Memory Hierarchy

The term memory hierarchy is used in computer architecture when discussing performance issues in computer architectural design, algorithm predictions, and the lower level programming constructs such as involving locality of reference. A 'memory hierarchy' in computer storage distinguishes each level in the 'hierarchy' by response time. Since response time, complexity, and capacity are related, the levels may also be distinguished by the controlling technology.

The many trade-offs in designing for high performance will include the structure of the memory hierarchy, i.e. the size and technology of each component. So the various components can be viewed as forming a hierarchy of memories (m₁,m₂,...,m_n) in which each member m_i is in a sense subordinate to the next highest member m_i-1 of the hierarchy. To limit waiting by higher levels, a lower level will respond by filling a buffer and then signaling to activate the transfer.

There are four major storage levels.

1. Internal – Processor registers and cache.
2. Main – the system RAM and controller cards.
3. On-line mass storage – Secondary storage.
4. Off-line bulk storage – Tertiary and Off-line storage.

This is a most general memory hierarchy structuring. Many other structures are useful. For example, a paging algorithm may be considered as a level for virtual memory when designing a computer architecture.