder's automatic gain control to settle itself down, eliminating hisses on the tape.^[36] A Schmitt trigger was added inside the ULA to reduce noise of the received signal.^[36] Originally, the team aimed for data transfer speed of 1000 baud, but succeeded in getting it to work at a considerably faster 1500 baud.^[36] Unlike the ZX81, the Spectrum was able to maintain its display during loading and saving operations, and programmers took advantage of this to show a splash screen whilst loading took place in the background.

As with the ZX81, the ZX Spectrum was manufactured in Dundee, Scotland, by Timex Corporation at the company's Dryburgh factory.^[5][37] Prior to the manufacture of the ZX81, however, Timex had little experience in assembling electronics and had not originally been an obvious choice of manufacturing subcontractor. It was a well-established manufacturer of mechanical watches but was facing a crisis at the beginning of the 1980s; profits had dwindled to virtually zero as the market for watches stagnated in the face of competition from the digital and quartz watches. Recognising the trend, Timex's director, Fred Olsen, determined that the company would diversify into other areas and signed a contract with Sinclair.^[38]

The ZX Spectrum was officially revealed before journalists by Sinclair at the Churchill Hotel in Marylebone, London, on 23 April 1982.^[32][39] Later that week, the machine was officially presented in a "blaze of publicity" at the Earl's Court Computer Show in London,^[40][41] and the ZX Microfair in Manchester.^[42] The ZX Spectrum was launched with two models: a 16KB 'basic' version, and an enhanced 48KB variant.^[2] The former model had an undercutting price of £125, significantly lower than its main competitor the BBC Micro, whilst the latter model's price of £175 was comparable to a third of an Apple II computer.^[43][44] Upon release, the keyboard surprised many users due to its use of rubber keys, described as offering the feel of "dead flesh".^[32][45] Sinclair himself remarked that the keyboard's rubber mould was "unusual", but consumers were undeterred.^[46]

Despite very high demand, Sinclair Research was "notoriously late" in delivering the ZX Spectrum. Their practice of offering mail-order sales before units were ready ensured a constant cash flow, but meant a lacking distribution.^[26] Nigel Searle, the newly-appointed chief of Sinclair's computer division, said in June 1982 the company had no plans to stock the new machine in WHSmith, which was at the time Sinclair's only retailer.^[47] Searle explained that the mail-order system was in place due to there being no "obvious" retail outlets in the United Kingdom which could sell personal computers, and it made "better sense" financially to continue selling through mail-order.^[48] The company's conservative approach to distributing the machine was criticised,^[26] with disillusioned customers telephoning and writing letters.^[49][50] Demand sky-rocketed beyond Sinclair's planned 20,000 monthly unit output to a backlog of 30,000 orders by July 1982. Due to a scheduled holiday at the Timex factory that summer, the backlog had risen to 40,000 units. Sinclair issued a public apology in September that year,^[32] and promised that the backlog would be cleared by the end of that month.^[49] Supply did not return to normal until the 1982 Christmas season, however.^[51]

Production of the machine rapidly increased with the arrival of the less expensive Issue 2 motherboard, a redesign of the main circuit board which addressed hardware manufacturing defects that affected production of the first model.^[52] Sales of the ZX Spectrum reached 200,000 in its first nine months,^[53] rising to 300,000 for the whole of the first year.^[54] By August 1983 total sales in Britain and Europe had exceeded 500,000,^[55] with the millionth Spectrum manufactured on 9 December 1983. By this point, an average of 50,000 units were being purchased each month.^[56]

In July 1983, an enhanced version of the ZX Spectrum was launched in the United States as the Timex Sinclair 2068. Advertisements described it as offering 72 kilobytes of memory, having a full range of colour and sound for a price under $200.^[57] Despite the improvements upon its British counterpart, sales proved poor and Timex Sinclair collapsed the following year.^[32][58][59]

A crucial part of the company's marketing strategy was to implement regular price-cutting at strategic intervals to maintain market share. Ian Adamson and Richard Kennedy noted that Sinclair's method was driven by securing his leading position through "panicking" the competition. While most companies at the time reduced prices of their products while their market share was dwindling, Sinclair Research discounted theirs shortly after sales had peaked, throwing the competition into "utter disarray".^[60] Sinclair Research made a profit of £14 million in 1983, compared to £8.5 million the previous year. Turnover doubled from £27.2 million to £54.5 million, which equated to roughly £1 million for each person employed directly by the company.^[61]

