Acorn's Machine Operating System (MOS) or OS was a computer operating system used in the Acorn BBC computer range. It included support for four-channel sound and graphics, file system abstraction, and digital and analogue I/O including a daisy-chained fast expansion bus. The implementation was single-tasking, monolithic and non-reentrant.Versions 0.10 to 1.20 were used on the BBC Micro, version 1.00 on the Electron, version 2 was used on the B+, and versions 3 to 5 were used in the BBC Master Series range.The final BBC computer, the BBC A3000, was 32-bit and ran RISC OS. Its operating system used portions of the Acorn MOS architecture and shared a number of characteristics (commands, VDU system) with the earlier 8-bit MOS.Versions 0 and 1 of the MOS were 16KiB in size, written in 6502 machine code, and held in ROM on the motherboard. The upper quarter of the 16-bit address space (0xC000 to 0xFFFF) is reserved for its ROM code and I/O space.Versions 2 to 5 were still restricted to a 16KiB address space but managed to hold more code and hence more complex routines, partly because of the alternative 65C102 CPU with its denser instruction set plus the careful use of paging.