Computer Architecture
The Anatomy of Modern Processors

The Memory Hierarchy

Processors use memory of various types to store the data on which a program operates. Data may start in a file on a magnetic disc ("hard disc"), be read into semiconductor memory ("D-RAM") for processing, transformed and written back to disc. As part of the transformation process, individual words of data will be transferred to the processor's registers and thence to the ALU. Transfers from semiconductor memory to registers will usually pass (transparently to a programmer) through one or more levels of cache.

Memory designers are able to trade speed for capacity - the fastest memory (registers) having access times below 10ns but the lowest capacity (10s of words) and the slowest (magnetic tape) having access times of several seconds but the highest capacity (10s of GBytes). Thus the memory in a system can usually be arranged in a hierarchy from the slowest (and highest capacity) to the fastest (and lowest capacity).

Discrepancy between Processor and Bus Frequencies

This figure (source: David Chapman, Motorola) shows the ratio between processor clock frequence ("Core Frequency") and bus frequency for the last dozen years. It can be seen that the ratio has started to increase dramatically recently - as processor frequencies have continued to increase and bus frequencies have remained fixed or increased at a slower rate. The bus frequency determines the rate at which information can be transferred between a processor and its bulk memory.

The widening gap between processor and bus frequencies shows that effective management of this memory hierarchy is now critical to performing, posing a challenge for architects and programmers alike. In the following sections, we'll look at the technology, organisation and characteristics of memories at each level of the hierarchy.

Operation of the memory hierarchy

This diagram shows how a high performance CPU processes a memory request issued by the CPU:

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© John Morris, 1998