The following paragraphs give good insight on RISC design and its associated Register Window
Berkeley RISC added circuitry to the CPU to "help" the compiler by using the concept of register windows, in which the entire "register file" was broken down into blocks, allowing the compiler to "see" one block for global variables, and another for local variables.
The idea was to make one particularly common instruction, the procedure call, extremely easy to implement in the compilers. Almost all computer languages use a system known as an activation record or stack frame for each procedure—a modular unit of execution—that contains the address from which the procedure was called, the data (parameters) that were passed in, and space for any result values that need to be returned. In the vast majority of cases these frames are small, typically with three or fewer inputs and one or no outputs (and sometimes an input is reused as an output). In the Berkeley design, then, a register window was a set of several registers, enough of them that the entire procedure stack frame would most likely fit entirely within the register window.
In this case, the call into and return from a procedure is simple and extremely fast. A single instruction is called to set up a new block of registers—a new register window—and then, with operands passed into the procedure in the "low end" of the new window, the program jumps into the procedure. On return, the results are placed in the window at the same end, and the procedure exits. The register windows are set up to overlap at the ends, so that the results from the call simply "appear" in the window of the caller, with no data ( Function locals and parameters ) having to be copied. Thus the common procedure call does not have to interact with main memory, greatly accelerating it.
On the downside, this approach means that procedures with large numbers of local variables are problematic, and ones with fewer lead to registers—an expensive resource—being wasted. There are a finite number of register windows in the design, e.g. eight, so procedures can only be nested that many levels deep before the register windowing mechanism reach its limit; once the last window is reached, no new window can be set up for another nested call.
