AMD "long mode" deficiencies ( Of what use 64-bit "GeneralPurpose" registers?)

"Yousuf Khan" writes:

In the original 32-bit implementation, I think 12 bits were
reserved for system use. That way if they ever page any of these pages out
to virtual memory (disk), then the system software can remember where that
page is located on the disk to get it back.

In the 80386, if the Present bit of a page table entry is 0 (so
that accessing the page will trigger an exception), all other 31
bits are available for operating-system use.