1980-ші жылдар басындағы британдық үй компьютері: Jupiter Ace

British home computer of the early 1980s

The Jupiter Ace by Jupiter Cantab was a British home computer released in 1982. The Ace differed from other microcomputers of the time in that its programming environment used Forth instead of the more popular BASIC. This difference, along with limited available software and poor character based graphic display, limited sales and the machine was not a success. Both had been on the design team for the ZX Spectrum: Altwasser worked on ZX81 development and hardware design of the Spectrum. Vickers adapted and expanded the 4K ZX80 ROM to the 8K ZX81 ROM and wrote most of the ROM for the Spectrum. The Jupiter Ace was named after an early British computer, the Pilot ACE, and went on sale on September 22, 1982 with a price of £89.95. The use of Forth rather than the more usual choice of BASIC, and the availability and success of the ZX Spectrum, as well as limited published software, the poor case and small initial memory all weighed against wider market acceptance. Eventually Jupiter Cantab ceased trading by the end of October 1983. The brand was then acquired by Boldfield Computing Ltd in 1984 that sold the remaining stock by mail order for £26.

Sales of the machine were never very large; the reported number of Aces sold before Jupiter Cantab closed for business was around 5,000. As of the early 2000s, surviving machines are uncommon, often fetching high prices as collector's items. Forth, while being structured and powerful, was considered difficult to learn, and a knowledge of BASIC acquired from familiarity with other home computers was of no practical help in learning it. A 1982 review stated that "The success of the Jupiter Ace will depend on the machine buying public's acceptance of another microcomputer language." Further, there was only a very limited range of published software either commercial programs or type in programs printed in hobby magazines for the machine, and these were restricted by the base model's small amount of RAM. Attempts to promote the Ace in the educational market also failed; doubts over whether Forth would be relevant for exam syllabuses, and the lack of support for Forth from teaching staff were key issues. Pupils were more interested in learning the widely used BASIC than a language used by only one (uncommon) machine with a peculiar RPN syntax. Finally, the tile based graphics compared poorly to the pixel based graphics of other machines which were also colour rather than the Ace's monochrome. This restricted sales largely to a niche market of technical programming enthusiasts.

The Jupiter Ace is often compared with ZX81 due to its similar size, low cost, and similar form factor. Internally its design is more similar to the ZX Spectrum although the Ace also had a dedicated video memory of 2 KB, partly avoiding the slow down when programs accessed the same bank (same chips) as the video memory. Like the Spectrum, the Ace used black conductive rubber keys although unlike the Spectrum, the keys had a conductive pad that was squashed directly onto tracks on the PCB rather than using a membrane. As a result, the keys would often stop working reliably until they were cleaned or the conductive material was refreshed. Audio capabilities were CPU controlled with programmable frequency and duration. Sound output was through a small built in speaker. As was common at the time, it used a common tape recorder instead of disk/tape drives. Similarly, a television was needed as a display but this was in black and white only, rather than the colour supported by competing models such as the Spectrum. A secondary (undocumented) edge connector on the back of the case made some video signals available, presumably for a forthcoming colour video card, but no official product that used this connector was ever released. The Jupiter Ace was based on the Zilog Z80, which the designers had previous experience of from working on the Sinclair ZX81 and ZX Spectrum. Both graphics and text could be displayed at the same time: (1) redefinition of the character tiles provided standard 256×192 graphics limited to the 128 available (definable) 8×8 chars, concurrent with plotting of 64×48 graphics. Internal speaker directly controlled by the CPU in single task mode, with control of sound frequency and duration in milliseconds. Storage was through a cassette tape interface at 1500 baud. Files could be used for either storage of Forth programs (compiled code) or raw dumps of memory.

The Ace had an 8 KB ROM containing the Forth kernel and operating system, and the predefined dictionary of Forth words in about 5 KB. The remaining 3 KB of ROM supported several functionalities: floating point numbers library and character definitions table, tape recorder access, decompiling and redefining newly re edited 'words' (i. e. routines). Some of the ROM was written in Z80 machine code, but some was also coded in Forth. The next 8 KB was built in RAM that was only partially decoded, with 2 KB of video RAM echoed twice, and 1 KB of user RAM echoed 4 times (with the same memory appearing at several different memory addresses). Using the lower address mirror of the video RAM would select CPU priority, resulting in some momentary random pixels on the screen when video subsystem and the CPU accessed the video RAM in the same clock cycle. Using the higher address would briefly pause the CPU on the interference, affecting the program timing and making this mode unsuitable for I/O operations. Since video RAM was partially separated from the main address and data busses, for the most part the video subsystem and the CPU could operate in parallel. The first 16 KB of the memory map was used for ROM, Video and User available RAM, leaving the second 16 KB of the memory map free for RAM extension and the topmost 32 KB undefined. One 1K bank allowed redefinition of most of its 128 ASCII based characters in 8×8 pixel bitmap format. The other 1K bank stored the full screen display of 24 rows × 32 columns of characters in black and white. Colour was intended to be achieved as expansion, but although a colour graphics board was designed, none was ever produced commercially. 1 KB RAM with the option of a 16 KB RAM Pack, and later a 32 KB one. A PCB was also marketed by Boldfield Computing that converted the edge connector to electrical compatibility with a Sinclair ZX81, allowing use of the ZX81 16K RAM pack.

Characteristic Value Processor Zilog Z80A clocked at 3.25 MHz. Operating system FORTH (as both the programming language and command line interface). Memory 2 KB for Video + 1 KB base expandable up to 49 KB (Video excluded). Video Independent sub system with 2 dedicated SRAM banks: Screen (1 KB) + Char Tiles (1 KB). Sound Internal speaker, CPU driven (non dedicated). Expansion 2 connectors: Main (CPU related) + Video (Screen + AV signal). Keyboard and Charset 40 key QWERTY keyboard (Symbols access with extra key). All chars in Charset (based on ASCII 1967) being redefinable. Character set Based on ASCII 1967 with extensions as for the ZX Spectrum character set including ↑, £ and ©. References to the Ace RAM sometimes include the separate 2 KB video memory, which was not available for programming, thus leading to some confusion. Similarly, it is sometimes argued that because of Forth's efficiency, the 1 KB standard RAM was in effect comparable to at least 2 KB on a BASIC system.

Avoiding sources was compensated by storing comments entered in the code with the compiled output, traditional compilation would discard such comments. The comments were then recovered on decompiling. As a result of "code is the source", modified words (edited) would demand actualization of all code using the one newly edited. This was done with the non standard REDEFINE command. Although not explicitly designed for such a purpose, the compiled Forth could be utilised for ROM extensions to the built in system. External ROMs were developed with Ace Forth to be used as control applications.

The original Jupiter Ace issue 1 was introduced in 1982, and came in a vacuum drawn case. Reportedly 5000 units were produced.