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If the first processor 102A then modifies the cache line of data in its cache 108A, then at that instant the corresponding memory line becomes stale.If a second processor, 102B for example, subsequently accesses the original data in the main memory 104, the second processor 102B will not find the most current version of the data because the most current version is in the cache 108A.For each cache-line address, cache coherence guarantees that only one copy of data in cache 108 can be modified.Identical copies of a cache line may be present in multiple caches 108, and thus be read by multiple processors 102 at the same time, but only one processor 102 is allowed to write, i.e., modify, the data.
Since this is a read request, memory 104 changes from the HOME state to the FRESH state, and the resulting after list 402AR is a read-only list with one entry 108A. 4B processor 102B requests a read permission to enter the read-only list 402B, which is the same list as 402AR of FIG. Cache 108B then becomes the head of the list 402BR receiving data line from head 108A.
The second type list is a read-write (sometimes called “owned”) list of caches for which the head-of-list processor 102 may have permission to write to its cache 108.
A list is considered “stable” after an entry has been completely entered into or completely deleted from the list.
The processors 102A, 102C, and 102N with invalidated cache data D0 in their respective caches 108 must fetch the updated version of cache line D1 if they want to access that data line again.
Normally and for illustrative purposes in the following discussion, cache coherence protocols are executed by processors 102 associated with their related caches. Memory 104 has a pointer which always points to the head (cache 108A) of the list while the forward pointers Af, Bf, and Cf of caches 108A, 108B, and 108C respectively point forward to the succeeding caches 108B, 108C, and 108D (not shown).
A read-write processor seeking to update a cache line requires updating the list from read-only to read-write. 09/235,588, entitled “System and Method for Deleting Read-Only Head Entries in Multi-Processor Computer Systems Supporting Cache Coherence with Mixed Protocols,” filed on Jan. Cache 108 for each processor 102 is used to hold data that was accessed recently by that processor.