Parallel Computer Architecture

CHAPTER 8 Directory-Based Cache Coherence

resource is busy. NACKs are used just to indicate inappropriate requests and facili-

tate changes of state at the requestor; the difference is that in this case a request that

is NACKed will never succeed in its original form but may cause a new type of

request to be generated, which may succeed.

Finally, when a block needs to be written back upon a miss, an important perfor-

mance question is whether the miss should be satisfied first or the block should be

written back first. In discussing bus-based protocols, we saw that most often the

miss is serviced first and the block to be written back is put