MBR & active partition dual boot

Question: If the MBR 's "one and only job is to launch the bootsector code written to the active partition of your boot disk", then how/why is it possible to install a Windows (or any other) bootloader in the MBR? As the MBR just launches code in the active partition's boot sector surely it should not contain any OS-specific code?

I'd be grateful if someone could clear up my misunderstanding.


Knows where his towel is.
Staff member
bootmgr code is in the PBR

1.After pressing the power button, the PC’s firmware initiates a Power-On Self Test (POST) and loads firmware settings. This pre-boot process ends when a valid system disk is detected.
2.Firmware reads the master boot record (MBR), which locates the "active" partition and then transfers control to that partition's boot sector (PBR) which then starts Bootmgr.exe. Bootmgr.exe finds and starts the Windows loader (Winload.exe) from the Windows\System32 folder on the partition which contains the copy of Windows selected from the boot menu.
3.Essential drivers required to start the Windows kernel are loaded and the kernel starts to run, loading into memory the system registry hive and additional drivers that are marked as BOOT_START.
4.The kernel passes control to the session manager process (Smss.exe) which initializes the system session, and loads and starts the devices and drivers that are not marked BOOT_START.
5.Winlogon.exe starts, the user logon screen appears, the service control manager starts services, and any Group Policy scripts are run. When the user logs in, Windows creates a session for that user.
6.Explorer.exe starts, the system creates the desktop window manager (DWM) process, which initializes the desktop and displays it.
Thanks for that but my question refers specifically to the BCD Deployment page in EasyBCD which has an option under MBR configuration to install the Windows Vista+ bootmgr loader into the MBR. As you say the bootmanager is in the PBR, so what exactly does this EasyBCD option do?
Similarly the Linux grub bootloader is apparantly installed in the MBR by the Ubuntu installation process. In a dual boot system this means that you get the rather messy grub boot menu. To return to the Windows boot menu the EasyBCD Deployment option mentioned above can be used.


Mostly Harmless
Staff member
Hi Graham,

If you refer to this in-depth wiki article that we published, I think it should explain most of your questions: The BIOS/MBR Boot Process

Assuming you've already read that, in answer to your direct question: EasyBCD's MBR deployment feature loads the correct code to both the MBR of the startup disk and the bootsector of the active partition. (Not the MBR only)