Jupiter Ace

In September 1982 Your Computer ran a news story headed “£90 Jupiter Ace goes far faster with Forth”. Two engineers from the ZX Spectrum project, Richard Altwasser and Steven Vickers, were launching their own computer, and “the real surprise though is that the new machine will not use Basic.” Altwasser said that “there are many drawbacks to Basic, which is why we are using Forth — the language of the future.”

Every other home computer of the day started up in BASIC. The Jupiter Ace started up in Forth, a language in which the programmer builds a program by defining new words. It cost £89.95. Altwasser had designed the Spectrum’s hardware and Vickers had written most of its ROM. The Ace was what they built next, and it sold about five or six thousand.

Jupiter Cantab

Altwasser left Sinclair Research, and Vickers left Nine Tiles, the Cambridge firm that had written Sinclair’s ROMs under contract. Their company, Jupiter Cantab, worked from 22 Foxhollow, Bar Hill, outside Cambridge, and its advertising made the most of their past: they “played a major role in creating the ZX Spectrum and then formed Jupiter Cantab to develop advanced ideas in personal computing.” One advertisement in Computer & Video Games called the Ace “probably the fastest microcomputer in the universe”.

The machine was sold by mail order only, “all inclusive” at £89.95 with a power supply, leads for a television and cassette recorder, a software catalogue and a manual. The manual opens with the Pilot ACE, the computer the National Physical Laboratory built in 1950, which “occupied a space the size of a small kitchen”. The new Ace, it went on, “can store 3 Kilobytes of information … and it is small enough to rest in your lap.”

Fast facts

• Launched: summer 1982, mail order · Price: £89.95
• Designers: Richard Altwasser and Steven Vickers
• CPU: Zilog Z80A at 3.25 MHz (Jupiter Cantab’s figure; the circuit diagram shows a 6.5 MHz crystal)
• Memory: 8K ROM, 3K RAM; Jupiter Cantab promised a 16K expansion
• Display: 32 × 24 characters, black and white; all 128 character shapes redefinable in RAM
• Language: Forth in ROM, with floating-point arithmetic
• Sound: a built-in loudspeaker, played with BEEP
• Tape: 1,500 baud
• Keyboard: moving rubber keys, laid out like the Spectrum’s

Forth in ROM

Jupiter Cantab sold Forth on speed and size. Its advertisements in late 1982 claimed that Forth was “typically four times as compact and ten times as fast as BASIC”, and that “before the Ace all personal computers used BASIC and FORTH was only available to a privileged few.” Those figures were the company’s claim, not a measurement. With only 3K of RAM, the compactness mattered at least as much as the speed.

Forth works differently from BASIC. BASIC gives you a fixed set of keywords and asks you to arrange them in numbered lines. In Forth you type new words into the dictionary, each built from words already there, and the last word you define is your program. Numbers go on a stack, and words take them off and put results back.

Jupiter Cantab published a booklet to help BASIC users make the change: Penny Vickers’s The Forth Bridge, which set BASIC keywords beside their Forth equivalents. Some of the rows show how much the Ace left to the user. Against SIN it says “Not in FORTH, but the Ace manual gives a series definition of sine.” Against SQR there is no built-in word, only a Newton-Raphson square root to type in:

: 2OVER ( f1, f2 - f1, f2, f1) 4 PICK 4 PICK ;
: 2SWAP ( f1, f2 - f2, f1) 4 ROLL 4 ROLL ;
: SQR   ( f - square root of f) 1. 10 0 DO 2OVER 2OVER F/ F+ .5 F* LOOP 2SWAP 2DROP ;

A floating-point number takes up two places on the Ace’s stack, which is why these words move values in pairs. 2DROP is not built in either: the manual defines it as : 2DROP DROP DROP ; in its chapter on fractions and decimal points. Once typed, SQR works like any other word. The booklet describes Forth as a language that lets you “begin from readily learnable words and then advance step by step to programs of greater complexity — building word on word.”

