Question

Explain Goodman's write-once protocol for write-back cache with state diagram.​

Answer 1

Explanation:

In cache coherency protocol literature, Write-Once was the first MESI 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 [1] (1983). Cache coherence protocols are an important issue in Symmetric multiprocessing systems, where each CPU maintains a cache of the memory.

Answer 2

Answer:

Goodman's write-once protocol is a cache coherence protocol used in write-back caching systems. The protocol ensures that data is consistent and updated in a consistent manner across all caches in a multiprocessor system.

Explanation:

The state diagram for the write-once protocol consists of four states:

1. Invalid: This state indicates that the cache line is not present in the cache and must be fetched from memory.
2. Shared: This state indicates that the cache line is present in multiple caches and is read-only.
3. Exclusive: This state indicates that the cache line is present in a single cache and can be read or written.
4. Modified: This state indicates that the cache line has been modified and the updates must be written back to memory.

The write-once protocol operates as follows:

1. When a cache wants to write to a cache line, it first checks if the line is in the Exclusive or Modified state. If it is, the cache can write to the line.
2. If the cache line is in the Shared state, the cache sends a request to all other caches to invalidate their copies of the line. When all caches have invalidated their copies, the line becomes Exclusive and the cache can write to it.
3. If the cache line is in the Invalid state, the cache must first fetch the line from memory and then transition to the Exclusive state before writing to it.
4. When a cache wants to write back a modified line, it sends a request to memory to write the line back, and then transitions the line to the Shared state.

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