If a procedure references variables local to its containing procedure, the inner procedure must be given a designator to the outer procedure's stack frame for it to be able to access the outer procedure's variables. The designator to the outer procedure's stack frame is the stack frame designator.
Except when the outer procedure has been activated recursively, only one stack frame for the outer procedure exists. The existing stack frame is the one that is always used when the inner procedure references variables declared in the outer procedure.
If the outer procedure has been activated recursively, the entry value used to call the inner procedure determines which stack frame is to be used when the inner procedure references the outer procedure's variables. For example:
A: PROCEDURE RECURSIVE;
DECLARE X FIXED BINARY(15);
DECLARE E ENTRY VARIABLE STATIC;
IF FIRST-TIME
THEN E=B;
ELSE CALL F;
CALL G;
.
.
.
B: PROCEDURE;
.
.
.
X=5;
In this example, assume that A calls G and that G calls A, then A calls F, and F calls the entry value held by E. When E is called, an activation of B results, because B was assigned to E. When that activation of B references X, two stack frames containing an instance of X exist. Two stack frames exist because two calls of A are still active. However, because the first of these two activations of A assigned B to E, E contains a designator to the first stack frame of A. Consequently, B assigns 5 to the first instance of X. If B were called directly from A, B would always use the most recent stack frame of A when referencing X.
An older stack frame is used only when an entry name is assigned to an entry variable or passed as an argument to an entry parameter. When the entry name is thus passed or assigned, the current stack frame of its containing block is assigned as the second component of the entry value produced by that assignment.
If the procedure name being assigned or passed as an argument is not declared within the block that is assigning or passing it, but is declared in a containing block, the containing block's stack frame is found using the same principle as was used to find X in the previous example. Once found, a designator to that stack frame is assigned as part two of the entry value.
Each stack frame for a block A has a designator to a stack frame of the block that contains A. That designator is the designator that was supplied as the second part of the entry value used to activate A. Unless the activation of A was the result of calling an entry variable or entry parameter, and unless recursion has been used, the stack frame thus designated is also the immediately preceding stack frame.