CompTIA A+® Certification All-In-One For Dummies®, 2nd Edition

4.3. Understanding CMOS

As I mention earlier, CMOS contains the computer's inventory list and advanced setup options. It can be considered an inventory list because it contains a record of all the devices connected to the system, such as the floppy drive, the hard drive, memory, and so on. Not only does CMOS list the devices, but it also dictates their capacity — for example, whether the system has a 2GB or a 6GB hard drive.

During the boot process, the system compares its inventory list with what it detects during boot-up. If there are any discrepancies, the system typically gives you an error and takes you into CMOS right away, asking you to save the new change. For example, assume that your system has 512MB of RAM and you add another 512MB of RAM. When you power up the system, the system compares what it had in inventory (512MB) the last time it booted with what it sees now (1024MB). Because there is a difference, CMOS reports a memory size error and gives you the opportunity to save the new changes to CMOS. CMOS doesn't know whether the difference is for the better; it sees a difference, so it reports an error. Because CMOS detected the new memory, all you have to do is save the changes to the inventory list and reboot.

Looking at the previous example, if you choose not to save the settings, you will get the memory size error the next time you start the computer. To prevent ...