Acorn MOS
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| Acorn Machine Operating System | |
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| Company/ developer |
Acorn Computers |
|---|---|
| Programmed in | 6502 machine code (v0, v1) |
| Source model | Proprietary |
| Initial release | Late 1981 |
| Latest stable release | 5 / Early 1986 |
| Available programming languages(s) | BBC BASIC, 6502 Assembler |
| Update method | Replacement ROMs |
| Supported platforms | BBC Micro, BBC Master Series |
| Default user interface | Command line interface (v3) |
| Working state | Historic |
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 0.1 to 1.2 were used on the BBC Micro, version 2 was used on the B+, and versions 3 and 5 were used in the BBC Master Series range. (The final BBC computer, the BBC A3000, was 32-bit and ran RISC OS.) MOS was largely derived from the firmware used on earlier Acorn systems. Some portions of the Acorn MOS architecture were later utilised as a basis for the architecture of the 32-bit operating systems Arthur and RISC OS.
MOS is 16 kB in size and is held in ROM on the motherboard. It reserves the upper quarter of the 16-bit address space (0xC000 to 0xFFFF) for its ROM code and I/O space. Versions 0 and 1 were written in 6502 machine code. The BBC Master replaced the BBC Micro's 6502 CPU with a 65C102 which, with a denser instruction set, allowed for the same 16 kB of space to hold more code, allowing for an expanded operating system.
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[edit] User interface
The original MOS versions – 0.1 to 1.20 on the BBC Micro models A and B – did not have a user interface. 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 will not start up (it halts 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 on the BBC Master did feature a 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, 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 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 */. *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.
[edit] Extension
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 popular such ROM 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 (0x0 to 0xDFF) from the bottom of the memory map for operating system and language ROM workspace. On a cassette-only machine, 0xE00 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.
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 OS.
[edit] 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 cursor LDA #10: JSR OSWRCH \ x-coordinate LDA #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) command LDA #4: JSR OSWRCH \ command k=4, or move absolute LDA #0: JSR OSWRCH: JSR OSWRCH: JSR OSWRCH: JSR OSWRCH \ send (0, 0) as low, high byte pairs LDA #25: JSR OSWRCH \ begin PLOT LDA #1: JSR OSWRCH \ k=1 - draw relative LDA #0: JSR OSWRCH: JSR OSWRCH \ x = 0 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.
[edit] Sound
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 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) and synchronise count (as soon as the same synch count is received for that many channels, all the synchronised notes are played together).
[edit] 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).
[edit] MOS 3 and 5
MOS 3 and MOS 5 shipped with the BBC Master Series systems, in the Master 128 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.
MOS 5 shipped with the Master Compact, and was much altered with some functionality removed or highly amended.
[edit] References
- 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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