Clive Sinclair became a focal point during the ZX Spectrum's marketing campaign by putting a human face onto the business. Sinclair Research was portrayed in the media as a "plucky" British challenger taking on the technical and marketing might of giant American and Japanese corporations. As David O'Reilly noted in 1986, "by astute use of public relations, particularly playing up his image of a Briton taking on the world, Sinclair has become the best-known name in micros."^[62] The media latched onto Sinclair's image; his "Uncle Clive" persona is said to have been created by the gossip columnist for Personal Computer World.^[63] The press praised Sinclair as a visionary genius, with The Sun lauding him as "the most prodigious inventor since Leonardo da Vinci". Adamson and Kennedy wrote that Sinclair outgrew the role of microcomputer manufacturer and "accepted the mantle of pioneering boffin leading Britain into a technological utopia".^[64] Sinclair's contribution to the technology sector resulted in him being knighted upon the recommendation of Margaret Thatcher in the Queen's 1983 Birthday Honours List.^[65][66][67]

The United Kingdom was largely immunised from the effects of the video game crash of 1983, due to the saturation of home computers such as the ZX Spectrum.^[68][69][70] The microcomputer market continued to grow and game development was unhindered despite the turbulence in the American markets. Indeed, computer games remained the dominant sector of the British home video game market up until they were surpassed by Sega and Nintendo consoles in 1991.^[71] By the end of 1983 there were more than 450 companies in Britain selling video games on cassette, compared to 95 the year before.^[72] An estimated 10,000 to 50,000 people, mostly young men, were developing games out of their homes based on advertisements in popular magazines. The growth of video games during this period has been compared to the punk subculture, fuelled by young people making money from their games.^[73]

By the mid 1980s, Sinclair Research's share of the British home computer market had climbed to a high of 40 per cent.^[74] Sales in the 1984 Christmas season were described as "extremely good".^[75] In early 1985 the British press reported the home computer boom to have ended,^[76] leaving many companies slashing prices of their hardware to anticipate lower sales.^[75] Despite this, celebration of Sinclair's success in the computing market continued at the Which Computer? show in Birmingham, where the five-millionth Sinclair machine (a gold coloured QL) was issued as a prize.^[75]

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The ZX Spectrum's successor, the Sinclair QL, was officially announced on 12 January 1984, shortly before the Apple Macintosh went on sale.^[77] Contrasting with its predecessors, the QL was aimed at more serious, professional home users.^[78] It suffered from several design flaws; fully operational QLs were not available until the late summer, and complaints against Sinclair concerning delays were upheld by the Advertising Standards Authority (ASA) in May of that year. Particularly serious were allegations that Sinclair was cashing cheques months before machines were shipped. By autumn 1984, Sinclair was still publicly forecasting that it would be a "million seller" and that 250,000 units would be sold by the end of the year.^[79] QL production was suspended in February 1985, and the price was halved by the end of the year.^[80] It ultimately flopped, with 139,454 units being manufactured.^[81]

The ZX Spectrum+, a rebranded ZX Spectrum with identical technical specifications except for the QL-like keyboard, was introduced in October 1984 and made available in WHSmith's stores the day after its launch. Retailers stocked the device in high quantities, anticipating robust Christmas sales. Nevertheless, the product did not perform as well as projected, leading to a significant drop in Sinclair's income from orders in January, as retailers were left with surplus stock. Subsequently, an upgraded model, the ZX Spectrum 128, was released in Spain in September 1985, with development financed by the Spanish distributor Investrónica.^[82] The launch of this model in the UK was postponed until January 1986 due to the substantial leftover inventory of the prior model.^[83]

While the Sinclair QL was in development, Sinclair also hoped to repeat his success with the Spectrum in the fledgling electric vehicle market, which he saw as ripe for a new approach. On 10 January 1985, Sinclair unveiled the Sinclair C5, a small one-person battery electric recumbent tricycle. It marked the culmination of Sir Clive's long-running interest in electric vehicles.^[84] The C5 turned out to be a significant commercial failure, selling only 17,000 units and losing Sinclair £7 million. It has since been described as "one of the great marketing bombs of postwar British industry".^[85] The ASA ordered Sinclair to withdraw advertisements for the C5 after finding that the company's claims about its safety could not be proved or justified.^[86]

The combined failures of the C5 and QL caused investors to lose confidence in Sinclair's judgement. In May 1985, Sinclair Research announced their intention to raise an additional £10 to £15 million to restructure the organisation. Given the loss of confidence in the company, securing the funds proved to be a challenging task. In June 1985, business magnate Robert Maxwell disclosed a takeover bid for Sinclair Research through Hollis Brothers, a subsidiary of his Pergamon Press.^[87] However, the deal was terminated in August 1985.^[74] The future of Sinclair Research remained uncertain until 7 April 1986, when the company sold their entire computer product range, along with the "Sinclair" brand name, to Alan Sugar's Amstrad for £5 million.^[88] The takeover sent ripples through the London Stock Exchange, but Amstrad's shares soon recovered, with one stock broker affirming that "the City appears to have taken the news in its stride".^[89] Amstrad's acquisition of the brand name saw the release of three ZX Spectrum models throughout the late 1980s, each with varying improvements.^[90]

By 1990, Sinclair Research consisted of Sinclair and two other employees down from 130 employees at its peak in 1985.^[91] The ZX Spectrum was officially discontinued in 1992, after ten years on the market.^[51][3] Sinclair Research thereafter continued to exist as a one-man company, marketing Sir Clive Sinclair's inventions until his death in September 2021.

