Editing Acorn Electron (section)

====Acorn Plus 1====
[[File:Acorn Electron and Plus 1.jpg|thumb|left|250px|An Acorn Electron with Plus 1 expansion unit attached]]
The Acorn Plus 1 added two ROM cartridge slots, an analogue interface (supporting four channels) and a [[IEEE 1284|Centronics parallel port]], priced at £59.90.<ref name="electronuser198407p1" /> The analogue interface was normally used for joysticks, although trackball<ref name="electronuser198510wh">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume3/Electron-User-03-01/page/n21/mode/1up | title=One way to have designs in the palm of your hand | magazine=Electron User | volume=3 | issue=1 | date=October 1985 | access-date=24 January 2021 | last1=Waddilove | first1=Roland | page=22 }}</ref> and graphics tablet<ref name="electronuser198603wh">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume3/Electron-User-03-06/page/n35/mode/1up | title=Try a tablet for pixel resolution | magazine=Electron User | volume=3 | issue=6 | date=March 1986 | access-date=24 January 2021 | last1=Nixon | first1=Robin | page=36 }}</ref> devices were available, and the parallel interface was typically used to connect a printer. Game ROM cartridges would boot automatically. Languages in paged ROM cartridges would take precedence over BASIC.<ref name="electronuser198409a">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume1/Electron_User_01-12/page/n47/mode/1up | title=The Electron and the Plus 1 - made for each other | magazine=Electron User | date=September 1984 | access-date=14 January 2021 | page=48 }}</ref> (The slot at the front of the interface took priority if both were populated.)

Access to ROM occurs at 2&nbsp;MHz until RAM access is required, so theoretically programs released on ROM can run up to twice as fast as those released on tape or disc. Despite this, all of the games released on ROM were packaged as ROM filing system cartridges, from which the micro would load programs into main memory in exactly the same way as if it were loading from tape.<ref name="electronuser198411_rom">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume2/Electron-User-02-02/page/n22/mode/1up | title=Snappy ROM | magazine=Electron User | date=November 1984 | access-date=4 September 2022 | last1=Waddilove | first1=Roland | pages=23 }}</ref> This meant that programs did not need to be modified for their new memory location and could be written in BASIC but gave no execution speed benefits.<ref name="acornuser198407">{{cite magazine |url=https://archive.org/details/AcornUser024-Jul84/page/n19/mode/1up |title=Expansion Factor |magazine=Acorn User |date=July 1984 |access-date=24 October 2020 |last1=Smith |first1=Bruce |page=18 |quote=The filing system used by the ROM cartridge system is in fact the *ROM filing system (RFS) as used on the Beeb, making it a simple task for commercial establishments to blow their own cartridge ROMs. *CAT, LOAD and CHAIN are all available with the RFS.}}</ref> Six ROM cartridge titles were announced for the launch of the Plus 1: three arcade games, one adventure game, one educational title, and the ''[[Acornsoft LISP|Lisp]]'' language implementation,<ref name="acornuser198406a">{{cite magazine |url=https://archive.org/details/AcornUser023-Jun84/page/n10/mode/1up |title=Six-pack send-off for Electron ROM box |magazine=Acorn User |date=June 1984 |access-date=25 October 2020 |page=9}}</ref> the latter being a genuine language ROM that "takes the place of the BASIC ROM" and is instantly available when switching on.<ref name="acorn_amp031" />

The cartridge slots provide additional control lines (compared to the lines available via the edge connector on the rear of the Electron) to ease implementation of ROM cartridges.<ref>Electron Advanced User Guide{{snd}} Appendix E</ref> Acorn described the hardware extension possibilities in promotional literature, giving an RS423 cartridge as an example of this capability of the Plus 1.<ref name="acorn_amp031">{{ cite book | url=http://www.4corn.co.uk/archive/docs/AMPAPP/150/AMP031%20-%20electron%20plus%201-opt.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.4corn.co.uk/archive/docs/AMPAPP/150/AMP031%20-%20electron%20plus%201-opt.pdf |archive-date=2022-10-09 |url-status=live | title=Electron Plus 1: New Dimensions in Capability | publisher=Acorn Computers Limited | access-date=7 February 2021 }}</ref>

