The first technology is selected for fast access time and necessarily has a high per-bit cost. Relatively little of the memory system consists of this technology. The second technology is selected for low per-bit cost and necessarily has a slow access time. The bulk of the memory system consists of this technology. The use of the hierarchy is coordinated by user software, system software, or hardware so that the overall characteristics of the memory system approximate the fast access of the fast technology, and the low per-bit cost of the low cost technology.
Memory hierarchies can, of course, consist of three or more technologies.
Discussion and analysis of these multilevel hierarchies is a fairly obvious generalization of the discussion and analysis given here.