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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. filing systems, application and utility software and drivers were made available as Sideways ROMs, and extra RAM could be fitted via the Sideways address space. The Advanced User Guide to the BBC Micro only refers to the Sideways address space as "Paged ROMs" because it predated the use of this address space for RAM expansion. The BBC B+, B+ 128 and BBC Master all featured Sideways RAM as standard. ==Sideways address space== (詳細は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. 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. Sideways ROMs 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sideways address space」の詳細全文を読む スポンサード リンク
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