| Release date | 1982 |
|---|---|
| Discontinued | 1984 |
| Operating system | ACE Forth |
| CPU | Z80 @ 3.25 MHz |
| Memory | 1 KB (Max=49 KB) |
The Jupiter Ace was a British home computer of the early 1980s, produced by a company, set up for the purpose, named Jupiter Cantab. The Ace differed from other microcomputers of the time[1] in that it used FORTH instead of the more common BASIC.
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Jupiter Cantab was formed by Richard Altwasser and Steven Vickers.[2] Both had been on the design team for the Sinclair ZX Spectrum. Altwasser did some work on the development of the ZX-81 and in the design of the hardware of the Spectrum. Vickers adapted and expanded the 4K ZX-80 ROM to the 8K ZX-81 ROM and wrote most of the ROM for the Spectrum. The Jupiter Ace was named after the early British computer, the ACE. The name was chosen to emphasize the "firsts" of using FORTH environment as more efficient for personal computers.
FORTH is a threaded code programming language that also acted as operating system. As such, "Forth was developed as the world's first and, at that time, only practical, fully Integrated (and explicitly Interactive) software Development Environment (IDE)"[3]
The Jupiter Ace system was adapted to the disk-less tape-using home computer hardware. On average, and for similar programs, ACE's FORTH was 5 times faster and used half the memory (which was a costly luxury at the time) of an equivalent program written in interpreted BASIC.[4] Forth programs tend to be more memory efficient than the bigger programs; as they become bigger, they reuse more previously-defined code.[5]
For such reasons FORTH was chosen to deliver better performance and [structured programming] flexibility.[6]
The Jupiter ACE is often compared with ZX81 due to its similar size, low cost, and similar form factor, although internally it is an independent design. The ZX81 used 75% of its Z80 CPU time to drive the video. In ACE the Z80 CPU was fully used for running programs. The ACE used dedicated video memory of 2 KB, leaving the 1 KB main memory free for user programming.
The Jupiter ACE was new to the market and the designers couldn't afford to use an uncommitted logic array (ULA), which were common in other computers (such as the ZX 81) to reduce component count, because these weren't economical in small quantities. Instead they reduced the number of chips required via clever design.
Like the Spectrum, the Ace used black rubber keys. Audio capabilities were CPU controlled with programmable frequency and duration. Sound output was through a small built-in speaker. A television was needed as a display - which was in black and white only.
Programs and data storage was by means of a common cassette tape, which was typical of home computers of the time.
References to the ACE RAM usually include the separate 2 KB video memory, which is not available for user programs, thus leading to some confusion.
One 1K bank allowed redefinition of most of its 128 characters ASCII based characters in 8×8 pixel bitmap format. The other 1K bank stored the full screen display of 24 rows x 32 columns of characters in black and white. So while the Ace had only one text video mode, redefinition of the character shapes could provide graphics in a low resolution of 64x48, and a higher resolution mode of 256x192 graphics, limited by the 128 available (definable) 8x8 chars. Both graphics and text could be displayed at the same time.
The font of the character set is identical to that of the Spectrum, but the display is white on black whereas the Spectrum has colour hardware. Although a colour graphics board was designed[8] none was ever produced commercially.
An internal speaker was directly controlled by the CPU in single task mode, with control of sound frequency and its duration in milliseconds.
Storage was through a cassette tape interface at 1500 baud.
Originally developed to receive ZX-81 add-ons, with a compatible expansion slot, it was actually delivered in a different configuration. A simple rewiring adapter could be used, but not very effectively due to power losses. Dedicated add-ons were needed and built by external companies, after the initial 16 KB by Jupiter Cantab.
The ACE had an 8 KB ROM containing the Forth kernel and operating system, and the predefined dictionary of Forth words in about 5KB. The remaining 3KB of ROM were used for tape control, floating point numbers library and character definitions table.[15] Some of the ROM was written in Z80 machine code, but some was also coded in Forth.
The next 8 KB were split in 2 blocks of 4KB each. The video subsystem access allowing two different priorities by the user to the 2KB VRAM, Regular or Overriding video. The 1 KB of user RAM was only partially decoded, so it echoed in the full 4 kB block address space it resides. So a 16 KB space was used for ROM, VIDEO, and USER, leaving free a 16 KB space for RAM extension plus 32KB space free for all possible usages.
The major visible difference from previous introductory computers, was Forth as its default programming language. Forth was considered well adapted to microcomputers with small memory and relatively low-performance processors. Forth allowed control structures to be nested to any level, limited only by available memory. Forth allowed implementation of machine code routines, if needed for a particular task. Forth also allowed recursive programming, if desired. On the downside, the usage of a data stack and the associated Reverse Polish notation were as unfamiliar as was structured programming.
ACE's Forth was based mostly on Forth-79, with some relevant differences.[16] Runtime error checking could be turned off to raise speed by 25% to 50%. A few extra words were named similar to known BASIC sound, video and tape commands, as behavior was the same. The implementation lacked less frequently used Forth words, but these were easily implemented if needed.
The ACE's dialect of FORTH introduced several innovations. It simplified usual Forth definer and compiler words creation, replacing the CREATE .. DOES>,[17] creation pair with:
As an operating system, it was adapted to Tape usage, saving/loading user vocabularies instead of the usual numbered programming blocks used in diskette systems.
The ACE's Forth could decompile its programs, unlike usual Forth systems. This decompiling ability had several advantages as a solution to the absence of the more flexible disk system used by Forth. It did not store the text of a Forth program, instead it compiled the code after editing and stored it in ready-to-run format. While this saved computer memory it also saved time in reading and writing programs from cassette tape. This tape-friendly and RAM-saving solution was unique to the Jupiter ACE Forth.
Though Forth delivered several advantages over the interpreted BASIC used on all other contemporary home computers, the weak box and small initial memory kept the sales low despite technical interest.[20]
1982 - Original Jupiter ACE in a yogurt-pot type of case - Reported 8000 units built.[21]
1983 - Jupiter ACE 4000 on stronger injection moulded case - Reported 800 units built.[22]
Sales of the machine were never very large; as of the early 2000s, surviving machines are quite uncommon, fetching high prices as collector's items. One main reason for low sales seems to have been the need to buy an extra 16 KB RAM-extension which almost doubled the ACE's price. It happened that the designed 4 KB of base RAM was not built in favour of the less costly 1 KB delivered. The absence of colour kept the ACE squarely in a niche market of programming enthusiasts.