Many people are asking, How the operating system gets started? Remember, the operating system is  the software that allows you use the computer. In order for any program to execute, it must be in the computer's memory. But the RAM of a computer loses its contents when the computer is switched off.

Many people are still asking, How then is the operating system loaded into RAM and executed when a computer is switched on? The answer lies in using ROM because ROM retains its contents permanently. The operating system itself is not stored in ROM. There is normally only a small amount of ROM available and it would not be large enough to hold an operating system. Also, since its contents cannot be modified, it would not be possible to correct any errors (bugs) in the software that unfortunately arise in complex programs like operating systems. Thus it would not be a good idea to store operating system in ROM, even if it is even if it was possible. Instead, what happens is that a small program is stored on the ROM which is used to get operating system loaded and executed. The operating system code is stored on disk. The ROM program loads a part of the operating system, called the bootstrap program from a fixed location on disk called the boot block into RAM  and switches control to this program. This bootstrap program in turns loads the rest of the operating system from disk into RAM and switches control to it. This process is called bootstrapping (booting) after the notion of pulling yourself up by your boot laces. One advantage of this technique is that it means that the same computer can use a different operating system by placing an appropriate bootstrap program in the boot block for the operating system you wish to boot up.

In summary, when you switch on (boot up) a computer, a program in ROM is executed which Loads in a second program from disk and starts it. This bootstrap program in turn loads the operating system program from disk and starts it. The operating system then starts the command interpreter to allow you give the computer commands. Each operating system has its own command interpreter which will be executed on booting.

IBM PCs and their compatibles are unusual in that the ROM does not only contain a bootstrap program as described above, but it also contains code for device drivers for the screen, keyboard and disk etc. This code is referred to as the ROM BIOS (Basic Input Output System). In other operating system this code will usually reside on disk with the rest of the operating system code. In addition, the ROM contains self test code to ensure that the hardware is functioning properly when the machine is switched on. This code is called power-on self-test code (POST code). This takes us to the steps in booting a computer system.
booting sequence

knowing the steps in the boot process will help you when troubleshooting or analyzing problem with your PC (Personal Computer). Below is the booting process.

1.When the PC is powered up, the CPU reset itself, clear out any leftover data and looks to an address in memory called F000 that is where the ROM BIOS chip is located. The ROM BIOS chip is what makes your PC IBM compatible.
When the CPU finds the ROM BIOS, it invokes the first program found in memory, which is POST, the power-on self test. This self test ensures that all your components are operating properly before you begin working with the computer.

2. As POST checks your secure computer it looks to a record of data stored in CMOS RAM that tells what kinds of components are in your PC. Specifically, it records what type of video card, floppy drives, hard disk, memory and so forth are contained in your PC. POST will test your computer based on what it believes in your PC. If the information is missing or incorrect the PC may not be able to recognize or use certain components in your system. It is important to keep a record of what specifically is inside your computer, and that you have a record of what is written into CMOS RAM.

3. If POST finds that there is a problem with your PC, it will display an error message or an error code that tells specifically what is wrong with the unit. If it cannot display such a message, it will keep in a specific pattern that indicate exactly what is wrong. If everything is OK with the computer, POST will sound one beep to the system speaker, indicating that all of  the test was normal with no errors.

4. The ROM BIOS will then look to the boot sector of either a floppy disk or a hard disk to find IO.SYS, the boot loader program of your operating system. If it can't find this file in that location, the PC will give an error message to the screen. When it does find the file, it loads the file into RAM, and then your operating system takes charge of the computer.

5. MSDOS.SYS then loads into memory (for both DOS and Windows operating system). It contains the bulk of the operating system code that makes DOS or Windows operate. Specifically, it lets you manage file names, execute programs and allows hardware and software to manage interrupt request  (IRQ).

6. Then CONFIG.SYS configure your system for the specific kinds of hardware that are contained in the system. Lines like DEVICEHIGH-C:\MOUSE.SYS tell DOS where to find the computer mouse driver, and whereto load it into memory. DOS and Windows both use a CONFIG.SYS file.

7. Then,COMMAND.COM loads into the memory. It is your command interpreter, and it allows you to input command at a DOS prompt. DOS and Windows both have COMMAND.COM as one of the system file.

8. Finally, AUTOEXEC.BAT loads into memory any program which to run every time you start or reboot your computer. Usually at the end of the AUTOEXEC.BAT file, the PC will call up Windows, or a menu program, or will allow you log into a network, so that you can begin working with the computer as you would wish. Both DOS and Win95 uses AUTOEXEC.BAT.
If any of these state does not occur in a normal manner, your PC will not boot as you would expect.

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