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Table of contents
1.
Introduction
2.
Compiler Design 
3.
Activation Records
4.
Example
4.1.
Implementation
4.1.1.
Output
4.2.
Explanation
5.
Components of Activation Records
5.1.
Return Address
5.2.
Parameters
5.3.
Control Link
5.4.
Access Link
5.5.
Saved Machine Status
5.6.
Local Data
5.7.
Temporaries
6.
Features of the Activation Record
7.
Caller and Callee
7.1.
Caller
7.2.
Callee
8.
Relation between Function call, Stack Call or Call stack, and Activation Record
8.1.
Function Call
8.2.
Activation Record
8.3.
Stack Call
8.4.
Relationship between Function Call, Stack Call, and Activation Record
9.
Difference between Activation Record and Call Stack
10.
Activation Record Organization and Structure
10.1.
Return address
10.2.
Parameters
10.3.
Local Variables
10.4.
Control Link
11.
Activation Records and Debugging Tools
12.
Memory Management in Activation Records
13.
Frequently Asked Questions
13.1.
What are the advantages and disadvantages of activation records in compiler design?
13.2.
What is the primary purpose of the return address in an activation record?
13.3.
What are stored in each activation record?
13.4.
What happens if a function call exceeds the available stack space for the activation record?
13.5.
What is the role of stack pointers in an activation record?
14.
Conclusion
Last Updated: Mar 27, 2024
Medium

Activation Record in Compiler Design

Author Rinki Deka
1 upvote
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Ashwin Goyal
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Introduction

Hey Ninjas, Have you ever wondered how programming languages manage function calls and store their variables effectively? How is there a seamless program flow and memory management during the execution of a function? The answer to all these questions is the activation record in compiler design.  The driver program uses activation records to execute a function. Its main aim is to manage the critical information required for the execution of a code, like variables, parameters, or addresses. Therefore it is necessary to understand the design and its usage of activation records in compiler design.

activation record in compiler design

To better understand the concept of activation record in compiler design, let’s first understand the basic idea of compiler design. 

Compiler Design 

A compiler is a computer program that takes the code from a developer (High-level language) and converts it into language that the computer understands and executes(machine learning language). It does not change the meaning of the code.  The compiler generates highly efficient code and uses the computer’s memory effectively.

compiler design diagram


Different steps involved in compiler design are lexical analysis, syntax analysis, semantic analysis, code generation, and optimization. 

Different compiler features make it compelling and unique, such as accuracy, compilation speed, maintaining the meaning of the code throughout, increasing the speed of the generated code, quickly detecting and handling errors, and many more. There are three types of compilers; 

  • The single-pass compiler passes the source code directly into the machine code in a single pass, for example, the Pascal language.
     
  • The Two-pass compiler reads the source code two times, first to understand its functionality and second to ensure its accuracy and effectiveness.
     
  • The Multipass Compiler processes the source code multiple times. It helps us to divide large programs into numerous small programs. 
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Activation Records

When a procedure gets called, the computer creates an activation record to store all the information needed to execute that procedure. This information includes the procedure's arguments, local variables, and return address. When the operation finishes executing, the computer deletes the activation record. Activation record is also known as stack frames or function call frames used by the compiler to manage the execution of a function or procedure.

activation records

Imagine a scenario where you have a program with multiple functions. A new activation function gets created whenever one of the functions gets called. The activation record is stored on the control stack whenever a process gets executed. The control stack is a runtime stack used to track live procedure activations. Its primary purpose is to determine which execution still needs to be completed. As the activation begins, the procedure name is pushed into the stack and will pop out as the activation ends. If there is a recursive procedure, then several activations are active at the same time. If there is a non-recursive procedure, one activation of the function is executed simultaneously.

activation record

We can also determine the size of all the activation records during the compile time. In the FORTRAN compiler, the static area holds all the activation records. When an executed called procedure completes, it will return control to the caller. When a procedure is called and executed, it eventually finishes its task and returns control to the part of the program called it. An activation tree resembling a tree-like structure represents the control flow. The activation tree depicts the sequence of procedure calls and their relationships. 

Example

Consider a simple example to understand the activation record concept better. Here in this code, we have taken a function named "addition" that returns the result of adding two numbers.

