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BIOS and CMOS. Chapter 5. Overview. In 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). Historical/Conceptual. We Need to Talk.

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bios and cmos


Chapter 5

  • In 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)
bridge introduction
Bridge Introduction
  • Data flows through the computer
    • Between CPU and RAM
    • Between CPU and video
    • Between CPU and other devices
  • Bridges are used to connect the pieces
    • Northbridge
      • Bridge closest to the CPU
    • Southbridge
      • The farther bridge
northbridge southbridge

Chip or chips that connect the CPU to video and/or memory


Handles all of the inputs and outputs to the many devices in the PC

Northbridge & Southbridge
  • A chipset is a set of Northbridge and Southbridge chips that work together
  • Explored in more depth in Chapter 7
data flow
Data Flow
  • Northbridge and Southbridgehave connectivity with all devices
  • Not the same in all systems



basic input output system bios
Basic Input/Output System (BIOS)
  • Bridges connect the devices
    • But 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 programs
  • These programs are collectively known as the basic input/output service (BIOS)
talking to the keyboard
Talking to the Keyboard
  • The keyboard talks to the external data bus
    • Uses the keyboard controller chip (8042)
  • Each program is called a service
  • Programs stored on Flash ROM chips are known as firmware
  • Programs stored on erasable media are called software

Keyboard controller chip



CompTIA A+Essentials


  • Separate chip from ROM BIOS
  • Volatile (kept alive by battery)
  • Stores only changeable data
    • Not programs
    • Acts as clock to keep data current
  • Customizable via SETUP program
  • Often on Southbridge
bios vs cmos


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



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)
cmos setup
CMOS Setup
  • Main menu
    • Access to all submenus
standard cmos features
Standard CMOS Features
  • Clock, hard drives, floppy drives
softmenu setup
SoftMenu Setup
  • Normally set to Default or Auto for all
advanced features
Advanced Features
  • POST, boot order
power management
Power Management
  • Use to enable/disable power-saving features
pnp pci
  • Rarely need to manipulate on today’s PCs
set password
Set Password
  • Locks access to CMOS settings
    • Prevents non-techs from changing key settings
  • Computer 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 CPU
    • Most devices with onboard BIOS use it only for internal needs (internal function) and use a device driver to talk to the CPU
device drivers
Device Drivers
  • A device driver is a file that contains the BIOS commands necessary to communicate with the devices they support
    • Loaded into RAM when the system boots
  • All devices come with their own device drivers
bios bios everywhere
BIOS, BIOS, Everywhere
  • All 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 post27
Power-On Self Test (POST)
  • The power-on self test (POST) is a special program stored on the ROM chip
    • Initiated when the computer is turned on or is reset
    • Checks out the system every time the computer boots
  • Communicates errors
    • Beep codes
    • Text errors
beep codes
Beep Codes
  • If video is determined to be missing or faulty
    • One long beep followed by three short beeps
  • If everything checks out
    • One or two short beeps
  • If RAM is missing or faulty
    • Buzzing noise that repeats until power turned off
  • More complicated beep codes may be found in legacy computers
    • Check motherboard manual for meaning
post cards
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
the boot process
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
the boot process32
The Boot Process
  • The last BIOS function called by POST is the bootstrap loader
  • The bootstrap loader loads the operating system from the boot sector
    • Searches 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

Care and Feeding of CMOS/BIOS

We have met the enemy and he is us.

- Pogo

losing cmos settings
Losing CMOS Settings
  • Common errors
    • CMOS configuration mismatch
    • CMOS date/time not set
    • No boot device available
    • CMOS battery state low
  • Common reasons for losing CMOS data
    • Jiggling the battery while doing other work
    • Dirt on the motherboard
    • Electrical surges
    • Faulty power supplies
    • Chip creep
losing cmos settings35
Losing CMOS Settings
  • If your system keeps resetting
    • Replace the battery
  • Common symptoms of low battery
    • Slow running clock
    • Clock resetting to January 1st
clearing cmos settings
Clearing CMOS Settings
  • To clear the CMOS settings, place the shunt on the CMOS jumper
    • Resets to factory settings
    • Resets password
flashing rom
Flashing ROM
  • Flash ROM chips can be reprogrammed
  • Download program from manufacturer
  • Copy program to floppy
    • Some programs will run within Windows
  • Boot to floppy and run program