11.2. Activation Record.   

When a procedure calls another procedure (even itself) the state of that procedure must be saved = its activation environment must be saved. In a structured language the space for this is taken from a stack so that the runtime stack may be composed of many activation records as procedures call other procedures. 

Remember that not just the return address is put on the stack

with recursion or nesting of procedures and hence deep

recursion can quickly cause a STACK OVERFLOW.

When the procedure returns, the stored activation record of the calling procedure is "reloaded" and execution continues from that point indicated by the INSTRUCTION POINTER.

The data frame is also reclaimed from the stack. 


   -------------

(A) Activation     (1) Pointer to current activation

    Environment        environment is at A.

    of current     (2) When procedure (2) is called, pointer

    procedure          moves to B and new environment is stored

   -------------   (3) When a return is executed the pointer

(B) Activation         at B returns the pointer to (A) and

    Environment        "dead" environment.

   -------------

                         Compare this with PL0.

                         What does a call do?

                         What does an OPR, 0, 0 do?


CALL:

          S[T+1]  BASE(L);

          S[T+2]  B;

          S[T+3]  P;

          B  T+1;

          P  a;


OPR 0,0:

          t  b-1;

          P  S[t+3];

          b  s[t+2];

Notice for the call that the top-of-stack pointer is not changed. Surely this is an error otherwise the first LIT or LOD will destroy the BASE(L) or B or P?
What is the answer?

How does RETURN reclaim the stack data frame? 

      Summary  of

               actions for a procedure activation


CALL:

 (1) Allocate space for data frame (activation record) of

     called procedure/function.

 (2)  Process parameters appropriately and place in reserved

     location in data frame (or static memory).

 (3)  Set up pointers to other environments to allow access to

     other data frames of textually enclosing

     blocks/procedures/functions.

 (4)  Save current environment and create new values for new

     activation.

 (5)  Stop execution of current procedure and start new.

RETURN:

 (1) STORE result in some place if function.

 (2)  "Reload" calling environment.

 (3)  Stop execution of called procedure and resume execution

     of calling procedure.