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Complementary Metal Oxide
Semiconductor. To perform its tasks, the BIOS need to know various
parameters (hardware configuration). These are permanently saved in a little
piece (64 bytes) of CMOS RAM (short: CMOS). The CMOS power is supplied by a
little battery, so its contents will not be lost after the PC is turned off.
Therefore, there is a battery and a small RAM memory on board, which never
(should...) loses its information. The memory was in earlier times a part of the
clock chip, now it's part of such a highly Integrated Circuit (IC). CMOS is the
name of a technology which needs very low power so the computer's battery is not
too much in use.
Actually, there is not a
battery on new boards, but an accumulator (Ni_Cad in most cases). It is
recharged every time the computer is turned on. If your CMOS is powered by
external batteries, be sure that they are in good operating condition. Also, be
sure that they do not leak. That may damage the motherboard. Otherwise, your
CMOS may suddenly "forget" its configuration and you may be looking
for a problem elsewhere. In the monolithic PC and PC/XT, this information is
supplied by setting the DIP (Dual-In-line Package) switches at the motherboard
or peripheral cards. Some new motherboards have a technology named the Dallas
Nov-Ram. It eliminates having an on-board battery: There is a 10 year lithium
cell epoxyed into the chip.
See Also : BIOS