Question

In Java

1. Discuss how Activation Records (stack frames in Java) work including how they help to pass parameters, return variables and save the state of the calling method.

Answer 1

Control stack or runtime stack is used to keep track of the live procedure activations i.e the procedures whose execution have not been completed. A procedure name is pushed on to the stack when it is called (activation begins) and it is popped when it returns (activation ends). Information needed by a single execution of a procedure is managed using an activation record or frame. When a procedure is called, an activation record is pushed into the stack and as soon as the control returns to the caller function the activation record is popped.

A general activation record consist of the following things:

• Local variables: hold the data that is local to the execution of the procedure.
• Temporary values: stores the values that arise in the evaluation of an expression.
• Machine status: holds the information about status of machine just before the function call.
• Access link (optional): refers to non-local data held in other activation records.
• Control link (optional): points to activation record of caller.
• Return value: used by the called procedure to return a value to calling procedure
• Actual parameters