REAL MODE Question

Hi all! I'm writing a driver for XP that reads the extended configuration space registers on the intel GMCH device. This needs to be done as memory-mapped IO so I get the MCH Base address from IO read and it gives a 64bit number. I don't know how to use it since real mode uses a 16bit segment and 16bit offset to point to an address. Anybody have ideas?

Thanks

Target system is intel chipset4 (82Q45)