CMOS and RTC

There is other start-up information that normally stays the same but that we might want to change once in a while. This includes info about the various pieces of hardware connected to the system, which disk drive to check first for the operating system and that sort of thing. This data can’t be stored on the hard drive because we need it to boot up. It can’t be stored in RAM because it will be lost at power-off, and it can’t be stored in the BIOS because we might need to change it.
The problem is solved by a type of RAM chip that uses very low power, and it is connected to a battery. This type of low-power memory chip is called CMOS. It stands for the type of technology used in the chip, which is Complementary MetalOxideSubstrate. This is probably more than you need to know, but I’m a fanatic about defining things. By the way, since batteries don’t last forever, if you leave your computer unplugged for about 5 years you’ll find it needs a bit of trickery to get it to boot again, because the CMOS information will be gone.
There is another feature in the computer that has the same requirements as CMOS, and that is the date and time function. This obviously needs to change very minute, but we don’t want to lose track when the computer is turned off. The circuitry for this is called the RTC or Real Time Clock, and for convenience it is usually included in the same chip with the CMOS. A little trickle of juice from the CMOS battery keeps the clock running, and when you turn the computer on again it knows exactly what time and day it is. Convenient, isn’t it?