ROM is a type of memory circuitry that holds the computer's startup routine. ROM is housed in a single integrated circuit—usually a fairly large, caterpillar-like DIP package—which is plugged into the motherboard. Whereas RAM is temporary and volatile, ROM is permanent and non-volatile. ROM circuitry holds "hard-wired" instructions that remain in place even when the computer power is turned off. The instructions in ROM are permanent, and the only way to change them is to replace the ROM chip. In a personal computer, ROM contains a small set of instructions called the ROM BIOS (basic input/output system). These instructions tell the computer how to access the hard disk, find the operating system, and load it into RAM. Once the operating system is loaded, the computer can understand your input, display output, run software, and access your data. CMOS MEMORY In order to operate correctly, a computer must have some basic information about storage, memory, and display configurations. RAM goes blank when the computer power is turned off, so configuration information cannot be stored there. ROM would not be a good place for this information either because it holds data on a permanent basis. If, for example, your computer stored the memory size in ROM, you could never add more memory—well, you might be able to add it, but you couldn't change thesize specification in ROM. To store some basic system information, your computer needs a type of memory that's more permanent than RAM, but less permanent than ROM. CMOS is just the ticket. CMOS (complementary metal oxide semiconductor memory) is a type of chip that requires very little power to hold data. It can be powered by a small, rechargeable battery that's integrated into the motherboard. The battery trickles power to the CMOS chip so that it can retain vital data about your computer system configuration even when your computer is turned off. When you change the configuration of your computer system—by adding RAM, for example—the data in CMOS must be updated. Some operating systems recognize such changes and automatically perform the update. You can manually change CMOS settings by running the CMOS setup program.