
CHAPTER 8 Directory-Based Cache Coherence
to the memory looked up all the tags to determine the states of the block in the
dif-
ferent caches (Tang 1976; Tucker 1986). Of course, the tag copies at main memory
had to be kept up-to-date. Since the directory was centralized in these early
schemes, they are called centralized
directory
schemes,
The value of directories is that they keep track of which nodes have copies of a
block, eliminating the need for broadcast. This is clearly very valuable on read
misses since a request for a block will either be satisfied at the main memory or the
directory will tell it exactly where to go to retrieve th