Memory Controller Control Registers?

Hi, this is my 1st post here. If I am making some mistakes on the posting, I will change it later.

For an older system, I can change some registers with "RW" attribute. For example, C0REFRCTRL on 82P45 after getting the MCHBAR was able to be modified.

However, for Nehalem, I can't write to MC_CHANNEL_0_REFRESH_TIMING even it is listed as RW.

Shoud I learn more about PCI protocol? Or which document should I study it again?

Any hints will be appreciated.

BR,

3putt