Sideways Address Space
The BBC Micro and Master-series machines used the 8-bit 6502 and 65C102 processors with a 16-bit address space. The address space was split into 32 KiB RAM (0x0000 to 0x7FFF), 16 KiB Sideways address space (0x8000 to 0xBFFF) and 16 KiB operating system space (0xC000 to 0xFFFF).
The Sideways address space is a bank-switched (referred to by Acorn as "paged") address space that allows access to one 16 KiB bank at a time. Each bank can be ROM or RAM. On both the BBC Micro and the BBC Master, there are ROM sockets on the motherboard (four on the BBC Micro) which take "Sideways ROMs". The BBC Micro shipped with a single ROM, containing BBC BASIC. Further ROMs can be added to the computer to add software that will remain available at all times; this permitted the addition of new filing systems to the OS (such as the Disc Filing System) and application and utility software. Software supplied as ROMs has two main benefits: it loads instantaneously, and it requires very little RAM to operate (and may use the dedicated paged ROM area of RAM that normal software keeps clear of). This allowed for application software to have more working space than would normally be possible, and for utility software such as debuggers to operate on software held in RAM.
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