Hardware Reference
In-Depth Information
to store BIOS Setup settings. Although it is called nonvolatile, the chip is actually volatile,
meaning that without power, the time/date settings and the data in the RAM portion will in
fact be erased. Many consider this chip nonvolatile because it is designed using CMOS tech-
nology, which results in a chip that requires little power compared to other chips. A small
battery can provide that power when the system is unplugged. This battery-powered clock/
memorychipiscommonlyreferredtoas“the”CMOSRAMchip;althoughthatissomewhat
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
RAMchiptodeterminehowtoconfigurethesystem.ArelationshipexistsbetweentheBIOS
and CMOS RAM, but they are two distinct parts of the system.
Some systems used special versions of these chips made by Dallas Semiconductor, Bench-
marq,orOdin(suchastheDS12885andDS12887)thatincludeboththeRTC/NVRAMchip
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 mother-
board, 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,orPlatform Controller Hubcomponent of
the motherboard chipset.
Motherboard ROM BIOS
All motherboards have a special chip containing software called the
ROM BIOS
. This
ROMchipcontainsthestartupprogramsanddriversthatgetthesystemrunningandactas
theinterfacetothebasichardwareinthesystem.Whenyouturnonasystem,thePOSTin
theBIOSalsoteststhemajorcomponentsinthesystem.Additionally,youcanrunasetup
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.
ThemotherboardROMcontainsacollectionofprogramsembeddedinoneormorechips,
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
—The POST tests your computer's processor, memory, chipset, video adapter,
disk controllers, disk drives, keyboard, and other crucial components.
