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, Sun Microsystems SPARC, and Intel Itanium.
Context.
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.
Implementation.
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.
Application in CPUs.
In the Berkeley RISC design, only eight registers out of a total of 64 are visible to the programs. The complete set of registers are known as the register file, and any particular set of eight as a window. The file allows up to eight procedure calls to have their own register sets. As long as the program does not call down chains longer than eight calls deep, the registers never have to be "spilled", i.e. saved out to main memory or cache which is a slow process compared to register access.
By comparison, the Sun Microsystems SPARC architecture provides simultaneous visibility into four sets of eight registers each. Three sets of eight registers each are "windowed". Eight registers (i0 through i7) form the input registers to the current procedure level. Eight registers (L0 through L7) are local to the current procedure level, and eight registers (o0 through o7) are the outputs from the current procedure level to the next level called. When a procedure is called, the register window shifts by sixteen registers, hiding the old input registers and old local registers and making the old output registers the new input registers.
The common registers (old output registers and new input registers) are used for parameter passing. Finally, eight registers (g0 through g7) are globally visible to all procedure levels.
The AMD 29000 improved the design by allowing the windows to be of variable size, which helps utilization in the common case where fewer than eight registers are needed for a call. It also separated the registers into a global set of 64, and an additional 128 for the windows. Similarly, the IA-64 (Itanium) architecture used variable-sized windows, with 32 global registers and 96 for the windows.
In the Infineon C166 architecture, most registers are simply locations in internal RAM which have the additional property of being accessible as registers. Of these, the addresses of the 16 general-purpose registers (R0-R15) are not fixed. Instead, the R0 register is located at the address pointed to by the "Context Pointer" (CP) register, and the remaining 15 registers follow sequentially thereafter.
Register windows also provide an easy upgrade path. Since the additional registers are invisible to the programs, additional windows can be added at any time. For instance, the use of object-oriented programming often results in a greater number of "smaller" calls, which can be accommodated by increasing the windows from eight to sixteen for instance. This was the approach used in the SPARC, which has included more register windows with newer generations of the architecture. The end result is fewer slow register window "spill" and "fill" operations because the register windows overflow less often.
Criticism.
Register windows are not the only way to improve register performance. The group at Stanford University designing the MIPS saw the Berkeley work and decided that the problem was not a shortage of registers, but poor utilization of the existing ones. They instead invested more time in their compiler's register allocation, making sure it wisely used the larger set available in MIPS. This resulted in reduced complexity of the chip, with one half the total number of registers, while offering potentially higher performance in those cases where a single procedure could make use of the larger visible register space. In the end, with modern compilers, MIPS makes better use of its register space even during procedure calls.