Question

4. Answer all of the following: (a) What information is stored in activation records? (b) Which of these information items are only in block-structured pro- gramming languages and why? (c) Name two programming languages where activation records are usu- ally stored on the stack and two where the activation records are usu ally on the heap. Page 2 of 7 Programming Languages Autumn 2012 (d) Describe why, in the latter two programming languages, it is necessary for the activation records to be stored on the heap.

Answer 1

As per the HomeworkLib policy on multiple questions, answer to first question is being given here:

Activation Records (AR):

To understand AR, let us first understand the call stack. A call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack". Although maintenance of the call stack is important for the proper functioning of most software, the details are normally hidden and automatic in high-level programming languages. Many computer instruction sets provide special instructions for manipulating stacks.

A call stack is composed of stack frames (also called activation records or activation frames). These are machine dependent and ABI-dependent data structures containing subroutine state information. This data is sometimes referred to as CFI (Call Frame Information).^] Each stack frame corresponds to a call to a subroutine which has not yet terminated with a return. For example, if a subroutine named DrawLine is currently running, having been called by a subroutine DrawSquare, the top part of the call stack might be laid out like in the following picture.

Stack Pointer top of stack Locals of DrawLine stack frame for DrawLine subroutine Frame Pointer一一 → Return Address Param eters for DrawLine Locals of DrawSquare Return Address Param eters for DrawSquare stack frame for DrawSquare subroutine

Fig: Call stack layout for upward-growing stacks

A diagram like this can be drawn in either direction as long as the placement of the top, and so direction of stack growth, is understood. Furthermore, independently of this, architectures differ as to whether call stacks grow towards higher addresses or towards lower addresses. The logic of the diagram is independent of the addressing choice.

The stack frame at the top of the stack is for the currently executing routine. The stack frame usually includes at least the following items (in push order):

• the arguments (parameter values) passed to the routine (if any);
• the return address back to the routine's caller (e.g. in the DrawLine stack frame, an address into DrawSquare's code); and
• space for the local variables of the routine (if any).