Chapter 5BIOS and CMOS

MELJUN CORTESMELJUN CORTES

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OverviewIn this chapter, you will learn to

Explain the function of BIOS

Distinguish among various CMOS setup utility options

Describe option ROM and device drivers

Troubleshoot the power-on self test (POST)

We Need to Talk

Historical/Conceptual

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Bridge IntroductionData flows through the computer

Between CPU and RAMBetween CPU and videoBetween CPU and other devices

Bridges are used to connect the piecesNorthbridge

Bridge closest to the CPU Southbridge

The farther bridge

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Northbridge & Southbridge

NorthbridgeChip or chips that

connect the CPU to video and/or memory

SouthbridgeHandles all of the inputs

and outputs to the many devices in the PC

• A chipset is a set of Northbridge and Southbridge chips that work together

• Explored in more depth in Chapter 7

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Data FlowNorthbridge and Southbridge

have connectivity with all devicesNot the same in all systems

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Chipset

Northbridge

Southbridge

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Basic Input/Output System (BIOS)

Bridges connect the devicesBut without a program, they don’t know how

to communicate

A special kind of program is required to enable the CPU to talk to other devices

A Flash ROM chip stores these programsThese programs are collectively known as

the basic input/output service (BIOS)

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Talking to the Keyboard

The keyboard talks to the external data bus Uses the keyboard controller chip (8042)

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BIOSEach program is called a servicePrograms stored on Flash ROM chips are known

as firmwarePrograms stored on erasable media are called

softwareKeyboard controller

chip

CMOS

Essentials

CompTIA A+Essentials

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CMOSSeparate chip from ROM BIOS

Volatile (kept alive by battery)

Stores only changeable data Not programsActs as clock to keep data current

Customizable via SETUP program

Often on Southbridge

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BIOS vs. CMOSBIOS

Programs

Non-volatile (stays same after power off)

Can be changed by “flashing”

Typically 64 K of programs (though Flash ROM is much bigger)

Often a separate chip

CMOSData

Volatile (kept alive with battery)

Changed via CMOS setup

Typically 128 K of data (though chip size is typically 64 K)

Often on Southbridge

Updating CMOS

• Updated via BIOS program• Three primary BIOS brands

– American Megatrends (AMI), Award, Phoenix– To enter setup, press key combination

(may be Del, ESC, F1, F2, CTRL-ALT-ESC, CTRL-ALT-INS, CTRL-ALT-Enter, or CTRL-S)

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CMOS Setup • Main menu

– Access to all submenus

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Standard CMOS Features• Clock, hard drives, floppy drives

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SoftMenu SetupNormally set to Default or Auto for all

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Advanced FeaturesPOST, boot order

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Power ManagementUse to enable/disable power-saving

features

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PnP/PCIRarely need to manipulate on today’s PCs

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Set PasswordLocks access to CMOS settings

Prevents non-techs from changing key settings

Option ROM and Device Drivers

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BYOBComputer makers could not predict all the new

types of hardware Ways to bring your own BIOS (BYOB) were

invented:Option ROM is a BIOS chip embedded on the

adapter card itself – (i.e., video cards)Most new hardware devices use device drivers to

tell the BIOS how to talk to the CPUMost devices with onboard BIOS use it only for

internal needs (internal function) and use a device driver to talk to the CPU

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Device Drivers A device driver is a file that contains the

BIOS commands necessary to communicate with the devices they supportLoaded into RAM when the system boots

All devices come with their own device drivers

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BIOS, BIOS, EverywhereAll hardware needs a program to allow CPU

to communicate with it

Could be on motherboard ROM

Could be on PC card ROM

Could be loaded via a driver

Could be loaded into RAM at boot

Power-On Self Test (POST)

CompTIA A+Technician

IT Technician

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Power-On Self Test (POST) The power-on self test (POST) is a special

program stored on the ROM chipInitiated when the computer is turned on or is

resetChecks out the system every time the computer

boots

Communicates errorsBeep codesText errors

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Beep Codes If video is determined to be missing or faulty

One long beep followed by three short beeps

If everything checks outOne or two short beeps

If RAM is missing or faultyBuzzing noise that repeats until power turned off

More complicated beep codes may be found in legacy computersCheck motherboard manual for meaning

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Text-Based Error Message

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POST Cards POST cards are devices that monitor POSTs and

report on the hardware that may be causing problems

Turn the PC off, plug in the card, and reboot

POST error codes do not fix the computer – they just tell you where to look

If all else fails, replace the motherboard

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The Boot Process The CPU is the first component initialized

when the computer is turned on

It reads a special wire called power good once the power supply provides the proper voltage to the CPU

Every CPU has a built-in memory address with the first line of the POST program on the system ROM

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The Boot Process The last BIOS function called by POST is the

bootstrap loader

The bootstrap loader loads the operating system from the boot sectorSearches the floppy, CD-ROM, or the hard drive Boot order set in CMOS

The bootstrap loader generates an error if it cannot find the boot sector on the bootable disk

Care and Feeding of CMOS/BIOS

We have met the enemy and he is us.- Pogo

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Losing CMOS SettingsCommon errors

CMOS configuration mismatchCMOS date/time not setNo boot device availableCMOS battery state low

Common reasons for losing CMOS dataJiggling the battery while doing other workDirt on the motherboardElectrical surgesFaulty power suppliesChip creep

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Losing CMOS SettingsIf your system keeps resetting

Replace the battery

Common symptoms of low battery

Slow running clock

Clock resetting to January 1st

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Clearing CMOS SettingsTo clear the CMOS settings, place the shunt

on the CMOS jumperResets to factory settings

Resets password

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Flashing ROMFlash ROM chips can be reprogrammed

Download program from manufacturer

Copy program to floppySome programs will run within Windows

Boot to floppy and run program

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