The lower 16 KiB of the ROM map (0x8000 to 0xBFFF) is reserved for the active Sideways address space paged bank. The Sideways system on the BBC Micro allows for one ROM at a time from sockets on the motherboard (or expansion boards) to be switched into the main memory map. Software can be run from ROM this way (leaving the RAM free of user program code, for more workspace) and the OS can be extended by way of such ROMs. The most prevalent sideways ROM after BASIC is the Acorn Disc Filing System used to provide floppy disc support to the machine.