Acorn MOS

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Machine Operating System (MOS)

Developer	Acorn Computers
Written in	8-bit 6502 machine code (v0, v1) 65C02 machine code (v2–v5)
Working state	Discontinued
Source model	Closed source
Initial release	Late 1981; 43 years ago (1981)
Final release	5 / Early 1986; 38 years ago (1986)
Marketing target	Personal computers
Available in	English
Update method	Replacement ROMs
Platforms	BBC Micro, Acorn Electron, BBC Master series
Kernel type	Monolithic
Default user interface	Command-line interface (v3, v4, v5)
Succeeded by	ARX (discontinued) Arthur, renamed RISC OS

The Machine Operating System (MOS)^[2] or OS is a discontinued computer operating system (OS) used in Acorn Computers' BBC computer range. It included support for four-channel sound, graphics, file system abstraction, and digital and analogue input/output (I/O) including a daisy-chained expansion bus. The system 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.

The final BBC computer, the BBC A3000, was 32-bit and ran RISC OS, which kept on portions of the Acorn MOS architecture and shared a number of characteristics (e.g. "star commands" CLI, "VDU" video control codes and screen modes) with the earlier 8-bit MOS.

Versions 0 to 2 of the MOS were 16 KiB in size, written in 6502 machine code, and held in read-only memory (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 3 to 5 were still restricted to a 16 KiB address space, but managed to hold more code and hence more complex routines, partly because of the alternative 65C102 central processing unit (CPU) with its denser instruction set plus the careful use of paging.