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Cache attribute write back/write allocate for Cortex-M4

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What is different between write back with write allocate and write with non write allocate on Cortex-M4.

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    Cache policies:

    Allocation policy

    A read allocate policy allocates a cache line only on a read.

    A write allocate policy allocates a cache line for either a read or write which misses in the cache (and so might more accurately be called a read-write cache allocate policy). For both memory reads which miss in the cache and memory writes which miss in the cache, a cache linefill is performed. This is typically used in combination with a write-back write policy on current ARM processors

    Write policy

    Write-through. With this policy writes are performed to both the cache and main memory.

    Write-back. In this case, writes are performed only to the cache, and not to main memory.(When a write happens which updates the cache, but not main memory, the dirty bit is set. If the cache later evicts a cache line whose dirty bit is set (a dirty line), it writes the line out to main memory.)

    Conclusion: write-back with write allocate - write to cache and not to main memory, later when cache eviction happens - main memory will be updated with data from evicted cache line.

    write-through with write allocate - write to both the cache and main memory (performance of processor is degraded because of wasting time to wait write operations)

    write with non write allocate - write directly to main memory

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    Cache policies:

    Allocation policy

    A read allocate policy allocates a cache line only on a read.

    A write allocate policy allocates a cache line for either a read or write which misses in the cache (and so might more accurately be called a read-write cache allocate policy). For both memory reads which miss in the cache and memory writes which miss in the cache, a cache linefill is performed. This is typically used in combination with a write-back write policy on current ARM processors

    Write policy

    Write-through. With this policy writes are performed to both the cache and main memory.

    Write-back. In this case, writes are performed only to the cache, and not to main memory.(When a write happens which updates the cache, but not main memory, the dirty bit is set. If the cache later evicts a cache line whose dirty bit is set (a dirty line), it writes the line out to main memory.)

    Conclusion: write-back with write allocate - write to cache and not to main memory, later when cache eviction happens - main memory will be updated with data from evicted cache line.

    write-through with write allocate - write to both the cache and main memory (performance of processor is degraded because of wasting time to wait write operations)

    write with non write allocate - write directly to main memory

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