CMOS RAM (Complementary Metal Oxide Semiconductor Random Access Memory)

CMOS (pronounced see-moss) stands for complementary metal-oxide semiconductor. This is a type of memory chip with very low power requirements, and in PCs it operates using small batteries. In PCs, CMOS is more specifically referred to as CMOS RAM. This is a tiny 64-byte region of memory that, thanks to the battery power, retains data when the PC is shut off.      The function of CMOS RAM is to store information your computer needs when it boots up, such as hard disk types, keyboard and display type, chip set, and even the time and date. If the battery that powers your CMOS RAM dies, all this information is lost, and your PC will boot with the default information that shipped with the motherboard. In most cases, this means you´ll have no access to your hard disks until you supply CMOS with the necessary information. Without access to your hard disks, you won´t be able to boot your operating system.      Fortunately, today´s CMOS RAM is protected by nickel cadmium batteries, which the computer´s power supply recharges. Even so, it´s an extremely good idea to keep a copy of all the information stored in CMOS, in case disaster strikes.      The information stored in CMOS is required by your computer´s Basic Input/Output System, or BIOS (pronounced bye-oss). Your PC contains several BIOSes--the video BIOS that interfaces your CPU and video card, for example--but the most fundamental is the system BIOS. The system BIOS is stored on a ROM (read-only memory) chip on the motherboard and is copied at boot time to a 64K segment of upper system RAM for faster system access (RAM is faster than ROM). The role of the system BIOS is to boot the system, recognize the hardware devices, and locate and launch the operating system. Once the operating system is loaded, the BIOS then works with it to enable access to the hardware devices.

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Last Update: 10:36 PM 6/29/01