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Saturday, November 15, 2008

How Windows XP Starts

After the computer starts and hands off the process to the operating system, Windows XP continues to load in the following manner:

1. The MBR a small program, as it can't do much work loads NTLDR  into memory. This MBR is typically found on the first sector of a hard drive. 

2. NTLDR switches the control to a file named BOOT.INI and then reads the contents of it. The BOOT.INI file contains information on the different boot sectors that exist on your computer.

3. If a computer has multiple bootable partitions, NTLDR uses the information in the BOOT.INI file to display a menu. That menu contains options on the various operating systems that you can load. If a computer has only one bootable partition, NTLDR bypasses the menu and loads Windows XP.

4. NTLDR opens a program into memory named NTDETECT.COM which performs a complete hardware test on your computer. After determining the hardware that is present, NTDETECT.COM gives that information back to NTLDR.

5. NTLDR then loads the version of Windows XP that we select in Step 3. Then is finds the NTOSKRNL file in the System32 folder. NTOSKRNL is the root program of the Windows operating system: the kernel. 

6. NTLDR passes the control of the boot process to the kernel and to another file named HAL.DLL. HAL.DLL controls the hardware abstraction layer (HAL), which is the protective layer between Windows and a computer’s hardware that enables such stability in the Windows XP environment.

7. NTOSKRNL handles the rest of the boot process which loads the additional files that make up the core Windows XP operating system.

8. Next, Windows verifies whether there is more than one hardware profile configured for the computer. If there is more than one profile, Windows displays a menu from which to choose. If there is only one hardware profile, Windows bypasses the menu and loads the default profile.

9. After Windows knows which hardware profile to use, Windows next loads all the device drivers for the hardware on your computer. By this time, you are looking at the Welcome To Windows XP boot screen.

10. Finally, Windows starts any services that are scheduled to start automatically. While services are starting, Windows displays the logon screen.

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