Editing Jupiter Ace (section)

=== 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>