Hardware Reference
In-Depth Information
commonly referred to as “the” CMOS RAM chip; although that is somewhat misleading
(almost all modern chips use a form of CMOS technology), the term has stuck. Most
RTC/NVRAM chips run on as little as 1 micro amp (millionth of an amp), so they use little
battery power to run. Most lithium coin cell batteries can provide power to one of these chips
for five years or more before they die and the information stored (including the date and time)
is lost.
When you enter the BIOS Setup, configure settings, and save them, the settings are written to
the storage area in the RTC/NVRAM chip (otherwise called the
CMOS RAM chip
). Every time
your system boots up, software in the BIOS reads the parameters stored in the CMOS RAM
chip to determine how to configure the system. A relationship exists between the BIOS and
CMOS RAM, but they are two distinct parts of the system.
Some systems used special versions of these chips made by Dallas Semiconductor,
Benchmarq, or Odin (such as the DS12885 and DS12887) that include both the RTC/NVRAM
chip and the battery in a single component. However, those are uncommon in modern systems
today. Although the so-called CMOS RAM chip started out as a discrete chip on the
motherboard, in modern systems it is no longer a separate chip, but instead included as one of
the functions in the South Bridge, I/O Controller Hub, or Platform Controller Hub component
of the motherboard chipset.
Motherboard ROM BIOS
All motherboards have a special chip containing software called the
ROM BIOS
. This ROM chip
contains the startup programs and drivers that get the system running and act as the interface to the
basic hardware in the system. When you turn on a system, the POST in the BIOS also tests the major
components in the system. Additionally, you can run a setup program to store system configuration
data in the CMOS memory, which is powered by a battery on the motherboard. This CMOS RAM is
often called
NVRAM
(
nonvolatile RAM
) because it runs on about 1 millionth of an amp of electrical
current and can store data for years when powered by a tiny lithium battery.
The motherboard ROM contains a collection of programs embedded in one or more chips, depending
on the design of your computer. That collection of programs is the first thing loaded when you start
your computer, even before the OS. Simply put, the BIOS in most PCs has four main functions:
•
POST
—POST tests your computer's processor, memory, chipset, video adapter, disk
controllers, disk drives, keyboard, and other crucial components.
•
Setup
—The system configuration and setup program is usually a menu-driven program
activated by pressing a special key during the POST. It enables you to configure the
motherboard and chipset settings along with the date and time, passwords, disk drives, and
other basic system settings. You also can control the power-management settings and boot-drive
sequence from the BIOS Setup, and on some systems, you can configure CPU timing and clock-
multiplier settings. Also, some systems offer a Windows-based application to access BIOS
Setup settings.
•
Bootstrap loader
—A routine that reads the first physical sector of various disk drives looking
for a valid master boot record (MBR). If one meeting certain minimum criteria (ending in the
signature bytes 55AAh) is found, the code within is executed. The MBR program code then
