IA64 general?purpose registers r0 to r127 have 65 bits because they include a special NaT (not a thing) bit in addition to the normal 64 data bits. What is the purpose of the NaT bit and how is it used?

What will be an ideal response?

You could regard the NaT bit as a tag bit that tells you something about the word it labels or qualifies. NaT or
not a thing tells you whether a word is valid, and is similar to the NaN bit in floating?point arithmetic. If a bit is
tagged with NaT set, then it does not have to be used in calculations.

Suppose you have an operation whose operand has its NaT bit set. The result will also have the NaT bit set (if
the source is invalid, the destination must also be invalid). The NaT is used with speculative loads. If a load
operation would lead to an exception because of a page fault (i.e., the operand is not currently in memory), the
NaT bit is set to indicate that valid data was not loaded. Later, if the NaT is detected, the correct value can be
loaded. If the load was speculative (not needed because it is part of an IF THEN ELSE that is not executed) the
cost of an unnecessary page fault has been avoided.