SPARC processors use register windows. Each register window is comprised of 8 in registers, 8 local registers, and 8 out registers (which are the in registers of the next window). There are also 8 global registers. The number of register windows ranges from 2 to 32, depending on the processor implementation.
Because drivers are normally written in C, the compiler usually hides the fact that register windows are used. However, it may be necessary to use them when debugging the driver. See "Debugging Tools" for more information on how register windows are used when debugging.