How PC CMOS Memory actually works (www.patmoss.com/pcref item 2.)
All PCs with 80286 or higher processors have 64 bytes of special memory
that is maintained by a battery, so that it keeps its information, even
when the computer is turned off. This PC memory is often referred to as
"CMOS memory". (CMOS stands for complementary metal oxide semiconductor.
CMOS is the substrate material used for the integrated circuit chip.)
This permanent storage is used to maintain the real-time clock and also
certain configuration data about the system and its peripherals. In the
Intel protected mode PC, CMOS memory is part of a Motorola MC146818 chip.
The 64 bytes of CMOS memory are not mapped into the CPU's address space.
Rather, they are accessed via the two I/O ports 70 and 71 hex (112 and
113 decimal). A program writes to a configuration address 00 to 3F hex
(0 to 63 decimal) via I/O port 70 hex and then writes a new byte value
or reads the current byte value at I/O port 71 hex.
CMOS memory contains the following information (in addition to power
saving, HD access, bus speed, and chip settings for newer BIOS chips):
a. Current date and time of day b. Hard drive #0/#1 type/size
c. Floppy drive #0/#1 type/size d. Memory size (base and extended)
Reference: The Peter Norton PC Programmer's Bible, pages 185-189
Microsoft Press, ISBN 1-55615-555-7
Re: PC CMOS "Disaster Prevention"
When the CMOS backup battery (eventually) fails, all of the hardware
configuration information that is contained in CMOS memory will be lost.
How can you prevent this from happening? Usually, you can't!
But, you can save a copy of CMOS memory beforehand to a CMOS emergency
floppy disk. Then when the CMOS backup battery fails you can:
a. Buy and install a fresh backup battery (such as from Radio Shack)
b. Boot the PC (usually from a bootable floppy disk)
c. Restore the CMOS information from the CMOS emergency floppy disk
d. Key in the correct date and time
And voila! Reboot and the PC is back to normal and ready to use again.
Note: The most important information that can be lost from CMOS memory
are the hard drive parameters. You should write this information
down beforehand (tracks, sectors, heads, write precomp, landing zone)
for each hard drive, and keep this information in a safe place.
Many newer motherboards come with a BIOS chip (such as AMI BIOS) that
will automatically determine hard drive characteristics of the PC.
Check the motherboard User Manual to be certain about this.
Three popular sources of software that will create and restore from a CMOS
emergency floppy disk are:
1. Norton Utilities 8.0 and up
2. PC-cillin 911 2.0 and up by CyberMedia
3. PC CMOS Disaster Prevention disk #495 from The Software Labs
Note: If you find errors/omissions, or have suggestions to improve this
documentation please send an email to patgmoss@yahoo.com. Thanks.
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