Chapter 4:

Motherboard and BIOS

Buses

Both the External Data Bus and the Address Bus connect all the parts on the motherboard

There are two of each of these buses, one of each between the CPU and the chipset, and the other two are between the chipset and everything else (except RAM)

BIOS

BIOS: Basic Input/Output Services A BIOS is a collection of programs

that tell the CPU how to communicate with the most basic parts of your computer

Each program is called a service These programs are tiny, usually 2

to 30 lines of code each

ROM A BIOS is stored on a ROM ROM = Read Only Memory ROM chips are nonvolatile,

meaning that they do not lose their data when they lose power

They are also read-only, which means that the programs on them can’t be changed

Hardware Groups Core Group - common, necessary,

unchangeable devices Examples are keyboard, chipset, support

chips CMOS group – common, necessary

devices that may be changed Examples are Hard Drive, Floppy Drive, RAM

Everything Else group – non-core devices such as mice, sound cards, and CD-ROM drives

System BIOS

The system BIOS is the collection of BIOS programs that interface with the Core group of hardware in a PC

The system BIOS comes with the motherboard, and is stored on the system ROM

System ROM Stores system BIOS Usually comes in a DIPP packaging, and

most have a shiny label The majority of system ROMs store 64k

of data Like all ROM chips, the system ROM is

nonvolatile Almost never fails, although when it

does, it can be catastrophic

CMOS CMOS – Complementary Metal Oxide

Semiconductor CMOS chip also acts as a real-time clock

for the computer CMOS chips do not store programs, they

store only the data read by BIOS to access changeable devices

Usually store 64k, but about 128 bytes are usually used

CMOS Setup In order to change CMOS data, you

need to use a program built into the system ROM called the CMOS setup program

It is started while the PC boots up, after it is turned on and before it loads an OS

Accessed in many different ways, look through your motherboard manual to find out how to access yours

CMOS Setup All CMOS Setup menus will appear

differently, but they do most of the same things

On the Standard CMOS Setup screen, the time/date, hard drive, and floppy drive settings can be changed

Other than that, all BIOS have different settings that can be changed, which are explained in your motherboard’s manual

More CMOS Because CMOS is changeable, it requires

power to keep its settings Power is usually supplied by a battery,

which is either 3.3 or 6 volts The battery is also usually Ni-Cad or Lithium

Ion cells, and last about 5-7 years When the battery dies, you either must buy

a new motherboard, or hope that your motherboard has a connector for an external battery, which most modern ones do not

Flash ROM ROM has a huge shortcoming:

Because it is read-only, you have to replace the ROM chip to update the BIOS

Because of this, Intel developed something called a Flash ROM

Flash ROM can be erased and updated using special command-line programs to accommodate things such as new processors

Also nonvolatile

Flash ROM

Because the Flash ROM is reprogrammable, data that is usually stored on a CMOS chip can now be stored on the System ROM

Although CMOS is virtually nonexistent now, the name “CMOS Setup” has stuck

Everything Else Group

Because IBM couldn’t predict every device that might be added to a PC, they gave a few ways to bring your own BIOS (BYOB)

These ways are: Option ROM Device Drivers

Option ROM The first method of BYOB is to put the BIOS on the

device itself A card with an option ROM has a chip on it looking very

much like the system ROM This is because it is a ROM, and has the device’s BIOS

stored inside it Most BIOS on option ROM display information when you

boot the system In the early days of the PC, all devices had option ROMs,

but today, option ROMs have been replaced by things such as device drivers, with one major exception: all video cards have their own BIOS for internal function

Device Drivers A device driver is a file stored on a PC’s

hard drive that contains all the BIOS commands to talk to a certain device

All operating systems load device drivers into RAM while they boot up, loading only the ones that are needed via the use of a “list” of device drivers

This list can be in a variety of formats: a text file, many text files, or, in the case of Windows 95/98/2000, a special database

Device Drivers

Without these files, the OS doesn’t know which device drivers to load

As far as the PC is concerned, if a device driver isn’t loaded for a device, that device doesn’t exist

Device Drivers

Some of the configuration files and their corresponding OS are: config.sys – MS-DOS, still loaded in

Windows 9x for DOS compatibility system.ini – Windows 9x/2000, not

required by Windows 2000 Windows Registry – Windows 9x/2000

Windows Registry The Windows Registry is an extremely

complex binary file It stores all the configuration data in

Windows 9x/2000 The Registry is rarely accessed directly The Control Panel and the Device

Manager provide a much more user-friendly way to access the Registry

If the registry is accessed directly, it is done using the Registry Editor

Control Panel The Control Panel contains applets that

allow the user to change a broad range of system settings

Each version of Windows has many applets in common with the others, but it also has its own unique applets

Some third-party software programs add their own applets, such as Apple Quicktime or Real Networks RealPlayer

Device Manager

Using the Device Manager, the user can change or remove the driver for any device installed in the computer

The device manager is not used to install new hardware, that is done with the Add/Remove Hardware applet

Registry Editor In Windows 9x, the Registry editor is a

program called REGEDIT.EXE In Windows 2000, it is a program called

REGEDT32.EXE REGEDT32 is much more powerful than

the Windows 9x REGEDIT Setting a value wrong in the Registry

can potentially kill any Windows system, so you must be careful when accessing it directly

POST POST – Power On Self Test Every time a PC boots, it initiates a special

program also stored on the system ROM called the Power On Self Test

The POST sends a message to every device telling each to check itself out

If a core device is not working, the POST will issue a series of short beeps through the system speaker that correspond to certain errors

If a non-core device is not working, a corresponding error will be displayed on the screen

Boot Process The first step in the boot process is to

initialize the CPU itself by charging a special wire called power good

As soon as the CPU is “woken up”, it automatically starts the POST

After POST is finished, the computer has to be able to find the programs on the hard drive to load an OS

This is done with a BIOS function called the bootstrap loader

Bootstrap Loader The bootstrap loader is little more than a few

dozen lines of BIOS code tacked on the end of the POST program that looks for an OS

First it searches the floppy drives to see if a disk is inserted, and if a disk is inserted, assumes there is an OS on the floppy

If a disk is not inserted, it proceeds to search the hard drive

If a disk is bootable, it will contain special code telling the computer where to locate the OS

A disk with a functioning OS is called a system disk or bootable disk

Motherboards

Motherboards come in a large variety of sizes

The most popular layout today is called ATX, followed distantly by the old AT formfactor

AT Motherboards Two different sizes of AT motherboards exist

Regular AT, or AT – the original size, about the size of two 8.5x11 sheets of paper placed next to each other

Baby AT – much smaller size of the AT motherboard The only dedicated connector on an AT

motherboard is the keyboard connector, so expansion cards must be used to add anything else, including serial and parallel ports along with PS/2 style mouse ports, leaving little room for other expansion cards

Because of this, a new formfactor was needed

ATX Motherboards ATX was designed to replace AT It had PS/2 keyboard and mouse connectors, serial

ports, a parallel port, and after their invention, USB ports onboard, eliminating the use of expansion cards to use these ports

Also uses a different power connector than AT boards

Enables better air movement with position of the power supply

Uses “soft power”, a feature that enables the computer to be turned on and off using software

ATX Motherboards ATX motherboards have three sizes:

Standard ATX – the original ATX formfactor

Micro ATX – about 30% smaller than ATX, but uses standard ATX connections

Flex ATX – even smaller than Micro ATX, usually use a special Flex ATX power supply