Register window

In computer engineering, register windows are a feature in some instruction set architectures to improve the performance of procedure calls, a very common operation. Register windows were one of the main features of the Berkeley RISC design, which would later be commercialized as the AMD Am29000, Intel i960, and Sun Microsystems SPARC.

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.

Register windows aim to solve this issue. Since every part of a program wants registers for its own use, several sets of registers are provided for the different parts of the program. If these registers were visible, there would be more registers to compete over, i.e. they have to be made invisible.

Rendering the registers invisible can be implemented efficiently; the CPU recognizes the movement from one part of the program to another during a procedure call. It is accomplished by one of a small number of instructions (prologue) and ends with one of a similarly small set (epilogue). In the Berkeley design, these calls would cause a new set of registers to be "swapped in" at that point, or marked as "dead" (or "reusable") when the call ends.

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