The central processing unit is a Zilog Z80, an 8-bit microprocessor, with a clock rate of 3.5 MHz. The original model Spectrum has 16 KB of ROM and either 16 kB or 48 kB of RAM.^[51]

Video output is channelled through an RF modulator, intended for use with contemporary television sets, to provide a simple colour graphic display. Text is displayed using a grid of 32 columns × 24 rows of characters from the ZX Spectrum character set, or from a custom set. The machine features a colour palette of 15 colours, consisting of seven saturated colours at two levels of brightness, along with black.^[92] The image resolution is 256×192 pixels, subject to the same colour limitations.^[93] To optimise memory usage, colour is stored separately from the pixel bitmap in a low resolution, 32×24 grid overlay, corresponding to the character cells. In practical terms, this means that all pixels within an 8x8 character block share one foreground colour and one background colour. Altwasser received a patent for this design.^[94]

An "attribute" consists of a foreground and a background colour, a brightness level (normal or bright) and a flashing "flag" which, when set, causes the two colours to swap at regular intervals.^[93] This scheme leads to what was dubbed "colour clash" or attribute clash, where a desired colour of a specific pixel could not be selected, but only the colour attributes of an 8x8 block. This became a distinctive feature of the Spectrum, requiring programs, especially games, to be designed with this limitation in mind. In contrast, other machines available at the same time, such as the Amstrad CPC or the Commodore 64, did not suffer from this limitation. While the Commodore 64 also employed colour attributes, it utilised a special multicolour mode and hardware sprites to circumvent attribute clash.^[95]

Sound output is produced through a built-in beeper capable of generating a single channel with ten octaves.^[96] It is controlled by a single EAR bit.^[97] By toggling it on and off, simple sounds are generated. This speaker, capable of producing just one note at a time, was governed by the BASIC command 'BEEP', where programmers could manipulate parameters for pitch and duration.^[98] Furthermore, the processor remained occupied exclusively with the BASIC BEEPs until their completion, limiting concurrent operations. Despite these constraints, it marked a significant step forward from the ZX81, which lacked any sound capabilities. Resourceful programmers swiftly devised workarounds; its rudimentary audio functionality compelled developers to explore unconventional methods such as programming the beeper to emit multiple pitches.^[99]

Later software became available that allowed for two-channel sound playback. The machine includes an expansion bus edge connector and 3.5 mm audio in/out ports, facilitating the connection of a cassette recorder for loading and saving programs and data. The EAR port has a higher output than the MIC and is recommended for headphones, while the MIC port is intended for attachment to other audio devices as a line-in source.^[100]

The ZX Spectrum integrated various design elements from the ZX81. The keyboard decoding and cassette interfaces were nearly identical, although the latter was programmed for higher-speed loading and saving. The central ULA integrated circuit shares some resemblance with that of the ZX81, but it features a fully hardware-based television raster generator with colour support. This enhancement indirectly provided the new machine with roughly four times the processing power of the ZX81, as the Z80 was relieved of video generation tasks. An initial ULA design flaw occasionally led to incorrect keyboard scanning, which was resolved by adding a small circuit board mounted upside down ("dead cockroach") next to the CPU in Issue 1 ZX Spectrums.^[44]

The machine's Sinclair BASIC interpreter is stored in 16 KiB ROM, along with essential system routines. The ROM code, responsible for tasks such as floating point calculations and expression parsing, exhibited significant similarities to ZX81, although a few outdated routines remained in the Spectrum ROM.^[101] The Spectrum's keyboard is imprinted with BASIC keywords. To input a command in BASIC, many keywords require a single keyboard stroke. Other keywords require a change of keyboard mode by a few keystrokes.^[102]

The BASIC interpreter is derived from the one used on the ZX81. A BASIC program for ZX81 can be entered into a ZX Spectrum with minimal modifications. However, Spectrum BASIC introduced numerous additional features, enhancing its usability. The ZX Spectrum character set was expanded compared to that of the ZX81, which lacked lowercase letters. Spectrum BASIC incorporated extra keywords for better graphics and sound functionality, and support for multi-statement lines was added. The built-in ROM tape modulation software routines for cassette data storage enable data transfers at an average speed of 171 bits per second (bit/s), with a theoretical peak speed of 256 bit/s. The tape modulation is significantly more advanced than the ZX81, with approximately four times faster average speeds.^[103]