Inside the machine

The Ace’s manual is frank about how the hardware works. The screen and the character shapes each live in 1K of RAM that can be reached through two ranges of addresses. Through the “front door”, at 8192 to 9215 for the screen, the computer serves the processor straight away, “even if it means neglecting the television picture”, and the neglect shows up “as momentary white dots”. Through the “back door”, at 9216 to 10239, the processor waits until the picture circuitry has finished, and “you don’t get any white dots.” Jupiter Cantab’s advertisements promised a “flicker-free display”. The manual explains how to get one.

Sound is just as direct. Reading one of the keyboard ports pushes the loudspeaker’s diaphragm in, and the BEEP word takes a pitch in units of 8 microseconds and a length in milliseconds. The manual uses it to play “Three Blind Mice”.

What went wrong

Jupiter Cantab’s first advertisements listed a printer interface “for around £20.00” and a 16K memory expansion “for around £30.00” as “available soon”. In 1983 the users’ club newsletter, Ace User, was writing: “We presume the lack of promised add-ons from Jupiter Cantab indicates a cash-flow problem.” Other firms filled the gap. Stonechip made 16K and 32K memory packs, Essex Microelectronics made memory and a sound board, and the club sold its own adaptor kit so that ZX81 memory packs and sound units could be used on the Ace.

Later in 1983 the newsletter confirmed that “Richard Altwasser has left Jupiter Cantab Ltd”. Geoffrey Walker joined Steven Vickers as co-director, to bring “much needed marketing expertise to the company.” By the next issue Jupiter Cantab had crashed. The editor, John Noyce, reported that about five or six thousand Aces had been sold, with around 1,500 more in stock and 800 of the Ace 4000, a version for the American market.

Looking back in 1990, ACE magazine put the failure down to the language: the Ace was “the only home computer to sport Forth as its inbuilt language. This proved to be its downfall.” John Kennedy, who had owned one, blamed the build as well in CU Amiga in 1998. The case was thin white plastic held together with plastic rivets. The keyboard was part of the main circuit board, its tracks shorted by conductive pads, and “each key worked for about three months”. His summary: “the cheapness of design, the use of FORTH, the lack of colour and any possibility of games software spelled the end for the Ace.”

After Jupiter Cantab

The Cambridge firm Boldfield Limited bought the bankrupt stock and kept selling it by mail order. Its 1984 advertisement offered the Ace with a 16K RAM pack for £44 plus VAT (“used to be £124.90”) and cut Jupiter software from up to £7.95 to £3. In October 1985 Your Computer reported that an “isolated colony of the long-extinct Jupiter Ace” had been found in Boldfield’s showrooms in the centre of Cambridge, “going for £35 a time”.

The users’ club kept going too. Ace User, edited by John Noyce in Brighton, ran a software catalogue that paid contributors “royalties of 20% of sale price … paid promptly every quarter”. It printed readers’ programs, hardware projects such as a colour board, and letters, including one from Steven Vickers correcting a club member on how the Z80 puts the port address on the address bus. In 1984 it became Forth User, and its editorial that year said: “Jupiter Cantab may have crashed in 1983, but the Ace lives on, indeed there has been a renewal of enthusiasm for it this year.” By then Boldfield had assembled a list of add-ons and produced new software for a computer whose maker no longer existed.

A footnote on the ROM

The club’s first newsletter, in December 1982, carried a postscript about Ian Logan, who had already documented the ZX81 and Spectrum ROMs. He had “apparently obtained a listing of the Rom by cross-loading from the Jupiter to the Spectrum. The cassette standards are apparently identical.”

See also

• Steven Vickers
• Sinclair Research: where Altwasser worked before.
• Sinclair ZX Spectrum: the machine Altwasser and Vickers built first, and then competed with.
• Ian Logan
• Z80
• Sinclair QL