Although a processor's architecture defines the instructions it can execute, its microarchitecture determines the way in which those instructions are executed. Of all the choices made in a processor design flow, microarchitectural decisions will have the greatest impact on the processor's performance and die area. Chapter 4 discussed how changes in architecture affect performance, but it is possible to build multiple processors that all support the same architecture with enormously different levels of performance. The difference between such implementations is their microarchitectures.

By definition, architectural changes are visible to the programmer and require new software to be utilized. New architectural registers or SIMD instructions improve performance for programs that use them but will have no impact on legacy software. Microarchitectural changes are not visible to the programmer and can improve performance with no change in software.

Because microarchitectural changes maintain software compatibility, processor microarchitectures have changed much more quickly than architectures. Like almost all other changes in the semiconductor industry, Moore's law has driven this progress. The microarchitectural concepts in use today were not totally unknown to past designers; they were simply impractical. As Moore's law allows larger and larger transistor budgets, old microarchitectural ideas that were too complex to be implemented become possible.

Imagine writing a word processing program. The program should provide as much functionality and ease of use as possible, but must be limited to 10,000 lines of code. With this restriction, this word processor will be a very crude program indeed. If after two years,...