Cache-Memory Coherence

Figure 6: Write-Through vs. Write-Back.

• In a single cache system, coherence between memory and the cache is maintained using one of two policies:
  1. write-through,
  2. write-back (see Fig. 6).
• When a task running on a processor P requests the data in memory location X, for example, the contents of X are copied to the cache, where it is passed on to P.
• When P updates the value of X in the cache, the other copy in memory also needs to be updated in order to maintain consistency.
• In write-through, the memory is updated every time the cache is updated,
• while in write-back, the memory is updated only when the block in the cache is being replaced.