Acorn MOS

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Acorn Machine Operating System
Acorn MOS Version 320.png
DeveloperAcorn Computers
Written in8-bit 6502 machine code (v0, v1) 65C02 machine code (v2–v5)
Working stateDiscontinued
Source modelClosed source
Initial releaseLate 1981; 38 years ago (1981)
Latest releasev5 / Early 1986; 33 years ago (1986)
Update methodReplacement ROMs
PlatformsBBC Micro, Acorn Electron, BBC Master Series
Default user interfaceCommand line interface (v3, v4, v5)

Acorn's Machine Operating System (MOS)[2] 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.

User interface[edit]

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.

Application programs on ROM, and some cassette and disc-based software also, typically provide a command line, useful for working with file storage such as browsing the currently inserted disc. The OS provides the line entry facility and obeys the commands entered, but the application itself oversees running the command prompt.

Cassette and disc based software typically relies on BBC BASIC's own user interface in order to be loaded, although it is possible to configure a floppy disc to boot up without needing to have BASIC commands executed; in practice, this was rarely performed.

In BBC BASIC, OS commands are preceded with an asterisk or passed via the OSCLI keyword, to instruct BASIC to forward that command directly to the OS. This led to the asterisk being the prompt symbol for any software providing an OS command line; MOS version 3 onwards officially uses the asterisk as the command prompt symbol. When referring to an OS command, they generally include the asterisk as part of the name, for example *RUN, *CAT, *SPOOL etc., although only the part after the asterisk is the command itself.

Unrecognised commands are offered to any "service" (extension) ROMs; filing system ROMs will often check to see if a file on disc matches that name, the same most other command-line interfaces do. The operating system call OSWORD with accumulator = 0 does however offer programs single line input (with ctrl-U for clear line and the cursor copying keys enabled) with basic character filtering and line length limit.

The MOS command line interpreter features a rather unusual idea: abbreviation of commands. To save typing a dot could be used after the first few characters, such as *L. for *LOAD and *SA. for *SAVE. *RUN was abbreviated to */ alone. *CAT, the command to catalogue (list) a cassette or disc, can be abbreviated right down to *..

Service ROMs[edit]

3rd party ROMs generally also support command abbreviation, leading to ambiguity where two service ROMs provide commands which are very similar in name but possibly different in functionality. In this case the MOS would prioritise the command from the ROM in the higher numbered ROM slot – i.e. 7 would take precedence over 6.

Some 3rd party suppliers would get around this by prefixing their star commands with other letters – for example Watford Electronics ROMS would have their star commands prefixed with "W" therefore making them unique.


The lower 16 KiB of the read-only memory map (0x8000 to 0xBFFF) is reserved for the active Sideways paged bank. The Sideways system on the BBC Micro allows for one ROM at a time from sockets on the motherboard (or expansion boards) to be switched into the main memory map. Software can be run from ROM this way (leaving the RAM free of user program code, for more workspace) and the OS can be extended by way of such ROMs. The most prevalent sideways ROM after BASIC is the Acorn Disc Filing System used to provide floppy disc support to the machine.

During a reset, every paged ROM is switched in and asked how much public and private workspace it needs. Each ROM is allocated a chunk of private workspace that remains allocated at all times, and a single block of public workspace, equal to the size of the largest request, is made available to the active ROM. During operation, the paged area is rapidly switched between ROMs when file system commands are issued and unrecognised commands are put to the OS.