The original ZX Spectrum is remembered for its rubber chiclet keyboard, diminutive size and distinctive rainbow motif. It was originally released on 23 April 1982 with 16 KB of RAM for £125 (equivalent to £557 in 2023) or with 48 KB for £175 (equivalent to £780 in 2023);^[105] these prices were reduced to £99 (equivalent to £422 in 2023) and £129 (equivalent to £550 in 2023) respectively in 1983.^[106] Owners of the 16 KB model could purchase an internal 32 KB RAM upgrade, which for early "Issue 1" machines consisted of a daughterboard. Later issue machines required the fitting of 8 dynamic RAM chips and a few TTL chips. Users could mail their 16K Spectrums to Sinclair to be upgraded to 48 KB versions. Later revisions contained 64 KB of memory but were configured such that only 48 KB were usable.^[107] External 32 KB RAM packs that mounted in the rear expansion slot were available from third parties. Both machines had 16 KB of onboard ROM.^[26]

An "Issue 1" ZX Spectrum can be distinguished from Issue 2 or 3 models by the colour of the keys – light grey for Issue 1, blue-grey for later machines.^[108] Although the official service manual states that approximately 26,000 of these original boards were manufactured,^[109] subsequent serial number analysis shows that only 16,000 were produced, almost all of which fell in the serial number range 001-000001 to 001-016000.^[110] An online tool now exists to allow users to ascertain the likely issue number of their ZX Spectrum by inputting the serial number.^[111]

These models experienced numerous changes to its motherboard design throughout its life; mainly to improve manufacturing efficiencies, but also to correct bugs from previous boards. Another issue was with the Spectrum's power supply. In March 1983, Sinclair issued an urgent recall warning for all owners of models bought after 1 January 1983.^[112] Plugs with a non-textured surface were at risk of causing shock, and were asked to be sent back to a warehouse in Cambridgeshire which would supply a replacement within 48 hours.^[112][113]

Development of the ZX Spectrum+ began in June 1984,^[114] and was released on 15 October that year at £179.^{[115][116][117]} It was assembled by AB Electronics in South Wales and Samsung in South Korea.^[118] This 48 KB Spectrum introduced a new QL-style case with an injection-moulded keyboard and a reset button that functions as a switch shorting across the CPU reset capacitor. Electronically, it was identical to the previous 48 KB model. The machine outsold the rubber-key model two to one,^[114] however, some retailers reported a failure rate of up to 30%, compared with a more typical 5–6% for the older model.^[117] In early 1985, the original Spectrum was officially discontinued, and the ZX Spectrum+ was reduced in price to £129.^[75]

In 1985, Sinclair developed the ZX Spectrum 128 (codenamed Derby) in conjunction with their Spanish distributor Investrónica (a subsidiary of El Corte Inglés department store group).^{[119][120][121]} Investrónica had helped adapt the ZX Spectrum+ to the Spanish market after their government introduced a special tax on all computers with 64 KB RAM or less,^[122] and a law which obliged all computers sold in Spain to support the Spanish alphabet and show messages in Spanish.^[123]

The appearance of the ZX Spectrum 128 is similar to the ZX Spectrum+, with the exception of a large external heatsink for the internal 7805 voltage regulator added to the right hand end of the case, replacing the internal heatsink in previous versions. This external heatsink led to the system's nickname, "The Toast Rack".^[124] New features included 128 KB RAM with RAM disc commands, three-channel audio via the AY-3-8912 chip, MIDI compatibility, an RS-232 serial port, an RGB monitor port, 32 KB of ROM including an improved BASIC editor, and an external keypad.^[99]

The machine was simultaneously unveiled for the first time and launched in September 1985 at the SIMO '85 trade show in Spain, with a price of 44,250 pesetas. Sinclair later presented the ZX Spectrum 128 at The May Fair Hotel's Crystal Rooms in London, where he acknowledged that entertainment was the most common use of home computers. Due to the large number of unsold Spectrum+ models, Sinclair decided not to start it selling in the United Kingdom until January 1986 at a price of £179.^[125][70]

The Zilog Z80 processor used in the Spectrum has a 16-bit address bus, which means only 64 KB of memory can be directly addressed. To facilitate the extra 80 KB of RAM the designers used bank switching so the new memory would be available as eight pages of 16 KB at the top of the address space. The same technique was used to page between the new 16 KB editor ROM and the original 16 KB BASIC ROM at the bottom of the address space.^[126]

