Saturday, 22 December 2007

Removing CHS based access from windows boot loaders

Recently, I had troubles to migrate my Windows installation from VMWare to VirtualBox. When booting the vmware created partition in virtualbox, I got "NTLDR not found". So I sharpened the knives and got down to business with vmware's gdb interface and virtualbox's internal debugger. Tracing the execution showed that the BIOSes of the two products reported different geometries on the INT 13h interface. The generic method contained in the boot loader to read a sector from disk is "clever" as it checks whether the sector is below the maximum sector index that is reachable with the CHS geometry reported by the BIOS. If not, it uses the LBA interface of the BIOS. If yes, the cleverness of the boot loader suddenly vanishes. Instead of using the BIOS reported geometry to break the absolute sector down into its CHS components, the boot loader uses a geometry stored in the so called BIOS parameter block. That's a section of the first sector embedded into the boot loader that hard codes such values as head per cylinder and sectors per heads into the boot loader. If the hard coded values are different from the ones used by the BIOS, the calculation produces wrong values. So, if you move your partition to a BIOS that exposes a different geometry to the boot loader than is hard coded in the boot loader the whole thing blows up. Brilliant Microsoft design, as ever.

My solution is to override the check in the boot loader, so that LBA based access is always used and the CHS code is never touched. This way I'm able to use my partition under vmware (which uses heads=15) and virtualbox (which uses heads=255) simultaneously. Here is my boot loader patcher for FAT32 and NTFS based boot loaders: killchs.c. Use on your own risk. Chances are good that you can restore you boot loader with mbrfix if it breaks your boot loader.

Btw: VirtualBox is available under the GPL, and not only this makes it much more sexy to work with, it is also much faster than VMWare, at least that's my impression. There is also a commercial distribution of VirtualBox.