MOS allocates a 3.5 KiB block of memory (0x0000 to 0x0DFF) from the bottom of the memory map for operating system and language ROM workspace:

Address Usage
0x0000-0x00FF current language workspace, misc system workspace
0x0100-0x01FF CPU stack
0x0200-0x02FF OS vectors, system variables, other workspace
0x0300-0x03FF VDU driver workspace, cassette system workspace, keyboard input buffer
0x0400-0x07FF allocated to the current language ROM, or to the Tube. BASIC uses 0x0400-0x046B for the single-letter integer variables @% through Z% (4 bytes per letter), which are therefore accessed more quickly than are variables on the heap.
0x0800-0x08FF sound system (stores ADSR envelopes and the tone buffer; if sound is not in use then a small amount of user data can be stored here), printer buffer, ENVELOPE definitions
0x0900-0x09FF cassette and serial input buffers (user data can be placed here if cassette is not in use, so many small machine code utilities were assembled into page 9)
0x0A00-0x0AFF cassette and serial output buffers, also used for small machine code utilities.
0x0B00-0x0BFF user-defined keys (strings that are inserted when function keys are pressed, set by the *KEY command)
0x0C00-0x0CFF user-defined characters for non-teletext modes (user data can be placed here if the display is in teletext mode)
0x0D00-0x0DFF Non-Maskable Interrupt service routine, Networking workspace, Mouse/trackerball workspace, Indirect vectors to paged ROMs and page ROM workspace pointers


On a cassette-only machine, 0x0E00 is the start of user program memory. With OS extension ROMs fitted such as the a filing system ROM, more memory is allocated above this point; DFS ROMs generally use another 2.75 KiB to cache the disc catalogue and manage random access buffers. A network filing system ROM (for Econet) allocates another 0.5 KiB on top of this. This is a serious problem because MOS does not support relocation of machine code, which must be run from the address at which it was assembled, so some programs which assumed a fixed start of user program memory could overwrite MOS workspace. The problem was alleviated in versions 3 to 5 by allowing ROMs to allocate workspace in an alternative RAM bank at 0xC000 to 0xDFFF which was present in Master series computers, though old ROMs could continue to allocate blocks of main memory.

The OS also maintains a vector table of all its calls which can be updated to hook any OS calls for user extension. By altering or 'hooking' these vectors, developers could substitute their own routines for those provided as defaults by the MOS.

Text, graphics, printing[edit]

The MOS permits textual output intended for the screen to be directed instead to the printer, or both at once, allowing for very trivial printing support for plain text. Graphics printing is not supported and has to be written separately.

Graphics and in general all screen output is handled in a very unusual way. The ASCII control characters are almost entirely given new significance under MOS: known as the "VDU drivers", because the documentation described them in relation to the VDU statement in BBC BASIC, they are interpreted as video control characters. VDU 30 (i.e. ASCII 30) moves the cursor to (0, 0), VDU 4 and 5 select whether text should be drawn at the graphics or text cursor, VDU 12 clears the screen and VDU 14 and 15 turn scroll lock on and off. Thus, pressing ctrl-L will clear the screen and ctrl-N will enable scroll lock. VDU 2 and 3 toggle whether screen output is echoed to the printer. The BBC BASIC VDU statement VDU x [, y[, z...]] is equivalent to the conventional BASIC PRINT CHR$(x) [; CHR$(y)[; CHR$(z)...]] and many of the control codes (such as 12 for "clear screen" and 7 for "beep") have the same functions as on other contemporary machines.

Many more control characters take parameters: one or more characters that follow are used solely for their bit value as a parameter and not as a control code. VDU 19 handles palette remap; the following five bytes represent the palette entry, the desired colour and three reserve bytes. VDU 31 locates the text cursor to the location held in the following two bytes. VDU 17 sets the text colour and 18 the graphics colour. VDU 25 uses the succeeding five bytes to move the graphics cursor and plot solid and dashed lines, dots and filled triangles, the documented extent of graphics in MOS 0 and 1. The first byte is the command code, followed by the x and y co-ordinates as two byte pairs. Other graphic functions – such as horizontal line fill bounded by a given colour – were available by use of undocumented or poorly documented command codes.

