The guide on this page is just one in a series that explains only what you really need to know about boot sequences and the chain of programs that are involved. This page is specific to using Windows' own bootmanagers to manage a dual-boot machine. For you to completely follow what is shown here you may have to read the first article in the series, which will give you the key to the graphics and introduce you to the individual boot files.

We have used Windows XP and Win-7 to illustrate our example, but it could just as easily be any of the legacy family of XP/2K based operating systems in dualboot with any of the new generation of Windows from Vista onwards. All Windows Vista, Win-7 and Win-8 based operating systems will in standard and native install circumstances on an MBR styled hard drive in a BIOS based machine produce the configuration of boot files shown below. The new UEFI based computers with GPT styled hard drives are somewhat different and much of what is shown and described here does not apply to them.
An Introduction to UEFI Firmware and GUID Partitions.

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If you use Microsoft's default and recommended procedures on an XP machine to create and format a second partition and then install Windows-7 you will end up with the configuration shown in the following graphic. The existing XP install will be modified to support the method of operation that ntldr (NT Loader) and bootmgr (bootmanager) require to coexist and provide a working dual-boot system. When booting Windows-7 the same 5 boot sequence programs will run in the same order as they do when Windows-7 is the only OS on a single partition drive.

For Windows-7 the boot sequence is:- BIOS - IPL - PBR - bootmgr - winload.exe --> Windows-7

You can see that in this dual-boot situation Windows-7 has placed its bootmgr and BCD (boot configuration data) files inside the partition of the existing XP install. Also notice that the PBR of the XP partition has been replaced with a Windows-7 PBR so that it will be bootmgr that is loaded next in the boot sequence instead of ntldr. On bootup of the machine it will be bootmgr that will halt the boot process and offer a boot menu. On selecting Windows-7 the bootmgr will jump directly to winload.exe in the Windows-7 System32 folder, skipping the unrequired PBR at the start of the Windows-7 partition.

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When you select XP from the dual-boot menu the boot process will have changed slightly from when XP was the sole operating system on the drive because bootmgr will have inserted itself in the chain. This adds an extra step to the boot sequence and gives XP 5 programs in its boot chain, instead of the 4 it originally had.

For Windows XP the boot sequence is now:- BIOS - IPL - PBR - bootmgr - ntldr --> Windows XP

The reason XP normally only has 4 programs in its boot chain is because, as you may remember from the previous page, the ntldr (NT Loader) is a dual purpose affair that carries out the functions of both a bootmanager and a bootloader. This is also the reason why the Windows-7 bootmanager (bootmgr) can't just directly start the XP bootloader and leave the XP bootmanager out of the picture. The bootmanager function of ntldr cannot be bypassed and this is why 2 bootmanagers are involved in booting XP in this particular Windows dual-boot configuration.

In our example here it will only be bootmgr that will halt the boot process and present a boot menu, where you can select either Windows-7 or XP. The XP bootmanager won't appear on screen because it only has one entry in its boot menu so there is no choice to be made. If however there had already been an existing dual or multi-boot system of legacy operating systems on the drive before the addition of Windows-7, then you would indeed see a second boot menu and so have to navigate two boot menu screens to get to your desired operating system.

With no bootmgr or BCD on the Windows-7 partition you would be unable to directly boot that Windows install by way of the partition's own PBR. Trying to do so with another bootmanager or by changing the Active partition so that the IPL would start the PBR of the Win-7 partition would just result in an error message from the PBR most likely saying that the bootmgr file was missing. The presence and integrity of the XP partition with the Win-7 boot files must be maintained for this dual-boot setup to continue to work. If you were to delete or format the XP partition or re-install XP you would remove all the Windows-7 components. A repair install of XP may actually retain the bootmgr and BCD files where they are, but it will replace the PBR of the partition with an XP one, which will take Win-7 out of the loop by starting the ntldr instead of bootmgr.

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The XP partition in our example has become what Microsoft calls the 'System' partition because it is now in control of booting an operating system that is on a different partition. Separate system and boot partitions have long been a feature of using a Windows bootmanager and it is something you should definitely familiarize yourself with if you intend multibooting the Microsoft way. The Windows System and Boot partitions.

From Windows-7 onwards the use of separate system and boot partitions has been made the preferred layout even for a machine that only has one operating system. During the install of Windows-7 the setup routines will favor the use of a system partition and if one does not already exist then setup will when possible create an extra small partition where it can place the bootmgr and BCD files. This behavior is going to be the normal Windows procedure on the new GPT styled hard drives and Microsoft have said they implemented the change even on MBR styled drives just so we can all get used to it. We don't object to this change as we have always said that if you must use a Windows bootmanager it should be on its own dedicated partition and not on one that also holds an operating system. Installing Windows to get the configuration you want.

If you use two individual hard drives for this dual-boot setup instead of partitions on a single drive the configuration of boot files shown above remains essentially the same. The XP install on the boot drive will receive the Win-7 boot files and have its PBR altered. The second drive with the operating system on it will be missing the bootmgr and BCD files and so will have to be booted through the first drive. This of course means that you can't remove the XP drive or swap the drives round to make the Windows-7 one the boot drive and expect it to boot. More on this in the relevant guide in the series.

You may like to make a copy of both bootmgr and BCD and place them in the root of the Windows-7 install. This will not only serve as a backup for these files that could be used to repair a damaged system partition, but it could also allow us to bypass or remove the system partition and boot Windows-7 directly by its own PBR. It is also possible to put the BCD and bootmgr on a boot disk and run them from there, which would let us overcome any damaged or missing boot files or system partition on the hard drive.
Backing up the BCD  Make a Windows Boot Disk/USB Key

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