BIOS and CMOS

Adding personality to your PC

We Need to Talk!

• The CPU needs some method to talk to the devices to tell them what to do

• The devices need some way to send data to, and receive data from the CPU

• We need to expand our logical model from the CPU chapter

CPUNorthbridge RAM

EDB

Address Bus

CPUNorthbridge RAM

EDB

Address Bus

Southbridge

keyboard

Hard Disk Drive

Video Card

CPUNorthbridge RAM

EDB

Address Bus

Southbridge

KeyboardController

Hard Disk DriveVideo Card

Chipset

Chipset

845 Chipset

925 Chipset

Keyboard

• How we communicate with the keyboard

Problems

• Different motherboards have different keyboard controllers

• We need specific code to talk to the controller

• We need a place to store this specific programming and the programming for other devices (optical drive, floppy, etc.)

Solution

• Use a ROM chip• Does not “forget” when power is removed,

like RAM does• Specific to motherboard and devices on it• Read only, code does not change• Can hold lots of programs, like a “chapter

book”• Called “firmware”

BIOBIOS

IBM’s BIOS

Patent “fence” around BIOS

Making a ROM chip

• We “burn” the code into the chip:

1 0

System ROM

• Holds all the support programming for basic devices

• Is given a specific address space, or range of memory addresses for its code

The First Megabyte

• Upper Memory

• Conventional Memory

Address 0

640 KB

1 MB

(384 KB)

Basic Hardware

• CPU• Mouse• Keyboard• Video card• RAM• Network card

• Sound Card• Floppy drive• Hard Disk drive• Zip drive• USB ports• CD-ROM drive

Basic Hardware

• CPU• Mouse• Keyboard• Video card• RAM• Network card

• Sound Card• Floppy drive• Hard Disk drive• Zip drive• USB ports• CD-ROM drive

Stored Information

• Stores access/write code for keyboard• Stores access/write code for system

speaker• Stores access/write code for other chips

(functions) on motherboard• Does not change (more on this in a

moment)

CMOS

• Complimentary Metal Oxide Semiconductor – how it was made

• Stores specific information about your system:– Amount of RAM present– Type of hard disk drive– Date and Time

• Needs battery power to “remember” when power is off

CPUNorthbridge RAM

EDB

Address Bus

Southbridge

KeyboardController

Mouse Video Card

Chipset

BIOSAndCMOS

Chipset

20

BIOS

• All devices need BIOS code to interoperate with CPU. Some is stored directly in System BIOS chip, most is stored elsewhere.

• One of the functions of BIOS is to provide the CMOS Setup Utility, or Setup, which allows us to change CMOS data.

BIOS information CMOS

information

CMOS or Setup

• Option to use this program is early in the boot cycle – long before system loads Operating System

• Different key(s) to enter different Setups• You should only get to Setup on purpose• Program stored in BIOS, but only edits

data in CMOS• First CMOS was set of switches!

Who makes BIOS?

• Award Software• Phoenix Technologies• AMI – American Megatrends Inc.• They write BIOS with lots of options;

motherboard maker picks sections to be used

• Long gone are IBM’s patents on BIOS

Same parent company

Setup

• Lots of screens and subscreens• You don’t need to know about all settings

on all screens• Motherboard book is mildly helpful on

some options• You should know generally where things

can be found

Date and Time and …

• Usually on the first screen, or first menu choice

• Lets you set Date, Time, floppy present (will autodetect it), memory count, hard drive(s) present (again, autodetected)

Here is where you set the boot

order

When adding a modem, you might want to turn both of

these off.

Old CMOS

• Stored the hard drive “geometry” which was NOT found on the drive’s label

• Old systems had a nasty habit of loosing CMOS data and requiring a trip to the repair shop to get fixed

• I spent many an hour hacking drive geometry values by trial and error

15

New CMOS

• Did you notice the “High Performance” option?

• Did you see the “Setup Defaults” option?• BIOS can “talk” to hard disk drive and get

all the information it needs.• About the worst that happens today is the

need to reset date and time.

Chip Evolution

• EPROM – Erasable Programmable ROM; used ultra-violet light (sunlight) to erase

• EEPROM – Electrically Erasable – what we use today

• Flash ROM – another name for EEPROM• NVRAM – Non-volatile RAM. Really not

so, still needs battery power

The Three-In-One

• We have BIOS that stores code and access program to CMOS

• We have CMOS that stores changeable information about our system

• We have RTC (Real Time Clock) that keeps track of date and time

• All three are now rolled into one chip

Easy way to spot this chip is by the

shinny label

About that Battery

• Should last 5 to 10 years• You can find them at Savon even• If your system forgets date and/or time, it’s

time to replace the battery• Beware the evil ESD!

More Battery

• Sometimes we WANT to reset CMOS:– A forgotten password– Too aggressive overclocking– Incorrect low-level settings on some screen

• Either move the jumper, or pull the battery• Do either for about 10 seconds• If you are to pull battery, make sure you

unplug system too

10

More BIOS

• Option ROM – put a BIOS chip on the card/device– Video is most common for this– SCSI cards– Network cards have socket, chip is extra

Software Solutions

• Device Drivers – very flexible, but must wait until OS starts loading.

• CONFIG.SYS – DOS utility program, runs before DOS really gets going

• SYSTEM.INI – Introduced with Win 1 to 3, another text file. Win 9x won’t boot without it; XP will

• The Registry – Introduced with Win 95. Binary database of information

Software Help

• Control Panel – Graphical “window” to the Registry

• Device Manager – Another graphical path to the Registry

• REGISTRY EDITOR(s):– REGEDIT– REGEDT32

Power On Self Test (POST)

• First thing to run when you start up computer

• Waits for “power good” signal from power supply, then starts the CPU

• FFFA is (almost) at the end of 1st meg of RAM, contains a “Jump Immediate” instruction to beginning of POST code

• CPU instructs our “short list” items to check themselves out with own diagnostic

POST, cont.

• Until video card is done, we are “in the dark” and could get “Beep Codes” for errors. Don’t memorize them. Constant beeping is either memory or video error.

• Once the video card has checked itself out, we could get error codes/messages on the monitor as other devices report status

5

POST, cont.

• About this time, we get the option to run CMOS Setup routine

• POST is done; control passes to “Bootstrap Loader.” It’s job is to find OS.– Looks for the “Boot Sector” on floppy/hard

disk drive(s)• A disk with functional OS is called a

“Bootable Disk” or “System Disk.” Could be floppy (DOS), CD or hard disk drive.

Boot Order

• Somewhere in CMOS Setup is the ability to set the boot order, or the order in which the bootstrap loader will search for an OS.

• I set my systems to: CD/DVD drive, floppy and then hard disk drive.

• Usually, if a bootable CD is found, you get the option to boot from it (by pressing any key). Do nothing and loader continues the search for an OS.

POST Cards

• Can be purchased; probably about extinct now.

• I could have used one about three times in my career so far.

• If another (known good) video card and/or memory won’t boot, motherboard is toasted.

Flashing the BIOS• Mess up the process and motherboard is

history• Won’t allow you to swap 486 for P4 chip• Most processes require a bootable floppy

with special program and very odd command line

• Some newer motherboards include Flash as Setup option.

• I would rather eat a bug

UEFI

• At least Sandy Bridge processor and current motherboard.

• 32/64-bit replacement for BIOS• Allows use of 3 TB+ drives without

partitioning it down• Good topic for an Internet search some

rainy Saturday