1980s Vintage Computers

Acorn BBC Computers - Tips

BBC Master

The BBC Microcomputer family are highly configurable, which led me to have many problems trying to get systems up and running. Thanks to the vast amount of support available I found answers either on line or in various manuals. Here is a summary.

Master Battery

The BBC Master uses a battery-backed CMOS memory chip to store configuration data. This includes setting the initial ROM (or ROM page for a large ROM) used on power-up or reset (in comparison the BBC B behaviour depends on which ROM is plugged into which socket). In many cases after the battery goes flat the machine can appear to be faulty, for example showing 'BBC MOS ADFS' and not responding to the keyboard. The first thing to do is replace the battery with a good one (see links). Next reset the CMOS by pressing 'R' then powering the machine on, machine should prompt to press Break to reset CMOS. Then press and hold down Ctrl+Space, then press Break. The machine is reset it should display 'This is not a language' and display a * prompt. To keep this setting type (where * is the machine operating system or MOS prompt):


This setting is saved in the battery-backed RAM, so next time the machine is powered on it should display 'This is not a language' and give the * prompt. This is fine in some situations (eg on a Master 512), you can then type Basic to get going. The commands to set the CMOS data are well documented, for example:


enables the Basic ROM.


sets Basic as the initial ROM language to use at power-up.

nb - *UNPLUG is the opposite of *INSERT.

Please refer to the Master manual on how to set the configuration for the required default file system (or Google 'configure noboot adfs').

Teletext Adapter

This relies on having a ROM to set-up and receive Teletext pages, I have used the ATS 3.0.1 ROM with success. This is bundled with BeebEm, search for the 'Acorn Advanced Teletext Manual' for instructions.

Disk formats

There are many different disk formats that were used on the BBC family, and when trying old disks it is not always obvious from the label which one was used. The disk density, number of tracks, and the filing system all have to be supported before the disk can be read, and some disks require a double sided drive too.

The BBC B could be fitted with either a 8271 or 1770 disk controller chip. The 8271 could only handle FM or single-density format, whilst the 1770 could handle either FM or MFM (single or double density), which doubles the capacity of a floppy. The Master came with a 1770 as standard.

5.25" disk drives were available with either 40 track or 80 track types, some 80 track drives had a switch to allow 40 track disks to be read. Some early drives were single sided, later ones were all double sided.

3.5" drives were also supported, on the BBC these are always 80 track double sided. 

There were two common filing systems, DFS and ADFS (plus a few non-Acorn ones).  On the BBC B changing filing system involves changing ROMs, again the Master has both types built in.

So when trying to read an old disk it is easiest to use a Master with a switchable drive to try the various combinations. Some combinations are highly unlikely, eg single density and ADFS.

The Z80 CP/M system used only one type of floppy, 80 track, double sided and single density (400KB capacity), and the Master 512 supported many formats including DOS disks, which were double sided and double density, 40 track (360KB capacity) or 80 track (720KB capacity). 


The key-switches are very reliable, but if left unused for years some keys may not work. The answer is to pound away on the same key (up to a hundred times). If this doesn't cause any improvement then the switch can easily be replaced from a donor machine (I have some spare Master switches).

Acorn BBC Links

This page was last revised on: 16/06/12