At the top of the hierarchy there will be a small memory, perhaps only a few KB, built from the fastest chips. The bottom of the hierarchy will be the largest but slowest memory. The processor will be connected to the top of the hierarchy, i.e. when it fetches an instruction it will send its request to the small, fast memory. If this memory contains the requested item, it will respond, and the request is satisfied. If a memory does not have an item, it forwards the request to the next lower level in the hierarchy.
Some systems have separate caches for instructions and data, in which case it has a split cache .
An instruction buffer is a special cache for instructions that also performs other functions that make fetching instructions more efficient. The main memory is known as the primary memory . The low end of the hierarchy is the secondary memory . It is often implemented by a disk, which may or may not be dedicated to this purpose. The unit of information transferred between items in the hierarchy is a block .