the index bits for accessing the tag store

A student writes.  I am not sure of his question, but I will try anyway.
Perhaps he (or someone else) will respond.

	Hello Dr. Patt,

	During recitation with Rustam, we could not remember what was 
	your reason for not increasing the size of the index toward the 
	page frame number such that the size of PFN decreases. 
	Would you mind to reiterate that reasoning?
	
	Thanks,
	name withheld to...

	<name withheld to protect the pseudo-clever student who signed as shown> 
I am not sure I understand the question.  My guess is you are asking why 
we do not make the PFN smaller so we have more bits for index, and therefore 
won't have to wait for the TLB access.  Answer: Generally not our choice.  
Making the PFN smaller means (if I under you correctly) making the page
size larger.  That is part of the ISA, which we are given.  Our job is to
design the cache which is part of the implementation, given the ISA (which
fixes the page size).  So, with the page size fixed, we only have the number
of bits that are not translated, and we can not add more on our own.

The only degree of freedom, if we want to go the route of indexing from the
unmapped bits of the address, is to increase the associativity.

If this does not answer your question, please try again.

Yale Patt