The new sound chip and MIDI out abilities were exposed to the BASIC programming language with the command PLAY and a new command SPECTRUM was added to switch the machine into 48K mode, keeping the current BASIC program intact (although there is no command to switch back to 128K mode). To enable BASIC programmers to access the additional memory, a RAM disk was created where files could be stored in the additional 80 KB of RAM. The new commands took the place of two existing user-defined-character spaces causing compatibility problems with certain BASIC programs.^[127] Unlike its predecessors, it has no internal speaker, and can only produce sound from a television speaker.^[128]

The ZX Spectrum +2 marked Amstrad's entry into the Spectrum market shortly after their acquisition of the Spectrum range and "Sinclair" brand in 1986. This machine featured a brand-new grey case with a spring-loaded keyboard, dual joystick ports, and an integrated cassette recorder known as the "Datacorder," akin to the Amstrad CPC 464. The ROM was updated so that the screen on boot-up showed an Amstrad copyright message (©️ 1986 Amstrad in place of the previous ©️ 1982 Sinclair Research Ltd) However, it was largely identical to the ZX Spectrum 128 in most technical aspects. The machine retailed for £149.^[90]

The new keyboard did not feature the BASIC keyword markings seen on earlier Spectrums, except for the keywords LOAD, CODE, and RUN, which were useful for loading software. Instead, the +2 introduced a menu system, almost identical to that of the ZX Spectrum 128, allowing users to switch between 48K BASIC programming with keywords and 128K BASIC programming, where all words, both keywords and others, needed to be typed out in full (though keywords were still stored internally as one character each). Despite these changes, the layout remained identical to that of the 128.^[118]

The ZX Spectrum +3, which was launched in 1987, bore a resemblance to its predecessor but introduced a built-in 3-inch floppy disk drive instead of the cassette drive. Initially priced at £249,^[129] it later retailed for £199.^[130] It was the only Spectrum model capable of running the CP/M operating system without additional hardware. Unlike its predecessors, the ZX Spectrum +3 power supply utilised a DIN connector and featured "Sinclair +3" branding on the case.^[131]

Significant alterations caused a series of incompatibilities, such as the removal of several lines on the expansion bus edge connector. This resulted in complications for various peripheries. Additionally, changes in memory timing led to certain RAM banks being contended, causing failures in high-speed colour-changing effects. The keypad scanning routines from the ROM were also eliminated, rendering some older 48K and 128K games incompatible with the machine. The ZX Interface 1 was also rendered incompatible due to disparities in ROM and expansion connectors, making it impossible to connect and use the Microdrive units.^[118]

Production of the +3 was discontinued in December 1990, reportedly in response to Amstrad's relaunch of their CPC range, with an estimated 15% of ZX Spectrums sold being +3 models at the time. The +2B model, the only other model still in production at this point, continued to be manufactured, as it was believed not to be in direct competition with other computers in Amstrad's product range.^[132][133]

The ZX Spectrum +2A was a new version of the Spectrum +2^[134] using the same circuit board as the Spectrum +3.^[134][135] It was sold from late 1988 and unlike the original grey +2 was housed inside a black case.^[134][135] The Spectrum +2A/+3 motherboard (AMSTRAD part number Z70830) was designed so that it could be assembled with a +2 style "datacorder" connected instead of the floppy disk controller.^[136] The power supply of the ZX Spectrum +2A used the same pinout as the +3 and has "Sinclair +2" written on the case.^[137]

1989 saw the release of the ZX Spectrum +2B and ZX Spectrum +3B. They are functionally similar in design to the Spectrum +2A and +3,^[138] though changes to the generation of the audio output signal were made to resolve problems with clipping.^[139] The +2B board has no provision for floppy disk controller circuitry, while the +3B motherboard has no provision for connecting an internal tape drive. Production of all Amstrad Spectrum models ended in 1992.^[3]

Following Amstrad's acquisition of Sinclair Research's computer business, Sir Clive Sinclair formed the mail order company Cambridge Computer, which he used to release the portable Cambridge Z88 in February 1987.^[140][141] This ill-fated Spectrum clone incorporated a ULA that introduced the high-resolution video mode originally pioneered in the Timex Sinclair 2068. The Pandora was designed with a flat-screen monitor and Microdrives, intended as Sinclair's solution for portable business computing.^[142]

Timex of Portugal developed and produced several branded computers, including a PAL region-compatible version of the Timex Sinclair 2068, known as the Timex Computer 2048. This variant features distinct buffers for both the ULA and the CPU, significantly enhancing compatibility with ZX Spectrum software compared to the American model.^[143][144] Software developed for the Portuguese-made 2048 remained fully compatible with its American counterpart, as the ROMs were left unaltered. Timex of Portugal also created a ZX Spectrum "emulator" in cartridge form.^[145] Several other upgrades were introduced, including a BASIC64 cartridge enabling it to utilise high-resolution (512x192) modes.^[146] This model saw significant success in both Portugal and Poland.^[147]

