If no operating system is found, CMOS returns an error message to the screen and pauses the system. The two typical messages about a missing operating system are “Bad or missing command interpreter” or “Non-system disk or disk error”. These errors will show on any DOS or Windows machine and they indicate that you should try to reinstall the system files. On Windows 9x machines, you can try the SYS.COM utility. On a Windows NT or 2000 system, try using a bootable disk and an emergency repair disk.
Remember that CMOS errors can often be generated after someone has changed the settings for the computer. This situation can lead to anything from an inability to boot the machine to a simple pause at the POST screen. Some errors might allow the machine to continue booting after pressing a function key (often F1).
CMOS settings are stored in a sort of file and checksum validation works with CMOS memory much the way it works with files. Checksum validation means that a number is read when the file is created and stored and that number can be appended to (added on to) the file and stored for later checking. When the file is read, the same process runs and should generate the same number. If the new number doesn’t match the stored number, a checksum error occurs. The settings file has a stored checksum and when the CPU reads the CMOS internal file, the numbers should match.
There are many CMOS errors but some of the important ones to know include:
- CMOS display type mismatch – This error indicates that the video settings don’t apply to the actual monitor installed on the system. The first step is to re-enter CMOS and verify that the correct monitor has been selected.
- CMOS memory size mismatch – This error will appear on many machines when more memory has been added. Auto detection will usually reconfigure the CMOS with a pause during the POST to make sure that what CMOS saw matches reality.
- CMOS device mismatch error – Displays and hard disks are considered devices and anyone of the attached devices can generate this error. Most likely, the wrong device is listed in CMOS for the physical device attached to the machine.
- CMOS checksum failure – A checksum failure indicates that corruption exists in the CMOS memory. This can often happen with a bad battery or a loose connection to the battery. If changing and checking the battery doesn’t solve the problem, it might be a motherboard going bad. Checksum errors may also indicate a virus.