Additional peripheral cartridges were produced by companies such as Advanced Computer Products (and subsequently PRES) whose Advanced ROM Adaptor (ARA) and Advanced Sideways RAM (ASR) products provided [[Sideways address space|sideways ROM and RAM]] capabilities,<ref name="aeworld_acp">{{ cite web |url=http://www.acornelectron.co.uk/hardware/cats/acp_pres.html |title=Archive Catalogue Details: ACP/PRES |website=Acorn Electron World |access-date=6 March 2016  |archive-url=https://web.archive.org/web/20160306153913/http://www.acornelectron.co.uk/hardware/cats/acp_pres.html |archive-date=6 March 2016 |url-status=dead}}</ref>  allowing ROM- or EPROM-based software to be accessed to provide languages, utilities and applications.<ref name="electronuser198702_cartridges">{{ cite magazine | url=https://archive.org/details/ElectronUserVol4/Electron%20User%20%20Vol%204%20No.5/page/n59/mode/1up | title=Cartridge survey | magazine=Electron User | date=February 1987 | access-date=8 April 2022 | last1=Waddilove | first1=Roland | pages=60 }}</ref> ROM or EPROM devices containing the software could be physically inserted into empty [[Zero insertion force|ZIF]] sockets, or the software would be loaded from ROM image files (typically provided on disk) into RAM devices fitted in such sockets. Such RAM could potentially be powered by a battery and thus be able to retain its contents when the computer itself was powered off. Both such arrangements exposed the software in the same sideways memory region.<ref name="electronuser198706_acp_bbr">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume4/Electron-User-04-09/page/n36/mode/1up | title=Keep your memories fresh | magazine=Electron User | date=June 1987 | access-date=8 April 2022 | last1=Waddilove | first1=Roland | pages=37, 39 }}</ref>

Such cartridge support enabled the Electron to provide the same functionality as that offered by the expansion ROM slots under the keyboard and on the bottom-left of the BBC Micro B keyboard. However, the need to use cartridge sockets for other peripherals encouraged PRES to develop the Advanced Plus 6 (AP6): an internal RAM and ROM board for the Plus 1 providing six sockets that could be freely used for ROM, EPROM and RAM devices. Installation of the AP6 unit required some modifications to the Plus 1, undertaken either by the user or by PRES, and the product could also be enhanced with the Advanced Plus 7 offering battery-backed RAM support for two 16&nbsp;KB banks.<ref name="electronuser198805_ap6">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume5/Electron-User-05-08/page/n30/mode/1up | title=More room for your roms | magazine=Electron User | date=May 1988 | access-date=8 April 2022 | last1=Nixon | first1=Chris | pages=31 }}</ref>

The addition of the Plus 1 added a number of new *FX or OSBYTE calls that allowed the OS to read values from the analogue interface and write to the parallel interface.

The Plus 1 needed memory page &D for its workspace, and the unit added some processing overhead when enabled, both of these things causing issues with the loading and running of software, particularly cassette-based games. To disable the Plus 1, after pressing BREAK, the following commands could be issued:<ref name="electronuser198410p1">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume2/Electron-User-02-01/page/n60/mode/1up | title=Your Micro Power games and the Plus 1 | magazine=Electron User | volume=2 | issue=1 | date=October 1984 | access-date=24 January 2021 | page=61 }}</ref>

 *FX163,128,1
 ?&212=&D6
 ?&213=&F1
 ?&2AC=0

An official application note described a similar set of commands to "remove the Plus One completely from the address map disabling the Centronics and A/D ports (additionally disabling the RS423 cartridge if fitted)".<ref name="acorn_an013">{{ cite tech report | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/AN/013.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://chrisacorns.computinghistory.org.uk/docs/Acorn/AN/013.pdf |archive-date=2022-10-09 |url-status=live | title=Acorn Electron Plus One: Disabling the Centronics and A/D Port | publisher=Acorn Computers Limited | date=6 July 1992 | access-date=10 March 2021 | issue=1 | page=2 }}</ref>

=====Further developments=====

After Acorn's change of focus away from the Electron, and with a shortage of Plus 1 units available to purchase, Advanced Computer Products secured the rights to manufacture the unit under licence from Acorn,<ref name="abcomputing198708" /> obtaining the injection moulds and tooling, thus restarting production in 1987 after Acorn's own production of the unit had ceased in November 1985.<ref name="electronuser198703">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume4/Electron-User-04-06/page/n4/mode/1up | title=Plus 1 going back into production | magazine=Electron User | volume=4 | issue=6 | date=March 1987 | access-date=18 January 2021 | page=5 }}</ref> The Advanced Plus 2 (AP2) ROM was later sold by PRES as a replacement ROM for the Plus 1, of whose 8&nbsp;KB utility ROM only 3&nbsp;KB had been used, thus providing an opportunity for a more comprehensive ROM to be developed. The AP2 added ROM management commands familiar from the [[BBC Master]] series, support for various sideways RAM products from PRES, disc formatting and verification utilities for different ADFS versions, a command to disable the Plus 1 entirely, and improvements that made tape loading more reliable in "high memory" screen modes.<ref name="electronuser198807_ap2">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume5/Electron-User-05-10/page/n24/mode/1up | title=More power under your bonnet | magazine=Electron User | date=July 1988 | access-date=30 October 2022 | last1=Nixon | first1=Chris | pages=25 }}</ref>