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;							 3-Jun-98
;
; RCS: $Id$
; notes for Wednesday, 3-Jun-98

1. cache associativity
2. virtual memory (in 5 minutes)
3. show & tell

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Cache associativity.

1. We talked about adding rows to the cache & indexing using part of the
   address (each row has a tag, but the tag is smaller); we talked about
   adding blocking to the cache (multiple data with the same tag).

   The third dimension in which to expand a cache is to duplicate the
   whole schmoodle.  This dimension is called "associativity".  A
   "direct-mapped" cache (what we've been building (with or without
   blocking)) has associativity = 1.

   An n-way "set-associative cache" duplicates the direct-mapped hardware
   n times.  Conventionally this is drawn as n caches side by side, and
   you can say that an n-way associative cache has "n columns".  Also, in
   a set-associative cache, the rows are sometimes called "sets".
   
   	4-way set-associative w/128 sets, 8 bytes/block
         = 128 rows, 4 columns, 8 data bytes/block or 32 data bytes/row
         = 4KBytes total cache size.
   
   A "fully-associative cache" has 1 row and a column for every block in
   the cache.  Small, fully-associative caches are often used for
   special-purpose cache applications, but it's too expensive to make
   them very big (all those comparators & distributed mux circuitry).

2. Associativity adds a new ambiguity