Also known as a BIOS setup utility, a CMOS setup utility is software that edits settings for hardware in a computer's BIOS. In earlier models, users had to alter settings each time they added a new drive, but the addition of auto-detect features made this unnecessary. While technical support is sometimes still necessary to make an adjustment, users still have to make changes sometimes, such as the computer's boot order.
Even though this utility controls BIOS setup, the reason why it is sometimes called CMOS setup utility or CMOS RAM is because, originally, these settings dwelt in a CMOS memory circuit with battery backup. As technology advanced, BIOS setting moved for storage into flash memory that was not volatile.
Every time a computer starts up, it accesses the BIOS setup. Right after the user turns on the computer, a quick message appears (and then just as quickly disappears) asking the user to press a particular key, such as F1, F2, DEL or ESC, if the user wants to go to a different startup, other than the normal one, in order to make changes to the BIOS setup. If the user fails to hit that key in time, the computer proceeds to start normally.