Define write-through and write-back policy

Write-through: All write operations are made to main memory as well as to the cache, ensuring that main memory is always valid. The simplest technique is called write through. Any other processor-cache module can monitor traffic to main memory to maintain consistency within its own cache. The main disadvantage of this technique is that it generates substantial memory traffic and may create a bottleneck. 

Write-back: An alternative technique, known as write back, minimizes memory writes with write back; updates are made only in the cache. Wan an update, a dirty bit or use bit, associated with the line is set. Then, when a block is replaced, it is written back to main memory if and only if the bit is set. The problem with write back is that portions of main memory are invalid and hence accesses by I/O modules can be allowed only through the each. This makes for complex circuitry and potential bottleneck.