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Windows Boot Manager

Windows Boot Manager operating system select menu
Other names	BOOTMGR
Developer(s)	Microsoft
Operating system	Windows
Predecessor	NTLDR
Type	Bootloader
License	Proprietary

TheWindows Boot Manager (BOOTMGR) is thebootloader provided byMicrosoft forWindows NT versions starting withWindows Vista andWindows Server 2008. It is the first program launched by theBIOS orUEFI of the computer and is responsible for loading the rest of Windows.^[1] It replaced theNTLDR present in older versions of Windows.

Theboot sector or UEFI loads the Windows Boot Manager (a file namedBOOTMGR oneither the system or the boot partition), accesses the Boot Configuration Data store and uses the information to load theoperating system throughwinload.exe orwinresume.exe onBIOS systems, andwinload.efi andwinresume.efi onUEFI systems.^[2]

On system withBIOS firmware, the BIOS invokesMBR boot code from ahard disk drive at startup. The MBR boot code and theVBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find anactive partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition. The VBR boot code tries to find and execute thebootmgr file from an active partition.^[3]

On systems withUEFI firmware, UEFI invokesbootmgfw.efi from anEFI system partition at startup, starting the Windows Boot Manager.

Once launched the Windows Boot Manager reads the Boot Configuration Data to determine what operating systems are present and if it should present the user with a menu allowing them to select which operating system to boot. Before Windows Vista, this data was contained inboot.ini.

These menu entries can include:

• Options to boot Windows Vista and later by invoking winload.exe.
• Options to resume Windows Vista and later from hibernation by invoking winresume.exe.
• Options to boot a prior version of the Windows NT family by invoking its NTLDR.
• Options to load and to execute avolume boot record.

The operating system is loaded by individual boot loaders for each install of Windows, called the Windows Boot Loader.

The Windows Boot Manager invokeswinload.exe—the operating systemboot loader—to load the operating system kernel executive (ntoskrnl.exe) and coredevice drivers. In that respect, winload.exe is functionally equivalent to the operating system loader function ofNTLDR in prior versions of Windows NT. InUEFI systems, the file is calledwinload.efi and the file is always located at\windows\system32 or\windows\system32\boot.

If the computer has recentlyhibernated, thenbootmgr will instead invokewinresume.exe. In UEFI systems, the file is calledwinresume.efi and is always located at\windows\system32 or\windows\system32\boot.^[4]

Boot Configuration Data (BCD) is afirmware-independentdatabase forboot-time configuration data.^[5] It is used by Microsoft's Windows Boot Manager and replaces theboot.ini that was used byNTLDR.

Boot Configuration Data is stored in a data file that has the same format asWindows Registry hives and is eventually mounted at registry keyHKEY_LOCAL_MACHINE\BCD00000^[6] (with restricted permissions^[7]). For UEFI boot, the file is located at/EFI/Microsoft/Boot/BCD on theEFI System Partition. For traditional BIOS boot, the file is at/boot/BCD on the active partition.^[8]

Boot Configuration Data may be altered using a command-line tool (bcdedit.exe), using theRegistry Editor^[6] (regedit.exe), usingWindows Management Instrumentation, or with third-party tools such asEasyBCD, BOOTICE,^[9] or Visual BCD Editor.^[10]

Boot Configuration Data allows for third-party integration, so anyone can implement tools like diagnostics or recovery options

• Booting process of Windows NT
• Windows Vista I/O technologies

• Boot loaders
• Windows NT architecture
• Windows Vista

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