BBC BASIC contained aliases for the commonly used VDU codes (such as GCOL for VDU 18 or PLOT for VDU 25). Some statements were direct equivalents to VDU codes, such as CLS for VDU 12. Some statements were less exact equivalents as they incorporated functionality specific to BASIC as well as calling the OS routines; for example the statement MODE x would set screen mode x and adjust the BASIC system variable HIMEM according to the amount of memory the new mode left available for BASIC, while VDU 22, x would set the screen mode only, without altering HIMEM. This allowed a programmer to allocate a block of memory from BASIC – for example to load machine code routines into – by lowering the value of HIMEM at the start of a program, and still be free to switch screen modes without deallocating it as a side effect.

There is a single operating system command to write a character, OSWRCH, which is responsible for all text and graphics. For example, to move the cursor to (10, 15), you could write (in 6502 assembler):

LDA #31: JSR OSWRCH \ move text cursor
LDA #10: JSR OSWRCH \ x-coordinate
LDA #15: JSR OSWRCH \ y-coordinate

(LDA loads a value into the accumulator; JSR is "jump to subroutine".) On the third OS call, the cursor will move. The following code would draw a line from (0, 0) to (0, +100):

LDA #25: JSR OSWRCH \ begin "PLOT" (ASCII 25) command
LDA #4: JSR OSWRCH \ command k=4, or move absolute
 \ send (0, 0) as low, high byte pairs
LDA #25: JSR OSWRCH \ begin PLOT
LDA #1: JSR OSWRCH  \ k=1 - draw relative
LDA #100: JSR OSWRCH \ y = 100 (low byte)
LDA #0: JSR OSWRCH   \ high byte

In BBC BASIC you can perform the above as any of the following:

 VDU 25, 4, 0; 0; 25, 4, 100; 0;

 PRINT CHR$(25); CHR$(4); CHR$(0); ... etc.

 PLOT 4, 0, 0: PLOT 1, 0, 100

 MOVE 0, 0: DRAW 0, 100: REM absolute co-ords only!


Graphics in the Acorn MOS use a virtual graphics resolution of 1280×1024, with pixel positions mapped to the nearest equivalent pixel in the current graphics mode. Switching video resolution will not affect the shape, size or position of graphics drawn even with completely different pixel metrics in the new mode, because this is all accounted for by the OS.

Acorn MOS does provide two other OS calls that handle text output: OSNEWL and OSASCI. OSNEWL writes a line feed and carriage return to the current output stream. OSASCI forwards all characters directly to OSWRCH except for carriage return, which triggers a call to OSNEWL instead. The precise code for OSASCI and OSNEWL – five lines of 6502 assembler – is documented in the BBC Micro User Guide.

Acorn MOS implements character recognition so that text printed on screen in the system font can be selected with the arrow keys and input with the COPY key as though it was being typed. To activate screen editing the user moves the hardware cursor to the text to be read and the OS displays a second cursor in software at the original position. Pressing COPY copies one character from the hardware cursor to the software cursor and advances both, so that holding the key down copies a section of the text, the cursors wrapping around the vertical edges of the screen as necessary. If the screen scrolls during editing, the hardware cursor's position is adjusted to follow the text. The user can make changes to the text during the copy, and user-defined characters are recognised in graphics modes. Screen editing is terminated when RETURN or ESCAPE are pressed, which have their usual effects. Character recognition is made available to users in the API with a call to read the character at the current cursor position.


Sound generation is carried out through another OS call, OSWORD, which handles a variety of tasks enumerated via a task code placed into the accumulator. All OSWORD calls bear a parameter block used to send and receive multiple data; the address of this block is passed in the X and Y registers, with the low byte in X and the high byte in Y. There are four buffered sound channels – three melodic and one noise-based on the sound chip found in the BBC Micro. There is only one waveform for melodic channels; the supported note parameters are pitch, duration, amplitude, envelope selection and various control options. For the amplitude parameter, a zero or negative sets a static amplitude, and a positive value select an amplitude and pitch envelope (a predefined temporal variation) to apply to the note.

Control parameters was passed through the channel parameter, and include flush (the buffer is cleared and the channel silenced before the note is played), synchronise count (as soon as the same sync count is received for that many channels, all the synchronised notes are played together), and control over the Speech system upgrade where fitted.

