When you try to repair your computer, it is important to know the startup or boot process, that is, when you press the power button on your computer, your pc perfomrs a number of tasks before you can see the log-in screen. Since the boot sequence is somewhat complicated, let's have a look at them one after another in the following:
The boot sequence of Windows 7 and Vista is different from Windows xp that uses the NT kernel. The following is the sequence:
1. When the computer is turned on, it loads either the BIOS or the EFI, Extensible Firmware Interface (a specification that defines a software interface between an operating system and platform firmware).
2. For the BIOS system, the MBR of the boot disk is accessed. It can be a hard drive or external media. Then it loads the boot sector of the drive or of the relevant hard disk partition.
3. This boot sector then loads the rest of the boot blocks.
4. Finally, the boot sector loads the Windows Boot Manager with the name of BOOTMGR. The Windows Boot Manager first looks for an active partition, then accesses the Boot Configuration Data store and uses the information to load the operating system. The Windows Boot Manager will invoke winload.exe, the operating system boot loader, to load the operating system kernel, ntoskrnl.exe and boot-class, device drivers. You should note that the function of winload.exe is equivalent to the operating system loader function of NTLDR in Windows XP.
Now let's go a bit deeper of two major components: Windows Boot Manager and Boot Configuration Data.
Windows Boot Manager
The function of the Windows Boot Manager is to read the boot configuration data and display an operating system selection menu, as NTLDR in Windows XP, A big difference is that the Windows Boot Manager is invoked by pressing the space bar instead of the F8 function key. The F8 key still remains assigned for advanced boot options once the Windows Boot Manager menu appears.
To maintain a consistent boot experience on Extensible Firmware Interface systems that also have a boot manager of their own, the Windows Boot Manager, and hence all of the installed Windows operating systems that can be booted using it, appear as a single entry on the EFI boot manager menu. (On EFI systems, the Windows Boot Manager is an EFI application stored on the EFI System Partition). Microsoft only adds multiple entries to the Windows Boot Manager (BCD) menu itself, and sets the timeout of the EFI boot manager to two seconds.
Boot Configuration Data
Boot Configuration Data or BCD is a firmware-independent database for boot-time configuration data. It replaces the boot.ini that was used by NTLDR, and is used by Microsoft's new Windows Boot Manager.
Boot Configuration Data is stored in a data file which is formatted in the same way as a Windows registry hive. It is located either on the EFI System Partition (on machines that use Extensible Firmware Interface firmware) or in \Boot\Bcd on the system volume (on machines that use IBM PC compatible firmware).
Boot Configuration Data may be altered using a command-line tool, bcdedit.exe, by using Windows Management Instrumentation
Boot Configuration Data contain the menu entries that are presented by the Windows Boot Manager, just as boot.ini contained the menu entries that were presented by NTLDR. The following are some menu entries:
1. Options to boot Windows by invoking winload.exe.
2. Options to resume Windows from hibernation by invoking winresume.exe.
3. Options to boot a prior version of the Windows NT family by invoking its NTLDR.
4. Options to load and to execute a Volume Boot Record.
5. Boot Configuration Data allows for the third party integration so anyone can implement tools like diagnostics or recovery options.
