Acorn MOS allowed an extra 16 banks of sidways RAM, so you could get up to 256KB extra RAM, but you would need to buy an extra board. The B+ models (64/128KB)

Acorn MOS allowed an extra 16 banks of sidways RAM, so you could get up to 256KB extra RAM, but you would need to buy an extra board. The B+ models (64/128KB) also had an extra 32KB of Shadow RAM, 20KB of which shadowed the address space used for video and the other 12KB was for the MOS. So a theoretically maxed out Beeb would have 32+32+256=320KB of RAM. The cool thing about Sideways RAM is that you could use it for data, code, or to load ROM images which would work as normal ROMs.