Write-once (cache coherence)

In cache coherency protocol literature, Write-Once was the first write-invalidate protocol defined. It has the optimization of executing write-through on the first write and a write-back on all subsequent writes, reducing the overall bus traffic in consecutive writes to the computer memory. It was first described by James R. Goodman in (1983). Cache coherence protocols are an important issue in Symmetric multiprocessing systems, where each CPU maintains a cache of the memory.

States

In this protocol, each block in the local cache is in one of these four states:

Invalid: This block has an incoherent copy of the memory.

Valid: This block has a coherent copy of the memory. The data may be possibly shared, but its content is not modified.

Reserved: The block is the only copy of the memory, but it is still coherent. No write-back is needed if the block is replaced.

Dirty: The block is the only copy of the memory and it is incoherent. This copy was written one or more times. This is the only state that generates a write-back when the block is replaced in the cache.

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