Datasheet

Identifying Components of Motherboards 31
BIOS and POST
Aside from the processor, the most important chip on the motherboard is the Basic Input/
Output System (BIOS) chip, also referred to as the ROM BIOS chip. This special memory
chip contains the BIOS systems software that boots the system and allows the operating sys-
tem to interact with certain hardware in the computer, in lieu of requiring a device driver to
do so. The BIOS chip is easily identified: if you have a non-clone computer, this chip might
have on it the name of the manufacturer and usually the word BIOS. For clones, the chip
usually has a sticker or printing on it from one of the major BIOS manufacturers (AMI,
Phoenix/Award, Winbond, and so on). On later motherboards, the BIOS might be difficult to
identify, but the functionality remains, regardless of how it’s implemented. Figure 1.24 gives
you an idea of what a modern BIOS might look like. Despite the 1998 copyright on the label,
this particular chip can be found on motherboards produced as late as 2009. Notice also the
Reset CMOS jumper at lower left and its configuration silkscreen at upper left. You might
use this jumper to clear the CMOS memory, discussed next, when an unknown password, for
example, is keeping you out of the BIOS configuration utility. The jumper in the photo is in
the clear position, not the normal operating position. System bootup is typically not possible
in this state.
FIGURE 1.24 A BIOS chip on a motherboard
A major function of the BIOS is to perform a process known as power-on self-test
(POST). POST is a series of system checks performed by the system BIOS and other high-
end components, such as the SCSI BIOS and the video BIOS. Among other things, the
POST routine verifies the integrity of the BIOS itself. It also verifies and confirms the size
of primary memory. During POST, the BIOS also analyzes and catalogs other forms of
hardware, such as buses and boot devices, as well as manages the passing of control to the
specialized BIOS routines mentioned earlier. The BIOS is responsible for offering the user a
key sequence to enter the configuration routine as POST is beginning. Finally, once POST
has completed successfully, the BIOS selects the boot device highest in the configured boot
order and executes the master boot record (MBR) or similar construct on that device so
that the MBR can call its associated operating system and continue booting up.
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