OSWORD handles many functions other than sound, many of which do not have direct support in BASIC. They may be accessed from BASIC by setting up the parameter block, loading its address into X% and Y% and the task code into A%, and then calling the routine.

Other I/O and second processor support[edit]

The BBC Micro had support for a second processor connected via the "Tube", which allowed direct access to the system bus. The driver code for the Tube interface itself is not present in the MOS, usually being supplied by an external service ROM.

The OS has calls to handle reading and writing to all I/O (ports and screen memory) and programmers are strongly advised to use these by the Acorn documentation. The reason for this is that when a second processor is installed, user software is run from the separate memory map on the far side of the Tube processor bus, and direct access to memory-mapped I/O registers and video memory is impossible. However, for the sake of performance, many apps including many games write directly to main address space for I/O, and hence crash or give you a blank screen if a 6502 second processor is attached. One such performance-critical area is sprite support; the BBC Micro hardware does not support sprites and games must implement sprites in software. In practice, the widespread use of direct access in place of the OS calls very rarely caused difficulties. Second processor units were expensive and very little software was written to make use of them, so few people bought them, and those who did have them could simply switch them off or unplug the cable if a problem arose.

The MOS contains two built-in file systems: cassette and ROM. These are quite similar (try *ROM, *OPT 1 2, *CAT with a suitable ROM installed) and share a great deal of code. They feature a rudimentary copy protection mechanism where a file with a certain flag set cannot be loaded except to execute it.[5] (Prior to Amstrad's launch of a mass-market twin cassette recorder in 1987,[6] most home users did not have facilities to dub cassettes without loading the files into the computer for re-saving.) The Advanced Disc Filing System (ADFS), installed as standard in the Master series, has a similar mechanism.[7]


Releases 0 and 1[edit]

Versions for the BBC Micro family, starting at 0.10 and finishing at 1.20. Confusingly the Electron shipped with version 1.00 despite being released after the BBC Micro's version 1.20, because it was the first release of a ROM for the electron. The MOS version number was not intended as an API definition: the Electron ROM was not "based on" the BBC Micro ROM version 1.0 in any sense.

Release 2[edit]

This version is for the BBC Model B+, essentially the same as MOS 1.20 except with the addition of support for the sideways and shadow RAM present on the B+.

Releases 3 to 5[edit]

MOS 3 to MOS 5 shipped with the BBC Master Series systems, in the Master 128, Master ET, and Master Compact models respectively.

The initial release of MOS 3 expanded upon the facilities provided in MOS 2 on the B+ to support additional hardware, provide a command line facility and extend the VDU driver code with enhanced graphics plotting abilities. Two notable versions were made public: version 3.20 being the most common, and version 3.50 (although this had more functionality and bug fixes[8] it was not 100% compatible with some popular applications software[9] so was offered only as an optional upgrade).

MOS 4 was a stripped down version of MOS 3 intended for the similarly stripped down Master ET, and a few minor bugs fixed.

MOS 5 shipped with the Master Compact, and was much altered with some functionality removed or highly amended.


With the exception of MOS 3.50 where the space was reclaimed for more code, the area normally hidden by the input/output memory locations (the 768 bytes from 0xFC00-0xFEFF inclusive) in the MOS ROM contained a list of names of contributors to the system. This could be recovered by extracting the ROM and reading its contents in an EPROM programmer. Those who did not have such a device could access the ROM on a Master by setting a test bit of an access control register, then using a machine-code program to copy the ROM directly to text-mode screen memory.[10]

The full text of the credit string in MOS 1.20 is as follows (there are no spaces after the commas so as to save memory):

