Q. Use of Overlapped Register Windows?

Register file comprises 138 registers. Let them be called by register number 0 - 137. 

Diagram demonstrates the use of registers: when there is call to function A(f_A) that calls function B (f_B) and function B calls function C (f_C).

Figure: Use of three Overlapped Register Windows

Please note the functioning of registers: at any instance of time global registers and only one window of registers is visible and is addressable as if it were only set of registers. So for programming purpose there can be only 32 registers. Window in the above illustration though has a total of 138 registers. This window comprises: 

• Global registers that are shareable by all functions.
• Parameters registers for holding parameters passed from previous function to current function. They also hold results which are to be passed back.
• Local registers which hold the local variables as assigned by compiler.
• Temporary registers: They are physically same as the parameter registers at subsequent level. This overlap allows parameter passing without actual movement of data.