Implementation

#include <stdio.h>

// function to perform addition 
void addition(int a, int b) {
    int result = a + b;
    printf("Result: %d\n", result);
}

//main program
int main() {
    int x = 2;
    int y = 8;
    addition(x, y);
    return 0;
}

 

Output

output

Explanation

The 'main' function is the program's starting point. It consists of local variables like 'x' and 'y'. When the 'addition' function gets called from the main function, a new activation record gets created for the 'addition' function. The activation record of 'addition' is initialized with parameters with 'a' and 'b,' which have (2 and 8) values from the calling function. The result gets stored with the value of 10. Once the 'addition' function work is completed and reaches its end, then activation records are removed from the calling stack. Finally, when the 'main' function terminates, its activation record is removed from the stack. 

Components of Activation Records

There are different components of the activation record in compiler design:

components of activation records

Return Address

This address holds the location where the control should return upon task completion. This feature helps the program continue executing from the same point it was initially created. It is used by a calling function that will return a value to the same calling function.

Parameters

The calling procedure uses it to supply parameters to the called method. It stores actual parameters used to send input to the called system. The parameters can be passed by value or reference and stored in the activation record for the function to access.

Control Link

It points toward the activation record of the caller. It allows you to return and execute continuously. The system uses it to store information outside the local scope. The control link connects the activation record to the activation record of the caller

control link in activation records

Access Link

It stores the address of the activation record of the caller function.

access link

Saved Machine Status

The activation record consists of critical information about the program's state, which is just about to get called. It stores information like the return address or machine registers. The saved machine ensures the program can resume execution once the procedure call gets terminated.

Local Data

This field consists of local data for a particular function's execution. Local data consists of variables that serve the purpose of quick calculations or storing specific values of a currently used function.

Temporaries

It refers to the variables or storage locations used to store intermediate values within the procedure. When a function executes, it may perform different operations that require temporary storage. Once the procedure call completes and the control returns to the calling code, the system deallocates activation records and releases temporaries.

Features of the Activation Record

There are many features of activation records. Some of them are:

  • It follows a stack-like structure.
  • It helps in preventing return addresses for the control flow.
  • It has control and access links which can be used in nested functions calling.
  • It helps in storing specific data like parameters, local variables, etc.

Caller and Callee

While discussing the functional calls and activation records, the two terms "callee" and "caller" are used. These describe the relationship between the two functions involved during the execution flow.

caller and callee

Caller

The caller function is responsible for invoking another function which is the callee. The caller starts the function call by controlling the callee and providing necessary arguments or parameters.

Callee

The callee function gets executed when the caller function calls it. It receives control from the caller, performs its tasks, and returns a value to the caller.

The activation records facilitate the interaction between caller and callee records. It allows a good flow of communication and control, helps pass the parameters, and many more.

Relation between Function call, Stack Call or Call stack, and Activation Record

Function Call

When a function executes and subsequently calls another function, then this is known as calling a function. A function call involves transferring control from the caller to the callee by passing arguments.

Activation Record

stack frame is another term used to refer to an activation record. Compilers and runtime systems have used it to organize the execution of the functions. Activation records handle memory allocation and deallocation on the call stack.

Stack Call

It is a data structure that stores and manages function calls and activation records. It pushes the activation record on the stack and pops it out once completed.

Relationship between Function Call, Stack Call, and Activation Record

The activation record is created and pushed onto the call stack when a function gets called. The activation stores all the necessary information for that calling function, like variables and values. The stack call ensures that the control flow is working correctly or not. It checks the flow of memory allocation and deallocation. The activation record at the top of the stack represents the currently executing function. The activation record pops out of the call stack when the function call gets completed. The two most important parts of the program's control flow are an activation record and a call stack. Activation records store information, while the call stack organizes the called functions. 

Difference between Activation Record and Call Stack

The difference between the activation record and the call stack is:

Activation Record Call Stack
It stores specific information. It keeps track of the sequence of the function call.
Stores information like variables, parameters, return addresses, and many more. It manages the sequence of function calls.
Each function has its activation record. It consists of multiple activation calls stacked on top of each other.
It gets created when a function is called and removed from the memory when it completes its execution. The function is pushed onto the stack when it begins its execution and pops out when it gets completed.

