What the right way to access memory mapped register is?

I'm new to developing device driver, but I've run into trouble shortly after starting.

I'm making Intel High Definition Audio Controller driver, and I checked in the specification that the access on the memory mapped register would be necessary.

I found their addresses using devmgmt.msc and RWEverything, they're started at 0xF7FF8000.

When I wrote DriverEntry, I directly accessed that base address(e.g. (unsigned char*)0xf7ff8000) and used index and offset to access specific register, but when I started driver, BSOD occured at the point I read a value from one of them register(It was CorbSize register on offset 0x4e)

BSOD stop code was 0x00000050, "PAGE_FAULT_IN_NONPAGED_AREA". And kd showed me that f7ff8000 is freed (or corrupted) pool and there's Bad allocation size @f7ff8000, too large...

Are those error codes mean that address 0xF7FF8000 is unavailable data space? But I checked the contents on 0xF7FF8000 using RWEverything, and there were constantly changing values on that space, even though db 0xf7ff8000 command on kd showed me only ??-filled memory space...

I really wonder what the problem is, and what the right way to access their memory mapped registers is.

Sorry for my bad english skill :'(

Thanks!