Acorn MOS

Acorn MOS

Infobox OS
name = Acorn Machine Operating System

caption =
developer = Acorn Computers
source_model = Proprietary
kernel_type =
supported_platforms = BBC Micro, Acorn Electron, BBC Master Series
ui = Command line interface (v3, v4, v5)
family =
released = Late 1981
latest_release_version = v5
latest_release_date = Early 1986
latest_test_version =
latest_test_date =
marketing_target =
programmed_in = 6502 machine code (v0, v1) 65C02 machine code (v2-v5)
prog_language = BBC BASIC, 6502 Assembler
language =
updatemodel = Replacement ROMs
package_manager =
working_state = Historic
license =
website =

Acorn's Machine Operating System 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 re-entrant.

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.

User interface

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 a cassette or disc, can be abbreviated right down to *.

Service ROMs – which add new commands to the OS – 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.


The lower 16 kB 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 3½-kB block of memory (0x0000 to 0x0DFF) from the bottom of the memory map for operating system and language ROM workspace. 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 2¾ kB to cache the disc catalogue and manage random access buffers. A network filing system ROM (for Econet) allocates another ½ kB 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 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

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", 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.

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 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.

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:

LDA #31: JSR OSWRCH move text cursorLDA #10: JSR OSWRCH x-coordinateLDA #15: JSR OSWRCH y-coordinate

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) commandLDA #4: JSR OSWRCH command k=4, or move absoluteLDA #0: JSR OSWRCH: JSR OSWRCH: JSR OSWRCH: JSR OSWRCH send (0, 0) as low, high byte pairs

LDA #25: JSR OSWRCH begin PLOTLDA #1: JSR OSWRCH k=1 - draw relativeLDA #0: JSR OSWRCH: JSR OSWRCH x = 0LDA #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.


Sound support is done with 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, passed into the X and Y registers. 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, and amplitude. The amplitude parameter is normally negative; positive values select an envelope (a predefined temporal variation) to apply to the note.

Other meta parameters (passed through the channel code when using the SOUND command in BASIC) include flush (the buffer is cleared and the channel silenced before the note is played), synchronise count (as soon as the same synch count is received for that many channels, all the synchronised notes are played together), and control over the Speech system upgrade where fitted.

Other I/O and second processor support

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. The reason for this being 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 probably all assembled games write directly to main memory for I/O, and hence crash or give you a blank screen if a 6502 second processor is attached. One issue involved here is a lack of sprite support in the OS and a need to handle this manually from user code. The driver code for the Tube interface itself is not present in the MOS, usually being supplied by an external service ROM.

The MOS contains two built-in file systems: cassette and ROM - the ROM file system is quite similar to the cassette file system (try *ROM, *OPT 1 2, *CAT with a suitable ROM installed) and shares a great deal of its code.


Releases 0 and 1

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.

Release 2

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

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 it was not 100% compatible with some popular applications software 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.


*Watford Electronics, "The Advanced Reference Manual for the BBC Master Series", 1988
*British Broadcasting Corporation, "BBC Microcomputer User Guide", John Coll and David Allen, 1982

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