The sideways address space on the Acorn BBC Microcomputer, Electron and Master-series microcomputer was Acorn's bank switching implementation, providing for permanent system expansion in the days before hard disk drives or even floppy disk drives were commonplace.
The Electron's sideways address space was exposed only by the addition of a Plus 1 add-on or a third-party equivalent; the Plus 1 also introduced cartridge slots that were carried over into the BBC Master design as an alternative way to package ROMs.
These include language and service entry points, ROM type code, version number and a pointer to the copyright information.
During operation the OS talks to valid ROMs by jumping to the two entry points with a specific value of the accumulator set.
This provides a clean API for expanding the operating system and negotiating bank switching and RAM sharing.
Pure service ROMs typically only extend the features of the OS itself, without providing any application software.
The version of Acorn MOS on the BBC Master has a built-in command line and will present this if no default language ROM is configured.
The methods vary, but the two most common methods are removing chips from the board and placing them into an expansion board that occupies the chips' original sockets, and fitting a RAM module in a ROM socket with a flying lead connected to a write signal elsewhere on the motherboard.
Due to limited motherboard space, extra sideways sockets were made available by third-party expansion boards.