Modern motherboards will have a copy of the CMOS contents stored in flash, so when they turn on and notice the CMOS checksum is garbage, they can simply copy it back (ok it'll only survive until next power-off or the battery is replaced)
Thanks for that. very interesting. My first PC was an Acorn Atom built from the kit of bits. It had static ram chips and there were two for eack 1K bytes. I snapped the power pin off one chip, and they were about £6 each (in 1979 !!) so I carefully ground off the corner of the package to expose the inner connector grid and soldered a flying lead of wire-wrap wire to use it, and it worked.
The symptoms are suggestive the BIOS POST is stopping well before it gets to sniffing hard drives or anything else.
I don't have a good theory as to why a flat battery should stop a BIOS. One of my systems here, will say something like "Press F1 to enter BIOS" when a bad checksum on the CMOS (from the flat battery) is detected, and the user then loads up the settings as desired. And the settings stay there for that session, as long as +5VSB is present. Once you switch off at the back, the next time mains power is applied, you're back to the "F1 prompt".
On the small number of affected systems, there is no opportunity to press F1, because the BIOS has stopped the POST at that point. It's a black screen early in POST.
The people who suffer these, don't usually have any equipment to be offering clues as to what's up. Like a Port 80 card output, a check of the RESET signal to see if it's a Power_Good type issue, or whatever.
Paul
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.