In India, Deci Bells Electronics Limited^[148] based in Pune, introduced a licensed version of the Spectrum+ in 1988.^{[149][150][151]} Dubbed the "dB Spectrum+", it performed well in the Indian market, selling over 50,000 units and achieving an 80% market share.^[152]

Numerous unofficial Spectrum clones were produced, especially in Eastern Europe. Thousands of small start-ups in the Soviet Union assembled various clones, distributed through poster adverts and street stalls. Over 50 such clone models existed in total.^[153] In Czechoslovakia, the ZX Spectrum was released as the Didaktik Gama, sporting two switched 32 KB memory banks and 16 KB of slower RAM containing graphical data for video output, along with a 5.25"/3.5" floppy disk.^[154] There were also clones produced in South America, such as the Brazilian-made TK90X and TK95,^[155] as well as the Argentine Czerweny CZ models.^[156]

In the United Kingdom, Spectrum peripheral vendor Miles Gordon Technology (MGT) released the SAM Coupé 8-bit home computer in December 1989. It was designed to be fully compatible with the ZX Spectrum 48K, housing a Zilog Z80B processor clocked at 6 MHz and 256KB of RAM.^[157] By this point, the Amiga and Atari ST had taken hold of the market, leaving MGT in eventual receivership in June 1990.^[158] In his book Retro Tech, Peter Leigh considers the Sam Coupé to be the "true" successor of the ZX Spectrum.^[159]

In 2013, an FPGA-based redesign of the original ZX Spectrum known as the ZX Uno, was formally announced. All of its hardware, firmware and software are open source,^[160] released as Creative Commons licence Share-alike. The use of a Spartan FPGA allows the system to not only re-implement the ZX Spectrum, but many other 8-bit computers and games consoles.^[161] The device also runs modern open FPGA machines such as the Chloe 280SE.^[162] The Uno was successfully crowdfunded in 2016 and the first boards went on sale the same year.^[163]

In January 2014, Elite Systems, who produced a successful range of software for the original ZX Spectrum in the 1980s, announced plans for a Spectrum-themed bluetooth keyboard that would attach to mobile devices.^[164][165] The company used a crowdfunding campaign to fund the Recreated ZX Spectrum, which would be compatible with games the company had already released on iTunes and Google Play.^[166] Elite Systems took down its Spectrum Collection application the following month, due to complaints from authors of the original software that they had not been paid for the content.^[167] Wired UK described the finished device, which was styled as an original Spectrum 48k keyboard, as "absolutely gorgeous" but said it was ultimately more of an expensive novelty than an actual Spectrum.^[168] In July 2019, Eurogamer reported that many of the orders had yet to be delivered due to a dispute between Elite Systems and their manufacturer, Eurotech.^[169]

Later in 2014, the Sinclair ZX Spectrum Vega retro video game console was announced by Retro Computers Ltd and crowdfunded on Indiegogo with the backing of Clive Sinclair.^[170] The Vega, released in 2015, took the form of a handheld TV game^[170][171] but the lack of a full keyboard^[46] led to criticism from reviewers due to the large number of text adventures supplied with the device.^[172][173] Most reviewers branded the device cheap and uncomfortable to use.^[174][168] The follow-up, the ZX Spectrum Vega+ was designed as a handheld game console. Despite reaching its crowdfunding target in March 2016,^[175] the company failed to fulfil the majority of orders. Reviewing the Vega+, The Register criticised numerous aspects and features of the machine, including its design and build quality and summed up by saying that the "entire feel is plasticky and inconsequential".^[176] Retro Computers Ltd was placed into liquidation in 2019.^[177]

The ZX Spectrum Next is an expanded and updated version of the ZX Spectrum computer implemented with FPGA technology^[178] funded by a Kickstarter campaign in April 2017,^[179] with the board-only computer delivered to backers later that year.^[180] The finished machine, including a case designed by Rick Dickinson who died during the development of the project,^[181] was released to backers in February 2020.^[182] MagPi called it "a lovely piece of kit", noting that it is "well-designed and well-built: authentic to the original, and with technology that nods to the past while remaining functional and relevant in the modern age".^[183] PC Pro magazine called the Next "undeniably impressive" while noting that some features are "not quite ready".^[184] A further Kickstarter for an improved revision of the hardware was funded in August 2020.^[185]

In August 2024, Retro Games announced that they would be releasing a recreation of the ZX Spectrum titled "The Spectrum" which would include 48 built-in games, a save game option, rewind mode and pre-owned titles load up. It is set to be released on 22 November 2024.^[186][187]

Official peripherals: the ZX Printer, the ZX Interface 2, the ZX Interface 1 and the ZX Microdrive

