Programmers’ theoretical minimum: do you never hear “register window”?
Call a function is expensive because an amount of resources available need to be reserved to properly handling the call and the return values.
This mechanism is described through the concept of activation record. The caller requires spaces on the stack to pass the parameters to the called function, the return value and the program counter.
Due the fact that the stack is a limited and precious resource, the SPARC architecture has a set of registers solely dedicated to holding parameters in procedure activation. This is called register window.
Expert C Programming: Deep C Secrets by Peter van der Linden