"(C) 1981 Acorn Computers Ltd. Thanks are due to the following contributors to the development of the BBC Computer (among others too numerous to mention):- David Allen,Bob Austin,Ram Banerjee,Paul Bond, Allen Boothroyd, Cambridge,Cleartone, John Coll,John Cox,Andy Cripps, Chris Curry,6502 designers,Jeremy Dion,Tim Dobson,Joe Dunn,Paul Farrell, Ferranti, Steve Furber,Jon Gibbons,Andrew Gordon,Lawrence Hardwick,Dylan Harris, Hermann Hauser, Hitachi, Andy Hopper, ICL,Martin Jackson,Brian Jones,Chris Jordan,David King,David Kitson,Paul Kriwaczek, Computer Laboratory,Peter Miller, Arthur Norman,Glyn Phillips,Mike Prees,John Radcliffe,Wilberforce Road, Peter Robinson, Richard Russell,Kim Spence-Jones,Graham Tebby,Jon Thackray,Chris Turner,Adrian Warner, Roger Wilson,Alan Wright."


In interviews in 1993 and 2001, Acorn co-founder Hermann Hauser recounted that Microsoft's Bill Gates, having noticed that 1.5 million BBC Micros were sold, tried to sell MS-DOS to Acorn, but Hauser considered that adopting MS-DOS would have been a "retrograde step" in comparison with retaining Acorn's system.[11][12]


  1. ^ Cult of Mac (2017-06-29). "Rare iPhone P-series prototypes run Acorn OS 3:46". Retrieved 2019-01-05.
  2. ^ Coll, John (1982) [1982]. Allen, David (ed.). The BBC Microcomputer User Guide (PDF). London: British Broadcasting Corporation. p. 443. ISBN 0-563-16558-8. Retrieved 28 March 2017.
  3. ^
  4. ^ EUG a-os2
  5. ^ Bray, Andrew C.; Dickens, Adrian C.; Holmes, Mark A. (1983). The advanced user guide for the BBC Microcomputer. Cambridge: Cambridge Microcomputer Centre. p. 347. ISBN 0-946827-00-1. Archived from the original (zipped PDF) on 2006-01-14. Retrieved 19 April 2012.
  6. ^ point 6
  7. ^ "5. The filing system commands". The Advanced Disc Filing System user guide (PDF) (1st ed.). Acorn Computers. August 1985. p. 32. Retrieved 22 July 2018.
  8. ^ David Spencer (December 1989). "The New Master Mega-ROM". Beebug. RISC Developments Ltd. 8 (7): 6–7. fix bugs in the original MOS, such as the infamous CLOSE #0 bug in the DFS...ADFS has been speeded up by a factor of at least two...Format, Verify and Backup utilities are now included in the ROM...speed increases in programs that perform a lot of floating point...allows international characters to be entered from the keyboard...Acorn have decided not to include the alternative ROM as standard in new Masters. The reason, they say, is that while the new ROM is highly compatible, there will be some existing programs that don't work with it. In particular, the new 8-bit key handling may confuse some programs, and any program which directly accesses routines within the ROM will almost certainly fail. I feel that Acorn have made a wise decision, considering the amount of software already available for the Master, but I don't think that you should be put off by incompatibility - in practice most programs will work.
  9. ^ Derek Gibbons (November 1990). "Some Comments on the New Master ROM". Beebug. RISC Developments Ltd. 9 (6): 38–39. *FX225,2 highlights another problem area...because the action of this command has now been changed from that originally documented, such programs no longer work
  10. ^ David Holton (April 1992). "Mr Toad's Machine Code Corner: The Hidden Persuaders (Part 1)". Beebug. RISC Developments Ltd. 10 (10): 6. setting bit 6 of the access control register ACCON (&FE34) will select this part of the ROM and the names may be read ... On page F.2-3 of the Master Reference Manual (Part 1), where each bit of ACCON is given a separate name, bit 6 is called TST
  11. ^ Grossman, Wendy M. (May 1993). "Missing The Big Time". Personal Computer World. Retrieved November 6, 2012.
  12. ^ Woolley, Alice (June 6, 2001). "My Biggest Mistake: Yes, I could have been Bill Gates". The Independent. Retrieved 2 June 2017.
  • Watford Electronics, "The Advanced Reference Manual for the BBC Master Series", 1988