Several peripherals were developed and marketed by Sinclair. The ZX Printer, a small spark printer, was already on the market upon the ZX Spectrum's release,^[188] as its computer bus was partially backward-compatible with that of its predecessor, the ZX81. It uses two electrically charged styli to burn away the surface of aluminium-coated paper to reveal the black underlay.^[189]

The ZX Interface 1 add-on module, launched in 1983, includes 8 kB of ROM, an RS-232 serial port, a proprietary local area network (LAN) interface known as ZX Net, and a port for connecting up to eight ZX Microdrives — tape-loop cartridge storage devices released in July 1983, known for their speed, albeit with some reliability concerns.^[190][191] A revised version of these Microdrives was used on the Sinclair QL, although the storage format was electrically compatible but logically incompatible with the ZX Spectrum's. Sinclair Research also introduced the ZX Interface 2, which added two joystick ports and a ROM cartridge port.^[192] Although the ZX Microdrives were initially greeted with good reviews,^[193] they never became a popular distribution method due to fears over cartridge quality and piracy.^[194]

A plethora of third-party hardware add-ons were available throughout the machine's life. Some notable ones included the Kempston joystick interface,^[195] the Morex Peripherals Centronics/RS-232 interface, the Currah Microspeech unit for speech synthesis,^[196] Videoface Digitiser,^[197] the Cheetah Marketing SpecDrum, which was a drum machine,^[198] and the Multiface, a snapshot and disassembly tool from Romantic Robot.^[199] After the original ZX Spectrum's keyboard received criticism for its "dead flesh" feel,^[3] external keyboards became especially popular.^[200] In 1983, DK'Tronics launched a Light Pen compatible with some drawing software.^[201]

There were also many disk drive interfaces like the Abbeydale Designers/Watford Electronics SPDOS, Abbeydale Designers/Kempston KDOS, and Opus Discovery. The SPDOS and KDOS interfaces were bundled with office productivity software, including the Tasword Word Processor, Masterfile database, and Omnicalc spreadsheet.^[202] This bundle, along with OCP's Stock Control, Finance, and Payroll systems, introduced small businesses to streamlined computerised operations.^[51] In 1987 and 1988, Miles Gordon Technology released the DISCiPLE and +D systems. These systems had the capability to store memory images as disk snapshots, allowing users to restore the Spectrum to its exact previous state. Both systems were compatible with the Microdrive command syntax, simplifying the porting of existing software.^[203]

During the mid-1980s, Telemap Group Ltd launched a fee-based service that allowed ZX Spectrum users to connect their machines to the Micronet 800 viewdata service via a Prism Micro Products VTX5000 modem. Micronet 800, hosted by Prestel, provided news and information about microcomputers and offered a form of instant messaging and online shopping.^[204]

Screenshots from ZX Spectrum games Rebelstar (1984) and Laser Squad (1988)

Most Spectrum software was originally distributed on audio cassette tapes, intended to work with a normal domestic cassette recorder.^[26][205] Software was mainly distributed through print media such as magazines and books.^{[2][206][207]} To load software onto the machine, the reader would type the BASIC program listing by hand, run it, and save it to the cassette for later use. Some magazines distributed 7" 331⁄3 rpm flexi disc records, or "Floppy ROMs", a variant of regular vinyl records which could be played on a standard record player.^[208] Some radio stations would broadcast audio stream data via frequency modulation or medium wave so that listeners could directly record it onto an audio cassette themselves. ZX Spectrum-focused radio programmes existed in the United Kingdom, which were received over long distances on domestic radio receivers.^[209]

Different types of software released for the machine include programming language implementations, databases,^[210] word processors (Tasword being the most prominent^[211]), spreadsheets,^[210] drawing and painting tools (e.g. OCP Art Studio^[212]), 3D-modelling (e.g. VU-3D^[213][214]) and archaeology software.^[215] Over 24,000 different software titles were released for the ZX Spectrum throughout its lifespan.^[1] The ZX Spectrum had an extensive library of video games, including iconic titles such as Manic Miner, Jet Set Willy, Chuckie Egg, Elite, Sabre Wulf, Knight Lore, and The Hobbit. Many of the popular ZX81 titles were rewritten for the Spectrum to take advantage of the newer machine's colour and sound capabilities (Psion's Flight Simulation being a notable example). These games were instrumental in establishing the machine as a prominent gaming platform during the 1980s. Hardware limitations of the machine required a particular level of creativity from video game designers.^[216][217] From August 1982 the ZX Spectrum came bundled with Horizons: Software Starter Pack,^[218] a software compilation which included ten demonstration programs.^[219]

