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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0001:  Power Supplies - System Board

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VIII  BIOS
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VII  CMOS

This mouth full of acronym is pronounced see-moss, and is short for Complementary Metal Oxide Semiconductor (CMOS). What makes a CMOS different from an ordinary transistor is that it requires less power. A PC uses CMOS to hold data when the computer is turned off.

The CMOS retains data by using a small battery on the system board.

Sometimes, because the information in CMOS has retained while the computer is turned off, it may be referred to as Non-Volatile RAM (NVRAM). This is true only as long as the battery is in good shape and connected.

The question arises, what sort of information must be retained when the computers turned off? How about the current date and time? Do you have one floppy or two? Are your floppy drive(s) 5.25" or 3.5"? So tell me. Or more precisely, your computer wants to know what kind of hard drive is that anyway? These are all questions that must be asked, and answered, before you can do anything with your computer besides look at it sitting there.

The idea of a CMOS has been around since the AT or 80286 computers. Before then, switches or jumpers had to be set to tell the computer more about what was in it when it woke up with power. Today's modern computers have a bit more data in the CMOS. This information is titled: Extended System Configuration Data (ESCD). The ESCD has a special part of CMOS where certain settings are held. These settings are a bit more advanced than the date and time. Modern expansion cards use what is known as plug and play to simplify installation. Plug and play is discussed in detail later on. The CMOS works in tandem with another critical part of the PC, the BIOS.


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