If the new cache line replaces a current dirty exclusive or dirty shared cache line, the current cache line must be written back before the new line can be loaded in the primary and secondary caches.

The processor examines the coherency attribute (cache coherency attributes are described in Chapter 11) in the TLB entry for the page that contains the requested cache line, and executes one of the following requests:

• If the coherency attribute is exclusive, the processor issues a coherent read request that also requests exclusivity.
• If the coherency attribute is sharable or update, the processor issues a coherent read request.
• If the coherency attribute is noncoherent, the processor issues a noncoherent read request.

If the current cache line needs to be written back and the coherency attribute for the requested cache line is sharable or update, the processor issues a cluster. The cluster consists of a coherent block read-with-write-forthcoming request for the cache line that contains the data element to be loaded, followed by a block write request for the current cache line.

If the current cache needs to be written back and the coherency attribute for the page containing the requested cache line is exclusive, the processor issues a cluster consisting of an exclusive read-with-write-forthcoming request, followed by a write request for the current cache line.