Uses of Call Stack
Call site processing
Usually the call stack manipulation needed at the site of a call to a subroutine is minimal (which is good since there can be many call sites for each subroutine to be called). The values for the actual arguments are evaluated at the call site, since they are specific to the particular call, and either pushed onto the stack or placed into registers, as determined by the calling convention being used. The actual call instruction, such as "Branch and Link," is then typically executed to transfer control to the code of the target subroutine.
Callee processing
In the called subroutine, the first code executed is usually termed the subroutine prologue, since it does the necessary housekeeping before the code for the statements of the routine is begun. The prologue will commonly save the return address left in a register by the call instruction by pushing the value onto the call stack. Similarly, the current stack pointer and/or frame pointer values may be pushed. Alternatively, some instruction set architectures automatically provide comparable functionality as part of the action of the call instruction itself, and in such an environment the prologue need not do this.
Return processing
When a subroutine is ready to return, it executes an epilogue analogous to the prologue. This will typically restore saved register values (such as the frame pointer value) from the stack frame, pop the entire stack frame off the stack by changing the stack pointer value, and finally branch to the instruction at the return address. Under many calling conventions the items popped off the stack by the epilogue include the original argument values, in which case there usually is no further processing back in the caller. With some conventions, however, it is the caller's responsibility to remove the arguments from the stack after the return.
