As per every chipset datasheet I've read, it seems that this area of RAM is "missing" or "not seen" or "inaccessible" during normal runtime (operating system already take control) to the processor. I mean this area is "shadowed" by other hardware devices (not the RAM itself). It should be that way to maintain legacy compatibility (with DOS), hence the RAM in this area is totally unseen by the system unless it is in SMM. The devices that occupy this area during runtime includes part of the VGA memory mapped video buffer, etc.
This is my opinion and understanding of SMM. For those who are more knowledgeable, would you please corrrect if there are any misunderstanding of this point of view
