Amstrad CPC | Code Like It's 198x

The invitation promised that Einstein, Archimedes, William Shakespeare, Monet and Ravel would all have “something to say about Amstrad’s really rather remarkable new computer”. On a Thursday in April 1984 the press went to the Great Hall of Westminster School to hear them. Amstrad Action later recalled that the “cynical journos” expected Amstrad, “purveyor of cheap hi-fi’s, TVs and CB radios, to fall flat on his flash backside”. Instead they found four working machines and software already written for a computer nobody had seen. The historical namesakes were real people who happened to share those names, found by Amstrad’s public relations firm: Monet showed off the colour and Shakespeare the word processing.

The machine was the CPC464, for Colour Personal Computer with 64K of memory. Alan Sugar’s first big hit, at the end of the 1970s, had been the Tower System, a hi-fi whose front panel looked like three separate units but hid a single box of electronics that needed one plug. He built his computer the same way. The keyboard had a cassette deck built in, and the monitor came with the power supply, so the family television stayed free. In August 1984 Computer & Video Games reported the price: “The green model is only £229 complete with cassette recorder, 64k micro and monitor. The colour version will cost an extra £100.”

Fast facts

Launch: CPC464 unveiled at Westminster School, London, in April 1984. Public sources give both 11 and 12 April.
Price at launch: £229 with a green-screen monitor, £329 with a colour monitor.
CPU: Zilog Z80A clocked at 4 MHz, running at an effective 3.3 MHz (see below).
Memory: 64K of RAM (CPC464 and CPC664), 128K (CPC6128). The 464’s 32K ROM holds Locomotive BASIC and the firmware.
Display: a 6845 CRT controller and Amstrad’s custom gate array, reading 16K of the main RAM. Three modes trade resolution against colour: Mode 0 at 160×200 in 16 colours, Mode 1 at 320×200 in 4, Mode 2 at 640×200 in 2.
Palette: 27 colours. The hardware takes 32 values, but the firmware manual says “Colours 27..31 are not intended for use; they are merely duplicates of other colours available”.
Sound: a General Instrument AY-3-8912 clocked at 1 MHz: three tone channels and noise, in stereo through a socket on the back.
Storage: built-in cassette deck on the 464. The 664 and 6128 have a built-in drive for 3-inch discs holding 180K a side.
Models: CPC464 (April 1984), CPC664 (April 1985), CPC6128 (August 1985), then the 464 Plus, 6128 Plus and GX4000 console (1990).

A machine called Arnold

Amstrad had never designed a computer. Amstrad Action reconstructed the project in 1989 for the machine’s fifth birthday, and David Thomas told it again in his 1990 book Alan Sugar: The Amstrad Story. The idea dates from January 1983. Amstrad first contracted the work out, and the early design was built around the 6502, the processor in the BBC Micro and the Commodore 64. By the summer of 1983 it was clear that the contractors’ prototype had, in Amstrad Action’s words, “the makings of a very poor computer indeed”. It had no grey scales on the mono monitor and no palette, and one of the team then gave up and disappeared, leaving the project in disarray.

Amstrad turned to Ambit International, a small electronics firm in Brentwood that had done work for it before, and Roland Perry took over as project manager. He had a plastic case, a keyboard and a set of rules: nobody was to know who the client was, and the parts, already ordered in bulk, could not be changed. On 10 August 1983 he visited Locomotive Software, a young firm in Dorking run by Richard Clayton and Chris Hall. Clayton remembered Perry arriving with a case on which he “had letrasetted the word ‘Arnold’”, and “when we opened it up ‘Amstrad’ was printed quite clearly on the circuit board!” The code name let people guess that the client was GEC, run by Arnold Weinstock. Perry said later that nobody noticed for months that Arnold is an anagram of Roland.

Locomotive already had a BASIC for the Z80 and reckoned converting it to the 6502 would take far longer than the three months available. So the machine became a Z80 computer. Locomotive wrote the BASIC and the firmware, and Mark-Eric Jones of MEJ Electronics designed the hardware. Jones persuaded Sugar to put much of the logic into a custom gate array. According to Thomas he gave three reasons: fewer parts, more features for the price, and a machine that would be hard to copy, and the last one won Sugar over. The gate array was finished in February 1984.

The launch was run on the same timetable. Fifty prototypes went out to software houses at the end of 1983, carried to them by couriers Perry hired, so that games would be ready on launch day. Locomotive met Sugar only once during development, towards the end of November 1983, and Thomas says Sugar noticed straight away that the cursor could not be moved when the machine was switched on. Amstrad Action tells it as people at Amstrad pressing the cursor keys, seeing nothing happen and deciding the machine was broken. The software was changed, “and that, believe it or not, is why the cursor keys on your CPC work as they do”. Because parts took months to arrive, Amstrad had to order “perhaps as many as 100,000” machines before anyone knew whether it would sell. Thomas puts sales at about 200,000 in 1984, most of them in the run-up to Christmas.