Activation Record Organization and Structure

Activation record organization refers to how multiple activation records are arranged and managed during the execution of a program. Activation records are organized in a stack-like structure known as a call stack. New activation records are created and added to the top of the stack whenever a function gets called. The design of the activation record consists of different components:

Return address

The activation record's return address indicates where the control should resume execution after completing the task. This mechanism ensures that the program continues running from where it started.

Parameters

It holds the parameters required by the function or procedure during execution. The activation record stores the parameters passed to the function, allowing it to access them by value or reference.

Local Variables

Activation records also reserve memory space for local variables used within the function or procedure. 

Control Link

It points to the activation record of the caller.

The structure and organization manner depends on the programming language the developer uses. The activation record structure ensures that there is proper storage and control is there when the function gets executed.

Activation Records and Debugging Tools

Activation records, or stack frames, store information like variables, parameters, return addresses, and many more. The call stack organizes activation records in a stack-like structure in compiler design. Activation functions are placed in the stack when a function is called and pops out when it gets terminated.

Debugging tools are software designed to identify and fix errors that occur in the execution of the program. It helps the developer to monitor the execution of the program, specify the error and mark the point where the error has occurred.

The relationship between activation records and debugging is interconnected. During debugging, developers can use debugging tools to analyze the activation records at different points in the program execution. Developers can identify the flow of execution through various activation records and identify the point where the error has occurred or the sequence of function calls. 

Memory Management in Activation Records

When a function invokes a call, an activation record gets created to store the variables and other data that are local to that function. The function returns and destroys the activation record, releasing the memory it uses. Activation records play an important role in memory management because they help track which memory is being used by which functions. It helps to prevent memory leaks and other problems.

When a function invokes a call, it creates activation records and assigns memory for its local variables, providing each function with its own separate memory space for storing variables.

  • Activation records also allocate memory to the temporary data during the execution of a function. 
     
  • The call stack or runtime stack organizes the activation record in a stack-like structure. As functions get invoked, they create activation records pushed into the call stack and assign memory to variables within the activation memory. It is very effective for memory management and automatically gets deallocated. 
     
  • Apart from stack-based memory management, we can also use dynamic memory allocation for complex problems. It helps provide more flexibility to memory management using a data structure with variable sizes or needs a long time for processing.
     

Managing memory within activation records is essential for optimizing program performance and resource utilization. Activation records help programs run smoothly and reliably by ensuring that memory gets allocated and deallocated correctly. 

Frequently Asked Questions

What are the advantages and disadvantages of activation records in compiler design?

The main advantage of using an activation record is that it has very effective memory management, and the main disadvantage of using an activation record is that it can lead to stack overflow situations or gives a slower performance in some cases.

What is the primary purpose of the return address in an activation record?

The primary purpose of the return address in the activation record is to store the address to which the address must return after the execution of a function. It is beneficial in maintaining a proper control flow overall.

What are stored in each activation record?

In each activation record, return address, parameters, temporary variables, local variables, previous frame pointer, and space for bookkeeping purposes, like saved registers or exception handling information are typically stored.

What happens if a function call exceeds the available stack space for the activation record?

A function call exceeding the available stack space will lead to a stack overflow situation. It can lead to errors in code, terminate the code abruptly, or introduce other sorts of errors due to its insufficient stack space.

What is the role of stack pointers in an activation record?

The stack pointer in an activation record keeps track of the top of the stack. It enables us to determine the currently executing activation record. It helps in the allocation and deallocation of the memory.

Conclusion

In this article, we discover information stating that activation records store essential details such as variables, parameters, return addresses, and other relevant information. We also explore components associated with activation records in compiler design, like return address, parameters, control link, access link, saved machine status, local data, and temporaries. Activation Records are mainly stored in the call stack and help control the execution flow. We have also discussed the relationship between call stack, activation record in compiler design, and function call. This article explores various aspects of activation records in compiler design. We have also seen the critical difference between points of activation record and call stack in this article.

Do check out the link to learn more about such topic

Activation Tree in Compiler Design

Compiler Design

Phases of Compiler

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