Some ZX Spectrum games hold a number of industry-firsts and Guinness World Records. These include Ant Attack, the first home computer game to use isometric graphics,^[220] Turbo Esprit, the first open world driving game,^[221] and Redhawk, which featured the first superhero created specifically for a video game.^[222]

Initial reception of the ZX Spectrum was generally positive. Critics in Britain welcomed the new machine as a worthy successor to the ZX81; Robin Bradbeer of Sinclair User praised the additional keyboard functions the Spectrum had to offer, and lauded the "strength" of its ergonomic and presentable design.^[223] Tim Hartnell from Your Computer noted that Sinclair had improved on the shortcomings of the ZX80 and ZX81 by revamping the Spectrum's load and save functions, noting that it made working with the machine "a pleasure".^[224] Hartnell concluded that despite minor faults, the machine was "way ahead" of its competitors, and the quality far exceeded that of the BBC Micro.^[225]

Computer and Video Games' Terry Pratt compared the Spectrum's keyboard negatively to the typewriter-style used on the BBC Micro, opining that it was an improvement over the ZX81 but unsuited for "typists".^[226] In similar vein, David Tebbutt from Personal Computer World felt that the Spectrum's keyboard felt more like a calculator than typewriter, but praised its functional versatility.^[227] Likewise, Gregg Williams from BYTE criticised the keyboard, declaring that despite the machine's attractive price the layout "is impossible to justify" and "poorly designed" in several respects. Williams was sceptical of the computer's appeal to American consumers if sold for US$220 — "hardly competitive with comparable low-cost American units" — and expected that Timex would sell it for $125–150.^[228] A more negative review came from Jim Lennox of Technology Week, who wrote that "after using it [...] I find Sinclair's claim that it is the most powerful computer under £500 unsustainable. Compared to more powerful machines, it is slow, its colour graphics are disappointing, its BASIC limited and its keyboard confusing".^[70]

The importance of the ZX Spectrum and its role in the early history of personal computing and video gaming has left it regarded as the most important and influential computer of the 1980s.^[2][3][230] Some observers credit it as being responsible for launching the British information technology industry during a period of recession,^[231] while introducing home computing to the masses.^[232] As of 2024^[update], it is also one of the best-selling British computers of all time, with over five million units sold by the end of the Spectrum's lifespan in 1992.^[6][8] It retained the title of Britain's top-selling computer until the Amstrad PCW surpassed it in the 1990s, with eight million units sold by the end of the PCW's lifespan in 1998.^[7][8] The ZX Spectrum is affectionately known as the "Speccy" by elements of its fan following.^[233]

A number of notable games developers began their careers on the ZX Spectrum. Tim and Chris Stamper founded Ultimate Play the Game in 1982,^[234] who found success with their blockbuster hits Jetpac (1983), Atic Atac (1983), Sabre Wulf (1984), and Knight Lore (1984).^[235] The Stamper brothers later founded Rare, which became Nintendo's first Western third-party developer.^[236] David Perry, the founder of Shiny Entertainment, moved from Northern Ireland to England to focus on developing games for the ZX Spectrum.^[237]

Some programmers have continued to code for the platform by using emulators on modern PCs.^[238] A robust homebrew community continues into the present day, with several games being released commercially from new software houses such as Cronosoft.^[239] In 2020, a museum dedicated to the ZX Spectrum and other Sinclair products opened in Cantanhede, Portugal.^[240]

The Rubber-Keyed Wonder, a documentary about the ZX Spectrum created by Anthony and Nicola Caulfield, was funded through Kickstarter in 2023 and will premiere on 3 October 2024 in London.^[241]

The ZX Spectrum has been referenced numerous times in popular culture.

A running gag in Scott Pilgrim vs. the World features character Julie Powers being censored with the sound effects of the ZX Spectrum. The director of the film, Edgar Wright, who was a big fan of the Spectrum, stated that he always used to wait for Spectrum games to load when he was a teenager.^[242]

On 23 April 2012, a Google doodle honoured the 30th anniversary of the Spectrum. As it coincided with St George's Day, the Google logo was of St George fighting a dragon in the style of a Spectrum loading screen.^[243]

One of the alternate endings in the interactive film Black Mirror: Bandersnatch (2018) included the main character playing data tape audio that, when loaded into a ZX Spectrum software emulator, generates a QR code leading to a website with a playable version of the "Nohzdyve" game featured in the film.^[244]

• List of computer system emulators#Sinclair ZX Spectrum and clones
• List of ZX Spectrum games
• ZX Spectrum graphic modes
• ZX Spectrum character set
• Contended memory

Wikimedia Commons has media related to Sinclair ZX Spectrum.

• ZX Spectrum BASIC manual
• JSSpeccy 3 ZX Spectrum emulator in the browser
• World Of Spectrum ZX Spectrum Game Database
• Spectrum Computing ZX Spectrum Database