Reliability and fault-finding - PC Based Instrumentation and Control - page 397

Photo 13.1 Dual BIOS ROM (main and backup) fitted to a modern PC
Photo 13.2 CMOS batteries may need replacing if CMOS errors are
reported by the BIOS during system initialization
to clear the data stored in CMOS memory. This may be required when making
a flash upgrade to the BIOS data or when the BIOS password is forgotten (or
simply not passed on from one owner to another).
When the BIOS POST fails it will generate a beep code that motherboard
will send to the system speaker. Beep codes can be invaluable when a system
refuses to boot. Indeed, this will be the only information that you have to work
on unless you have access to a hardware diagnostic aid!