The BBC Master 128 has a CMOS NVRAM and battery system for storing configuration information. I assume that a dead or removed battery just leads to the NVRAM reverting to defaults or other non-harmful state.

Short of replacing the battery, can the BBC Master still be used to its full extent? How would one use it, and what MOS commands are needed?

3 Answers 3


I don't know how cleanly memory adopts a default state upon battery failure but even at worst you can still use the machine almost as normal: hold down R while switching on to reset the CMOS RAM. You'll be told that it is reset and prompted to press break for a warm reboot, at which point you should be back to factory defaults and safe at least until your next power cycle.

*STATUS (or *ST.) can be used to see what settings are currently stored; *CONFIGURE (or *CO.) can be used to set CMOS values. If status reports a line like:


i.e. use the ROM in slot 0 as the default language, then you can set that by:


i.e. use the ROM in slot 12 as the default language. The built-in BASIC ROM appears in slot 12.

NOTE: A BBC Master 128 with the default Megabit ROM chip will, upon resetting the CMOS and doing a warm reset, drop the machine into ADFS. If there are no disk devices available, then the BBC will hang until the user aborts by pressing CTRL+D+BREAK.

  • If I understand correctly, then not replacing the battery results in at least one feature being lost - the ability to control which ROM is "auto-started" when the Beeb is powered on. Any other significant features lost? What I'm getting at is what's the cost to the user to never replace the battery.
    – Brian H
    Commented Jun 16, 2017 at 15:24

It will work fine. As you imagined, on power-up will store default values in the NVRAM, which will later disappear on power-off. I have used a few like this in the deep and distant past.

I'm not quite sure what you mean by 'what MOS commands are needed'. But the above explanation may make than question irrelevant anyway.

  • Will I get dropped into BBC BASIC?
    – Brian H
    Commented Jun 14, 2017 at 19:14
  • I should imagine so. The Advanced Reference Manual doesn't say what happens but you could take a look into the MOS source code.
    – TonyM
    Commented Jun 14, 2017 at 19:20
  • @BrianH: You will go into BBC Basic if that is the first available language ROM. The OS will scan the ROMs and enter the first that it sees. This will be BASIC unless someone has been fiddling.
    – Chenmunka
    Commented Jun 15, 2017 at 7:35
  • This doesn't match my experiance, the master has the ability to soft-disable roms and at least on the ones I used with no battery it came up with all of them disabled. Commented Sep 19, 2018 at 18:36

In my experiance a master without a battery will drop you into a "* commands" prompt with the sideways roms disabled.

You can re-enable the roms manually with *configure, set up your preffered languages etc. Apparently as another poster has said there is also a reset CMOS hotkey which is probablly quicker (but which I did not know about when I was last dealing with BBC masters)

Personally I would recommend fitting a battery holder in which you can use normal AA batteries.

  • proper replacement lithium thionyl chloride batteries aren't that expensive, and they're less likely to leak if you forget about them for a decade than AAs
    – scruss
    Commented Jun 23, 2018 at 16:20

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