Re: [Gems-users] Necessity of NP state

["Mike Marty" <mikem@xxxxxxxxxxx>]

"no tag for the block" does mean a cache miss, but a cache miss can also occur due to "having a tag for the block, but insufficient coherence permission".  In the former case, the protocol must allocate a cache block (and possibly replace data) and then obtain appropriate permission.  In the latter case, a tag is already allocated, so the protocol only has to obtain coherence permission.

 

As for the CacheMemory::allocate() sets the permission as Invalid, this is what it should do when allocating a new block.  The if check is probably an artifact for when CacheMemory.h did not have a deallocate() method (we just set the tag to NotPresent instead). 

 

--Mike

 

----- Original Message -----

From: Juyoung Jung

To: gems-users@xxxxxxxxxxx

Sent: Friday, June 22, 2007 12:33 PM

Subject: Re: [Gems-users] Necessity of NP state

Thanks for your quick reply.

>Well the protocol needs to know if the cache has no tag for hte block and needs to allocate one.

>
>--Mike

Mike, does "no tag for the block" means simply cache miss? Then, cache controller has to replace the missed line with the demanded data from lower level, right?
Can I think of allocating one newly as the process of cache replacement?
The below is CacheMemory::allocate( ) method, and I am still wondering when cache's access permission is set to NotPresent instead of Invalid. In addition, why cache permission should be compared with Not present before allocating new cache entry? Could you give me more advice? Thanks for your time.

   Index cacheSet = addressToCacheSet(address);
   for (int i=0; i < m_cache_assoc; i++) {
     if (m_cache[cacheSet][i].m_Permission == AccessPermission_NotPresent) {      <=== When permission is set to Not present?
       m_cache[cacheSet][i] = ENTRY();  // Init entry
       m_cache[cacheSet][i].m_Address = address;
       m_cache[cacheSet][i].m_Permission = AccessPermission_Invalid ;                   <=== Why
       m_cache[cacheSet][i].m_LastRef = g_eventQueue_ptr->getTime();
       return;
     }
   }


Thank you.


- Young

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