Acorn MOS
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| 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 |
| Latest release | v5 / Early 1986 |
| Update method | Replacement ROMs |
| Platforms | BBC Micro, Acorn Electron, BBC Master Series |
| Default user interface | Command line interface (v3, v4, v5) |
Acorn's Machine Operating System (MOS) or OS is a discontinued 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 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, didn't run this operating system (but it also included its "asterisk" command line interface), was 32-bit and ran RISC OS. That 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 16 KiB 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 16 KiB 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.
The original MOS versions – 0 to 2 – did not have a user interface per se: applications were expected to forward operating system command lines to the OS on its behalf, and the BBC BASIC ROM supplied with the BBC Micro is the default application used for this purpose. The BBC Micro would halt with a "Language?" error if no ROM is present that advertises to the OS an ability to provide a user interface (so-called "language ROMs"). MOS version 3 onwards did feature a simple command-line interface, normally only seen when the CMOS did not contain a setting for the default language ROM.
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