Editing Acorn Electron (section)

====Jafa Systems====
Released in 1987 at a price of £89,<ref name="electronuser198907">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume6/Electron-User-06-10/page/n17/mode/2up | title=One man band | magazine=Electron User | date=July 1989 | access-date=10 November 2020 | last1=Leah | first1=Tony | pages=18–19 }}</ref> the Mode 7 Mark 1 Display Unit was a separate unit "about the size, shape and colour of the Plus One or a Slogger ROMbox"<ref name="acornuser198805_jafa">{{ cite magazine | url=https://archive.org/details/AcornUser070-May88/page/n154/mode/1up | title=Electron a la Mode | magazine=Acorn User | date=May 1988 | access-date=17 April 2021 | last1=Emblem | first1=Bernard | pages=153 }}</ref> that connected to the Electron's expansion connector and featured a [[Motorola 6845]] display controller and [[Mullard SAA5050]] character generator to replicate the main elements of the BBC Micro's Teletext display solution. This only used 1&nbsp;KB of memory for the display, with the expansion listening to display memory write accesses and buffering the data in its own memory.<ref group=note>Here, [https://git.redump.net/mame/tree/src/devices/bus/electron/mode7.cpp the MAME source code emulating the Mark 1 adapter] is informative.</ref> A ROM was included to extend the operating system to allow activation of Mode 7 as a genuine screen mode and to provide extra commands and to support keyboard shortcuts used on the BBC Micro to emit Teletext control sequences. To support the output of both the Mode 7 display and the existing video output, a lead connected the Electron's RGB output to the expansion, with the expansion providing only RF (television) output.<ref name="electronuser198708">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume4/Electron-User-04-11/page/n19/mode/1up | title=Teletext on tap | magazine=Electron User | date=August 1987 | access-date=10 November 2020 | last1=Waddilove | first1=Roland | page=20 }}</ref>

Conscious of the relatively high price of the Mark 1 unit,<ref name="electronuser198907" />{{rp|page=19|quote=Interestingly, when it came out the Mode 7 Adaptor at £85 cost nearly twice as much as the Electron was going for in many places under the heading of obsolete stock. Because of this at the end of last year John decided to produce a £25 simulator including a Mode 7 screen editor program.}} John Wike of Jafa devised and, at the end of 1988, introduced a software-based ''Mode 7 Simulator'', priced at £25, supplied on a ROM cartridge that rendered the Mode 7 display in a low-resolution, 8-colour graphics mode. Although cheap and effective in enabling use of some software that only used official operating system routines for text output, this solution proved very slow because the Electron had to be placed into the high-bandwidth Mode 2 display to be able to show eight colours at once. In doing so, the CPU spent a lot of time drawing representations of Mode 7 characters and graphics that in a hardware solution would be achieved without any demand on the CPU. It also used up 20&nbsp;KB of RAM for the graphics display rather than the 1000 bytes of a hardware Mode 7.<ref name="electronuser198905">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume6/Electron-User-06-08/page/n36/mode/1up | title=The software solution | magazine=Electron User | date=May 1989 | access-date=9 November 2020 | last1=Cusack | first1=Stephen | pages=37–38 }}</ref>

A conceptually similar predecessor to the software-based simulator was published by Electron User in early 1987, offering a monochrome Mode 4 simulation of the Teletext display, using the lower 25 character lines of the screen to show the Teletext output, reserving several lines at the top of the screen for a representation of Mode 7 used to prepare the eventual visual output. However, the program did not support direct access to Mode 7 memory locations. The author noted that a Mode 2 version would have been possible but would have required a redesigned character set and "too much memory".<ref name="electronuser198701m7">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume4/Electron-User-04-04/page/n25/mode/2up | title=Go Mode 7 the software way | magazine=Electron User | volume=4 | issue=4 | date=January 1987 | access-date=19 January 2021 | last1=Nixon | first1=Robin | pages=26–27 }}</ref>

