Sideways RAM (ASR) products provided sideways ROM and RAM capabilities, allowing ROM- or EPROM-based software to be accessed to provide languages, utilities and applications. ROM or EPROM devices containing the software could be physically inserted into empty 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. 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 KB banks. 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: *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)". 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, obtaining the injection moulds and tooling, thus restarting production in 1987 after Acorn's own production of the unit had ceased in November 1985. The Advanced Plus 2 (AP2) ROM was later sold by PRES