title: For Write-Back Cache Policy, why data should first be read from memory, before writing to cache?
body: <p>Caches with Write Back Cache, perform write operations to the cache memory and return immediately. This is only when the data is already present in the cache. If the data is not present in the cache, it is first fetched from the lower memories, and then written in the cache.</p>
  <p>I do not understand why it is important to first fetch the data from the memory, before writing it. If the data is to be written, it will become invalid anyways.</p>
  <p>I do know the basic concept, but want to know the reason behind having to read data before writing to the address.</p>
  <p>I have the following guess,</p>
  <p>This is done for Cache Coherency, in a multi-processor environment. Other processors snoop on the bus to maintain Cache Coherency. The processor writing on the address needs to gain an exclusive access, and other processors must find out about this.
  But, does that mean, this is not required on Single-Processor computers?</p>