CMOS

CMOS

CMOS (Complimentary Metal Oxide Semiconductor) is a small amount of memory in a special RAM chip. Its memory is
maintained with electric power from a small battery. Certain system data are stored in this chip. They must be read to make
the PC operable. There may be 100 to 200 bytes of data regarding date, time, floppy and hard disk drives, and much more.
CMOS data can be divided in two groups:
l   Data, which POST can not find during the system test.
l   Data, which contain user options.
For example, POST cannot by itself find sufficient information about the floppy drive(s). Floppy drives are so "dumb," that
POST cannot read whether they are floppy drives or not, nor what type. About the same goes for IDE hard disks, while EIDE
hard disks are a little more "intelligent," However, POST still needs assistance to identify them 100% correctly.
The same goes for RAM: POST can count how much RAM is in the PC. However, POST cannot detect whether it is FPM,
EDO or SD RAM. Since the CPU and BIOS reads data from RAM chips differently, depending on the RAM type, that type
must be identified.
Configuration
The PC must be configured, be supplied with this information. That is done in the factory or store, where it is assembled. This
information is stored in CMOS, where they stay. CMOS data only need to be updated, when different or additional hardware
components are installed. This could be a different type hard disk or floppy disks or an new RAM type, Often he user can do
this.
Other data in CMOS contain various user options. Those are data, which you can write to CMOS. For example, you can
adjust date and time, which the PC then adjusts every second. You can also choose between different system parameters.
Maybe you want a short system check instead of a long one. Or if you want the PC to try to boot from hard disk C before
trying floppy disk A, or vice versa. These options can be written to CMOS.
Many of the options are of no interest to the ordinary user. These are options, which regard controller chips on the system
board, which can be configured in different ways. Ordinarily, there is no need to make such changes. The system board
manufacturer has already selected the optimal configurations. They recommend in their manuals, that you do not change
these default settings.
We can conclude, that CMOS data are essential system data, which are vital for operation of the PC. Their special feature isthat they are user adjustable. Adjustments to CMOS are made during start-up.

CMOS BATTERY