As the nesting depth of procedure calls increases, there may not be sufficient register to accommodate the new procedure. In this case, the information of oldest procedure is stored back into memory and the saved window pointer keep tracks of the most recently saved window.

It is clear that N-Window register file can hold only N-1 procedure activations. The value of N need not be very large, because in general, the depth of procedure activation is small. In case of recursive call the depth of procedure call may increase. From survery, it is found that with 8 windows, a save or restore is needed on only 1% of the calls or returns.

Global Variables

The window scheme provides an efficient organization for storing local scalar variables in registers. Global variables are accessed by more than one procedure.

Two solutions to access the global variables:

1. Variables declared as global in an HLL can be assigned memory location by the compiler, and all machine instructions that reference these variables will use memory reference operands. This scheme is inefficient for frequently accessed global variables.
2. An alternative is to incorporate a set of global registers in the processor. These registers would be fixed in number and available to all procedures. In this case, the compiler must decide which global variables should be assigned to registers.