Locomotive BASIC and the firmware

The 464 switched on into Locomotive BASIC, and it was a BASIC with unusual reach. Four timers, counted in fiftieths of a second, could interrupt a running program: AFTER 250 GOSUB 60 calls a subroutine once after five seconds, and EVERY 50,1 GOSUB 30 calls another once a second. The sound commands queued notes so that music played while the program got on with something else. Games Computing’s review in October 1984 singled this out: “Interrupt handling is supported from Basic as standard”. The same reviewer found the screen editor “awful” and the compulsory spaces between keywords a trap at two in the morning.

Beneath the BASIC sat the firmware, a set of routines for the keyboard, screen, cassette and sound, all reached through jump blocks so that machine-code programs could use them too. Locomotive’s preface to the CPC464 Firmware manual, written in Dorking in May 1984, explains the aim: “to produce the most sophisticated computer possible within the twin constraints of minimum hardware cost and a limited timescale”. Bruce Godden wrote the manual. The deal Locomotive struck for the BASIC turned out less well for them. Chris Hall told Thomas that he did not expect the machine to last, so he accepted £15,000 a year after the first year, and was still receiving the same £15,000 at the end of the 1980s.

The gate array makes time regular

The CPC’s Z80 is clocked at 4 MHz and does not run at 4 MHz. The firmware manual explains why:

“Accesses to memory are synchronised with the video logic — they are constrained to occur on microsecond boundaries. This has the effect of stretching each Z80 M cycle (machine cycle) to be a multiple of 4 T states.”

The gate array holds the processor with its WAIT line until the next microsecond boundary, so that it “can only make one access to memory each microsecond”. The result is “an effective clock rate” of “approximately 3.3 MHz”, about a sixth slower than the crystal. What the programmer gets in return is time that does not vary. Every machine cycle becomes a whole number of microseconds, so an instruction takes the same time wherever the beam is and whatever memory it touches.

Its neighbours made the cost variable instead. On the Commodore 64 the VIC-II stops the processor for 40 to 43 of a line’s 63 cycles on every eighth line, and moving the screen moves those lines. On the ZX Spectrum the processor is held up only when it touches the 16K of RAM that holds the screen while the picture is being drawn, so code runs faster just by living higher in memory. The CPC charges on every access whether the video wanted the bus or not, and in exchange a routine takes as long on line 1 as on line 200. Raster code on a CPC can be timed in whole microseconds.

The gate array also allows the display mode to change partway down the screen. It “synchronises mode changing to the next horizontal fly-back in order to aid software that requires different parts of the screen to be handled in different modes”, the manual says. A game can show a 16-colour playing area above a sharper 2-colour or 4-colour status panel by changing mode on the right line.

The 664 and the 6128

In April 1985 Amstrad replaced the cassette deck with a disc drive and called the result the CPC664. It sold for £339 with a green screen and £449 in colour. The official Amstrad Computer User magazine renamed itself to make room for it, from issue 7, because “the way Amstrad are going, we could soon run out of room on the cover”. Inside Amstrad, Thomas writes, the 664 was code-named IDIOT: Includes Disk Instead Of Tape.

Four months later Amstrad announced the CPC6128, with twice the memory in a smaller case. Amtix! reported it at “£299 for the monochrome monitor and £399 for the colour version”, cheaper than the 664, and noted that it had first been launched in the United States, where Amstrad’s agent wanted a 128K machine to be taken seriously for business. Amstrad later said it had sold 70,000 of the short-lived 664, and owners were not pleased. Asked about it in 1989, Roland Perry told Amstrad Action he still would not wear his 664 sweatshirt in public: “I’d be lynched!”

France, Spain and Germany

The CPC’s biggest successes came abroad. In France, Amstrad’s subsidiary was run by Marion Vannier, who had started it in 1982 from a one-person office above a bakery in Sèvres. She advertised the CPC464 as “The Anti-Crisis Home Computer”, stressing that the family would no longer have to give up the television, and sold it for the same price as in Britain. Customers asked for it in computer shops that had never heard of Amstrad. Thomas records that Amstrad France had sold 30,000 computers by March 1985 and that its turnover rose eightfold in a year. In 1990 Amstrad’s marketing director, Malcolm Miller, told Amstrad Action: “Last year in France we had a 55 per cent share of the market.”

