To boot an operating system the bootloader has to first be found and then started. In Vista that's the job of bootmgr with the help of the BCD store to tell it where to look. In previous WinNT it is slightly different because ntldr is both bootmanager and bootloader and so it is the location of the main OS system files that ntldr has to find. Both ntldr and bootmgr have to first identify which hard drive they should look on. The ntldr does this with the aid of the boot.ini file, which lists the hard drives by number in the same order as the computer's BIOS sees them. The ntldr consults the boot.ini for the number of the drive that it wants and then checks the BIOS to find the location of that drive. In Vista the BCD store does not list hard drives by number at all but rather by their unique disk signature. When bootmgr queries the BCD for the drive it wants it is told the disk signature of that drive, so it then scans the connected drives till it finds the one with that signature. If no match is found then bootmgr cannot continue to look for the Vista bootloader (winload.exe) and hence displays the error message that winload.exe......is missing or corrupt.

So the disk signature is now a vital component in the boot process and has to be maintained for bootmgr to do its job. If a signature is changed then every boot option in the BCD for the affected hard drive will need to be updated to contain the new signature. If a signature is completely zeroed-out then a new one will have to be written and the BCD again informed. There are not many situations where the average PC user would ever do anything that could alter or damage the disk signature, so the whole process should remain transparent and trouble free. However, any imaging or cloning, installing OSes or dual/multi booting, could result in problems. Installing a bootmanager like XOSL that changes the disk signature, or updating the MBR with fdisk/mbr which zeros out the signature, would have obvious consequences.

In addition to the disk signature the BCD also holds details about the location of partitions on a hard drive, which is also used by bootmgr to find the whereabouts of winload.exe. If a partition's starting point on a drive (its offset) has moved, then bootmgr will not be able to find the partition and again will not be able to locate winload.exe. This is covered in more detail on the bootmgr and BCD page. How to fix the BCD to cure a winload.exe error is dealt with on the cloning page.


A Larger IPL
Historically the Microsoft MBR bootcode (the IPL - Initial Program Loader) mainly just checks the partition table to see which partition is marked as the active one on the disk and then passes control to the OS bootcode in the PBR (Partition Boot Record) of that active partition. It can generate only a few error messages, which you can see after the bootcode and before the disk signature. The Vista bootcode (in yellow) in the MBR has grown in size slightly over that of XP, but still only occupies no more than the space before the disk signature on the very first sector of the drive. I have looked for but not found one single byte in any of the free sectors between the MBR and the first partition.

The purpose of the extra code is not clear, but it seems it may be mainly to do with the BitLocker drive encryption system. This MS white paper asking computer suppliers not to hook their recovery systems into the MS bootcode gives some insight. I have tried replacing the Vista bootcode in the MBR with that of XP, Win98, Linux's Grub and with that of four other MBR bootmanagers. Unsurprisingly, in all cases Vista boots just fine. It's too early to say for sure but this suggests that the extra code is not actually a required part of the IPL and as in previous Windows it remains OS-Independent, as least when a Trusted Platform Module and BitLocker are not being used.