Tuesday, July 17, 2007

Basics of Booting a PC

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Booting of the Computer

So far we have talked about various things that we can do on a computer... Whether Offline or online. But a little of the most know about how a computer starts (no! not just smackin’ the switch/buttons and powering it on!). So in this post I am gonna talk about the very basics of Switching on and Booting of the COMPUTER.


We know about almost unlimited software. But, the first and the foremost software that comes into picture when the computer is switched on is BIOS. This is what happens when you switch on the computer.

Switching on the Computer:

When the computer is switched on,the current is passed on to the various cards. From here it goes to the chip called the BIOS chip. It is the one which wakes the computer and you see the various messages on the monitor. Once the BIOS has performed its house keeping activites, the control is shifted to the operating system (OS). But the booting of the PC is very interesting. Read on..

What is BIOS:

BIOS is Basic Input/Output System. In PCs, the boot process is very important. It is the process which makes the computer run and hands over the computer system to the OS for running. The importance of PCs BIOS is that it performs all the functions, the PC needs to get started. The BIOS contains the first instruction the PC needs to get started, programming that checks that PCs hardware is attached and ready and other routines to help the computer get up and running.

Booting of the computer:

Another of activities of the BIOS is to provide the interface that connects the CPU to the input and output devices attached to the PC. The BIOS relieves the PC from needing to know about how hardware devices are attached to the computer. When a new device is attached to the computer, only the BIOS configuration data needs to be updated rather the need of updating every piece of software in the PC as to where the hardware is attached and where its drivers are located.

When the PC boots,the PCs hardware cannot perform independent actions. It must have instructions to do anything at all. These instructions are carried in the form of PC’s software, which are blocks of instructions that guide the hardware to perform specific activities.

System Boot Sequence:

The process used to boot a PC by the action of BIOS is a fairly complex sequence of steps that verifies the configuration, checks the hardware and loads the software. The actual steps included in a particular BIOS’s boot sequence can vary by manufacturer but the following are the typical of the steps normally performed during the system boot sequence.

Ø When you turn on the PC’s power switch, the internal power supply initializes itself. The power supply doesn’t provide the power to the rest of the PC immediately. As soon as the power supply is able to supply reliable power to the motherboard, it transmits a “good power” signal to the motherboard’s chipset, which sends a system rest command to the processor. At this point, from all outward appearances the PC looks as if its still powered off.

Ø The system rest command sent by the motherboard’s chipset causes the CPU to read its first instruction from what is called the jump address. The jump address is always located in fixed preset location. Typically address FFFF0h is in system memory. The jump address contains the physical address of the BIOS’s boot program on the ROM BIOS chip.

Ø The CPU executes the first instructions, which copies the BIOS programs into system memory and starts running the BIOS.

Ø The BIOS then performs the POST (Power-On Self Test) process. The POST verifies and tests the hardware configuration stored in the BIOS configuration information. Should the POST detect any problems, it sounds beep codes, or flashes error messages about the nature of the problem and the Boot-Process stops.

Ø If the POST detects no problems, the boot process continues. At this point the system BIOS looks for video adapter’s BIOS and starts it. Virtually all the devices on the PC have their own BIOS. This is the first time that you will know the PC is booting; besides from the noises of the disk drives and single beep indicating all is well.

Ø The display of video adapter’s information is followed by information about the system BIOS itself. This usually includes info about the manufacturer and the version of BIOS programs.

Ø Any device BIOS routines are started. Next, the BIOS begins a series of tests on the system, including the amount of memory detected on the system which now can be displayed on the screen and any error messages can be flashed (because now BIOS has the use of the monitor, so it don’t need to make beeps anymore as earlier).

Ø With the device BIOSs loaded, the system BIOS checks if the devices listed in the CMOS (wait for the next post, its going to be about it) configuration data are present and functioning. Including their speeds, access modes and other parameters. In this sequence, the serial and parallel ports are assigned their identities (COM1, COM2, LPT1 etc.). As each device is passed, a message is displayed that it was found, configured and tested.

Ø If the BIOS supports Plug and Play (PnP) technology, any PnP devices are configured, info about them displayed on the screen, although it typically goes by much too fast to be read.

Ø At the end of the test and configuration sequence, the BIOS should display a summary data screen that details the PC as the BIOS sees it and indicating that the system is verified and ready for the use.

Starting the OS:

To start the Operating System, the BIOS must first find it. Included in the CMOS data is a parameter that indicates the disk drives and the order in they which should be accessed to find the operating system. In most cases, the sequence by default is first floppy disk drive, then the hard disk drive and then the CD ROM drive. This sequence can however be changed to reflect the sequence the user defines as desired.

If the first boot device is the Hard Disk Drive (HDD), the BIOS looks for the Master Boot record (MBR) to use to start the OS. If the boot device is a floppy disk drive (FDD),then the BIOS looks at the first sector on the Floppy Disk for the OS program. If the Boot Program is not found on the first device then the second device is looked for, then third and so on until the program is found. If no boot device is found, then the boot sequence stops and error message (“Error loading OS” or “No Boot Device avalable” , depending on the case) is displayed.

The PC should now be up and running and ready for use.

Recommended: Next time you boot up a PC, watch this sequence more closely to see if you recognize the actions taking place.


Ankur expressed...

Never knew all this ... thanks

steve expressed...

Excellent, easy to follow and understand article. Thanks for putting it together...

Just added you to my technorati favorites by the way.


Abhinav Sood expressed...

Thanks a lot,for this encourages me bring up more stuff like this. And thanks for the add in technorati. But dont know why,i am facing a problem. Inspite of being faved by many of my readers the techxtreme status at technorati is NO AUTHORITY.. I am seeing to that and hope to find some solution asap

RT expressed...

This is a good article! It's written the way a "how to" article should be written, not catering to a specific group.

Abhinav Sood expressed...

Thank you Richard ;)