Editing Jupiter Ace (section)

== Design ==
The Jupiter Ace is often compared with [[ZX81]] due to its similar size, low cost, and similar [[computer form factor|form factor]].<ref>{{cite web |title=Unpopulated Issue two PCB |url=http://jupiter-ace.co.uk/blankpcb.html |website=Jupiter Ace Archive}}</ref> Internally its design is more similar to the [[ZX Spectrum]] although the Ace also had a dedicated video memory of 2&nbsp;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 [[chiclet keyboard|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{{snd}} 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 [[tile-based video game|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 [[Central processor unit|CPU]] in single task mode, with control of sound frequency and duration in [[millisecond]]s.

Storage was through a cassette-tape interface at 1500 [[baud]]. Files could be used for either storage of Forth programs (compiled code) or raw [[core dump|dumps]] of memory.

=== Memory ===
The Ace had an 8 KB [[read-only memory|ROM]] containing the Forth [[kernel (operating system)|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).<ref>{{cite web | url=http://factor-h.net/_extra/ace_rom_project.zip| title=ACE ROM Project (E-Book)}}</ref> Some of the [[Read-only memory|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 [[address decoder|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 [[parallel computing|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,<ref>{{Cite news |last=Wike |first=John |date=April 1984 |title=Adding colour to the Ace |url=https://www.jupiter-ace.co.uk/hardware_colour_board.html |work=ETI |pages=41}}</ref> none was ever produced commercially.

1 KB RAM with the option of a 16 KB RAM-Pack, and later a 32 KB one. A [[printed circuit board|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.<ref>{{cite web |title=/pub/Vintage/Sinclair/80/Jupiter Ace/Peripherals/Motherboard (Boldfield) |url=https://k1.spdns.de/Vintage/Sinclair/80/Jupiter%20Ace/Peripherals/Motherboard%20(Boldfield)/ |website=The Sinclair ZX Computers Archive |access-date=8 December 2019}}</ref>

=== Specifications ===
{| class=wikitable
! Characteristic !! Value
|-
| [[Central processor unit|Processor]] || [[Zilog Z80A]] clocked at 3.25&nbsp;MHz.<ref>{{cite book|title=Tecnologías Libres para Síntesis de Imagen Digital Tridimensional|year=2006 |page=[https://archive.org/details/2006TecnologiasLibresParaSintesisDeImagen3D/page/40 40]|publisher=Self |isbn=978-84-689-9280-8|quote=Micro Z80 a 3.25 MHz [...], el Jupiter Ace [...]|url=https://archive.org/details/2006TecnologiasLibresParaSintesisDeImagen3D/page/40}}</ref>
|-
| [[Operating system]] || [[Forth (programming language)|FORTH]] (as both the [[programming language]] and [[command-line interface]]).
|-
| [[Static random-access memory|Memory]] || 2 [[kilobyte|KB]] for Video + 1 [[kilobyte|KB]] base expandable up to 49 KB (Video excluded). 
|-
| [[Video]] || Independent sub-system with 2 dedicated [[static random-access memory|SRAM]] banks: Screen (1 KB) + Char Tiles (1 KB).
|-
| [[Sound]] || Internal speaker, CPU-driven (non-dedicated).
|-
| [[Expansion pack|Expansion]] || 2 connectors: Main (CPU related) + Video (Screen + AV signal).
|-
| [[Computer keyboard|Keyboard]] and [[Charset]] || 40-key [[QWERTY]] keyboard (Symbols access with extra key).
All chars in Charset (based on [[ASCII-1967]]) being redefinable.
|-
| [[Character encoding|Character set]] || Based on [[ASCII-1967]] with extensions as for the [[ZX Spectrum character set]] including ↑, £ and ©.
|}

References to the Ace [[static random-access memory|RAM]] sometimes include the separate 2 [[kilobyte|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&nbsp;KB standard RAM was in effect comparable to at least 2&nbsp;KB on a BASIC system.