In Spain the machine was sold by Indescomp, the Madrid company of José Luis Domínguez, which also wrote games for it. Thomas puts Amstrad’s Spanish sales at £4.4 million in 1984-85 and £32.3 million the following year. Amstrad bought Indescomp outright in 1987, announcing the deal at the SONIMAG show in Barcelona, and the weekly Amstrad Semanal served the Spanish owners. In West Germany the CPC was sold under the name of Amstrad’s distributor, Schneider, and the market was worth £32.5 million to Amstrad in 1984-85, almost a quarter of its turnover.

Sugar later described the effect on the British market, where dealers who had been left with unsold computers after Christmas 1984 expected Amstrad to cut its prices. “They thought we were sitting on stockpiles,” he told Thomas. “But we had no inventory. It had gone to France and Spain.”

The Plus range and the GX4000

In 1990 Amstrad replaced the 464 and 6128 with the 464 Plus and 6128 Plus, in new cases, and added the GX4000, a games console built from the same hardware. ACE listed the prices in October 1990: £99 for the GX4000, £229 or £329 for the 464 Plus and £329 or £429 for the 6128 Plus, with mono or colour monitors. The new hardware added a larger palette, hardware sprites and smooth scrolling, but ACE pointed out that only cartridge software could use it; tape and disc programs ran as on the old machines. “To all intents and purposes, the CPC plus range is the old hardware with the new GX4000 bolted on.” It also wondered about releasing an 8-bit console “this close to the release of the Sega Megadrive”.

Sales

Amstrad’s own figures are the only ones public sources give. In May 1989 Amstrad’s Mike Walton told Amstrad Action that over two million CPCs had been sold, “the majority of those in the UK”. In September 1990 Malcolm Miller told the same magazine: “So far we’ve sold over two and a half million CPC machines.” Public sources do not give a date for the end of CPC production. By April 1993 Amstrad Action was reporting that major software houses were pulling out of the machine.

Sources

Amstrad Action, issue 44, May 1989, pp. 15–17, “A machine called Arnold” by Steve Carey: the launch invitation and Westminster School; the 6502 contractors; Ambit and Roland Perry; Locomotive Software and MEJ Electronics; the Arnold code name; the project timeline, including the gate array in February 1984; the prototypes for software houses; the cursor keys; the 100,000 machines ordered in advance; Mike Walton’s “over two million CPCs”; the 664 sweatshirt.
David Thomas, Alan Sugar: The Amstrad Story (Century, 1990): the Tower System (chapter “A Mug’s Eyeful”); Perry and Ambit (pp. 116–117); Locomotive Software and the Arnold code name (pp. 119–121); the gate array decision and the BASIC royalty (pp. 126–128); Locomotive’s meeting with Sugar (p. 130); the launch, the celebrity namesakes and prices (p. 134); about 200,000 sold in 1984 (p. 138); exports to France, Spain and West Germany and Sugar’s “It had gone to France and Spain” (pp. 140–141); Amstrad France (pp. 143, 152–154); Indescomp and Spanish sales (pp. 149–156); the CPC664, IDIOT, the 6128 and 70,000 664s (p. 168); Schneider and West Germany (p. 265); the purchase of Sinclair’s computer business in 1986 (p. 215).
Computer & Video Games, issue 34, August 1984: launch prices.
Games Computing, October 1984, “Amstrad unwrapped”, from p. 29: the monitor and power supply, interrupts in BASIC, the editor.
CPC464 Firmware (Bruce Godden, Locomotive Software; AMSOFT, 1984): preface; section 1.1, the hardware and the 3.3 MHz effective clock; section 7, the sound manager; appendix XII, the 4.00 MHz clock, ROM and RAM layout, screen memory, screen modes and mid-screen mode changes.
Amstrad CPC664 User Instructions (Amstrad, 1985): the 27 colours; 3-inch discs of 180K a side; the AFTER and EVERY commands.
Amstrad Computer User (CPC464 User), issue 6, May 1985: the CPC664 and the change of title. Issue 7, June 1985: CPC664 prices.
Amtix!, issue 0, October 1985: the CPC6128’s prices and American debut.
Mundo Amstrad, issue 2, December 1987, p. 4: Amstrad’s purchase of Indescomp.
Amstrad Action, issue 60, September 1990, p. 36, “Playing the market”: Malcolm Miller on CPC sales and the French market. Issue 63, December 1990: Roland Perry leaves Amstrad. Issue 91, April 1993: software houses leaving the CPC.
ACE, issue 37, October 1990, pp. 119–120: the GX4000, 464 Plus and 6128 Plus.