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On system with BIOS firmware, the BIOS invokes MBR boot code from a hard disk drive at startup. The MBR boot code and the VBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find an active partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition.
In Windows NT, the booting process is initiated by NTLDR in versions before Vista and the Windows Boot Manager (BOOTMGR) in Vista and later. [4] The boot loader is responsible for accessing the file system on the boot drive, starting ntoskrnl.exe, and loading boot-time device drivers into memory. Once all the boot and system drivers have been ...
The Windows boot manager is located at the \EFI\Microsoft\Boot\ subfolder of the EFI system partition. [23] On Windows XP 64-Bit Edition and later, access to the EFI system partition is obtained by running the mountvol command. Mounts the EFI system partition on the specified drive. Available on Itanium-based computers only. [24]
EasyBCD has a number of bootloader-related features that can be used to repair and configure the bootloader. From the "Manage Bootloader" section of EasyBCD, it is possible to switch between the BOOTMGR bootloader (used since Windows Vista) and the NTLDR bootloader (used by legacy versions of Windows, from Windows NT to Windows XP) in the MBR from within Windows by simply clicking a button.
Microsoft boot sectors, therefore, traditionally imposed certain restrictions on the boot process. For example, the boot file had to be located at a fixed position in the root directory of the file system and stored within consecutive sectors, [7] [8] conditions taken care of by the SYS command and slightly relaxed in later versions of DOS.
The VBR is often OS-specific; however, its main function is to load and execute the operating system boot loader file (such as bootmgr or ntldr), which is the second-stage boot loader, from an active partition. Then the boot loader loads the OS kernel from the storage device.
Rarely an experienced user would reinstall Windows. In the case of the former, the boot loader will most likely inform the user the name of the missing file, allowing for the user to supply and/or reinstall the missing files to resolve the boot problem. In the case of the latter, however, the computer will not be able to boot, even into safe mode.
Though NTLDR and boot.ini are no longer used to boot Windows Vista and later versions of Windows NT, they ship with the bootcfg utility regardless. This is to handle boot.ini in the case that a multi-boot configuration with previous versions of Windows exists and needs troubleshooting from within the later operating system.