A further refinement of the hardware solution was introduced in 1989 with the Mode 7 Mark 2 Display Unit, which retained the SAA5050 character generator but omitted the 6845 display controller, and was fitted internally in the Electron itself instead of being housed in an external unit, although some kind of ROM expansion unit was needed to hold the driver/utilities ROM. It used software to ensure that the SAA5050 was fed with the correct character data. A software ROM would put the machine into a two-colour, 40-column graphics mode (thus providing one byte per character), and as the ULA read display data from memory in the usual fashion, the SAA5050 would listen to the data it was reading and produce a Mode 7 interpretation of the same information,<ref name="jafamk2">{{ cite book | url=http://www.bygonebytes.co.uk/images/Mode7Manual.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.bygonebytes.co.uk/images/Mode7Manual.pdf |archive-date=2022-10-09 |url-status=live | title=Jafa Systems User Guide for the Mode 7 Mk 2 Display Unit | publisher=JAFA Systems | date=1989 | access-date=10 November 2020 }}</ref>{{rp|page=3}} this being achieved by fitting a board on top of the ULA connecting to its pins.<ref name="jafamk2" />{{rp|page=20|quote=Take the smaller of the Mode 7 Mk2 boards and lay it on the under side of the Electron board so that the holes in it fit over the pins of the ULA.}} When necessary the hardware would switch between the conventional Electron graphics output and the Mode 7 output being produced by the add-on, feeding it to the Electron's built-in video output sockets via the red, green and blue lines on the motherboard.<ref name="jafamk2" />{{rp|page=22|quote=The diagram on page 16 shows 4 pads around IC3 marked R, G, B and Sy. These now need to be wired to the similarly labelled points on page 18.}}

The disadvantage to this system is that while the SAA5050 would expect to be repeatedly fed the same 40 bytes of data for every display scanline of each character row, the ULA would read a different set of 40 bytes for every display scanline in order to produce a full graphics display. A software ROM worked around this by duplicating the data intended for a Mode 7 display in memory. Although this produced a Mode 7 that had less of an impact upon CPU performance than a software solution, gave the same visual quality as the BBC Micro, and supported direct access to Mode 7 screen addresses as well as accesses via operating system routines, it still used 10&nbsp;KB of memory for the display and reduced the amount of readily-usable application memory  (as indicated by HIMEM) by another 6&nbsp;KB.<ref name="jafamk2" />{{rp|pages=3–4|quote=The other, between &4000 and &6800, is the Mode 4 display area that is actually scanned by the Mode 7 Mk2 circuits.}}

However, with users increasingly able to rely on expansions such as the Slogger Master RAM board to provide more memory, and with this combination of expansions acknowledged throughout the user manual, the emphasis of the Mode 7 Simulator and Mark 2 Display Unit was arguably to deliver the actual display capabilities for those applications that needed them, instead of using Mode 7 as a way of economising with regard to memory usage, and to do so at a reasonable price. In this latter regard, the Mark 2 model was available as a kit costing £25 or as an assembled product (requiring some soldering) costing £49, with a fitting service available for £10.<ref name="electronuser1989">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume7/Electron-User-07-02/page/n5/mode/1up | title=Add-ons boost for Electron | magazine=Electron User | date=November 1989 | access-date=11 November 2020 | page=6 }}</ref>

The Jafa interfaces did not provide a Teletext or Viewdata reception capability, but the Mark 2 was explicitly stated to work in conjunction with the Morley Electronics Teletext Adapter.<ref name="jafamk2" />{{rp|page=15|quote=To receive TELETEXT you will require a Master RAM board, a User Port and a Morley Teletext Adapter with separate power supply.}} Meanwhile, the manual for the Mark 2 noted that the product would provide the functionality of a Viewdata terminal if combined with Jafa's RS423 cartridge.<ref name="jafamk2" />{{rp|page=2|quote=If you use the socket in the JAFA Systems RS423 cartridge then you will have a self-contained PRESTEL terminal.}}