The computer's BIOS is the most low-level code that it contains, and some of the most important. Without a BIOS, a computer cannot boot. Every CPU looks to some particular portion of memory for instructions when it's first turned on. The x86 PC architecture, like that of most computers, is designed so that ROM resides at the CPU's start-up location. That ROM contains code to help the computer boot from a floppy disk, hard disk, or some other device. The details are complex and vary from one computer to another, but a series of operations starting at the BIOS and ending with high-level programs in your OS leads to a running computer.
The x86 BIOS was originally designed to provide services to a running OS. For instance, the BIOS contains routines that an OS can use to read data from a hard disk and to display information on the screen. Linux doesn't use these routines in ordinary operation, though, because Linux is a 32-bit OS and these BIOS routines are 16-bit in nature. Aside from a transfer of control during the boot process, Linux makes little use of the BIOS. As I've said, that BIOS boot code is critically important.
The BIOS also contains routines to help you configure how your hardware operates. This portion of the BIOS is known as the CMOS setup utility because it sets options that are stored in complementary metal oxide semiconductor (CMOS) memory. This is a small area of nonvolatile memory on the motherboard. Options you can set in the CMOS setup utility include the order in which the BIOS looks for boot devices, memory timing characteristics (particularly on older motherboards), and what motherboard devices (EIDE ports, parallel ports, and so on) are active. Most CMOS setup utilities are broken down into a number of sub-sections. For instance, Figure 3.4 shows a typical main screen. Each option along the top (Main, Advanced, and so on) corresponds to a screen full of options.
Some CMOS setup settings affect how Linux handles the hardware in question, and so are important even in Linux. The details of CMOS configuration vary a great deal from one motherboard to another, both because the BIOSes come from different producers and because different motherboards support different features. Indeed, how you enter a BIOS's CMOS setup utility varies from one BIOS to another. Typically, you press a special key or key combination (Del and F3 are two common options) early in the boot process, typically while the motherboard is checking its RAM. (Most BIOSes present a prompt at the appropriate point in the boot process, so look for it, or consult your motherboard's manual.)
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