Tim:
>> I've come across computers that foul up when powered up with a dead
>> CMOS battery.  I get the impression that some BIOSs have parts of
>> themselves powered only by the battery, and don't get anything from
>> the main power supply.

Joe Zeff:
> Well, of course.  What do you think powers the CMOS when the
> computer's off?  However, the clock will start to run slow long enough
> before the battery dies as a warning.

I was thinking in the other direction:  That when mains power is
available, the main supply powers the CMOS instead of the battery, to
prolong its life.

It's a common enough circuit, in some devices.  A diode from supply to
clock and memory circuits, a diode from back-up battery to the same
circuit, the greatest voltage drives everything.

By "foul up when powered up with a dead CMOS battery" I meant while
powered up (from the mains) and with a dead CMOS battery...

-- 
tim@localhost ~]$ uname -rsvp

Linux 3.15.10-201.fc20.i686 #1 SMP Wed Aug 27 21:33:30 UTC 2014 i686

All mail to my mailbox is automatically deleted, there is no point trying
to privately email me, I will only read messages posted to the public lists.

George Orwell's '1984' was supposed to be a warning against tyranny, not
a set of instructions for supposedly democratic governments.

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