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Showing posts from 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 boo…