I just built a new(ish) machine and I stuck 6 gigs of RAM into it. I decided to give the RC of Windows 7 (64bit) a try. As I was using it I noticed the machine said it had 6 gigs of ram, but only 5 was usable. This drove me a little nuts because it was able to detect 6 gigs. And it clearly wasn't this issue "Dude, Where's My 4 Gigabytes of RAM?" because I was getting 5 gigs. I also noticed that the BIOS was only reporting 5 gigs as usable. I thought it might be a bad RAM chip so I started swapping them in and out. Eventually I determined they were all good. I noticed the Memory remapping feature in the BIOS so I gave that a try and Windows and the BIOS is reporting all 6 gigs are usable.
Why does this remapping need to be done? Shouldn't the BIOS automatically be using all of the ram?