Register Window - Abstract

Abstract

Most CPU designs include a small amount of very high-speed memory known as registers. Registers are used by the CPU in order to hold temporary values while working on longer strings of instructions. Considerable performance can be added to a design with more registers. However, since the registers are a visible piece of the CPU's instruction set, the number cannot typically be changed after the design has been released.

While registers are almost a universal solution to performance, they do have a drawback. Different parts of a computer program all use their own temporary values, and therefore compete for the use of the registers. Since a good understanding of the nature of program flow at runtime is very difficult, there is no easy way for the developer to know in advance how many registers they should use, and how many to leave aside for other parts of the program. In general these sorts of considerations are ignored, and the developers, and more likely, the compilers they use, attempt to use all the registers visible to them. In the case of processors with very few registers to begin with, this is also the only reasonable course of action.