SystemVerilog Associative Arrays: Basics and Beyond

Associative arrays are an integral part of SystemVerilog and are widely used in various applications. In this blog post, we will dive deep into associative arrays, learn their basics, and explore some practical examples to fully understand how they work.

Introduction to Associative Arrays

What are Associative Arrays?

Associative arrays are a collection of elements indexed by a unique key, which could be a string, integer, or any other data type. These arrays do not have any fixed size and can grow dynamically as needed.

Comparison between Dynamic Arrays and Associative Arrays

Dynamic arrays are indexed using consecutive integers, while associative arrays use unique keys as indices. Dynamic arrays have a size limit and need to be resized, whereas associative arrays can grow as required.

Declaration of Associative Arrays

Associative arrays can be declared using the [*] or [type] syntax. Here's an example:

real        myArray1[*];               // Associative array of real data type (unspecified index)
logic [7:0] myArray2[string];  // Associative array of 8-bit logic, indexed by string
event       myArray3[myClass];       // Associative array of event indexed by class myClass

Accessing Invalid Indices and Allocating Associative Array Elements

Handling Invalid Indices

If you try to access an invalid index in an associative array, SystemVerilog does not throw an error but returns a default value for the indexed data type. For example, an invalid index for an associative array of integers will return 0.

Allocating Elements in Associative Arrays

Adding elements to an associative array is simple. Just assign a value to an index:

myArray["apple"] = 5; // Adding an element to the associative array

Exploring Built-in Associative Array Methods

Num() and size()

These methods return the number of elements in an associative array:

int count = myArray.size();

Delete()

This method removes an element from the associative array:

myArray.delete("apple"); // Deleting an element from the associative array

Exists()

This method checks if an element exists in the associative array:

if (myArray.exists("apple")) begin
  // do something
end

First(), Last(), Next(), and Prev()

These methods allow you to traverse the associative array. Here's an example:

string key;
int value;
for (key = myArray.first(); key != ""; key = myArray.next(key)) begin
  value = myArray[key];
  // do something with the key-value pair
end

Associative Array Assignments, Arguments, and Literals

Assigning Associative Arrays

To copy an associative array to another, simply assign it:

int myNewArray[string];
myNewArray = myArray; // Copying associative arrays

Passing Associative Arrays as Arguments

To pass associative arrays as arguments to tasks or functions, use the input and output keywords:

function int SumElements(input int arrayArg[string]);
  int sum = 0;
  string key;
  for (key = arrayArg.first(); key != ""; key = arrayArg.next(key)) begin
    sum += arrayArg[key];
  end
  return sum;
endfunction

task main;
  int total = SumElements(myArray);
endtask

Associative Array Literals

You can declare and initialize an associative array using the {} syntax:

int myLiteralArray[string] = '{"apple": 5, "banana": 3};

Practical Examples and Common Use Cases

Example: Simple Gradebook System

In this example, we will create a simple gradebook system using associative arrays to store student grades.

module Gradebook;
  string students[] = {"Alice", "Bob", "Charlie", "Diana"};
  int grades[string];

  initial begin
    initializeGrades(students, grades);
    printGrades(grades);
  end

  function void initializeGrades(input string studentList[], output int grades[string]);
    foreach (studentList[i]) begin
      // Assign random grades between 60 and 100 to each student
      grades[studentList[i]] = $urandom_range(60, 100);
    end
  endfunction

  function void printGrades(input int grades[string]);
    string key;
    for (key = grades.first(); key != ""; key = grades.next(key)) begin
      $display("%s: %d", key, grades[key]);
    end
  endfunction
endmodule

In this example, we use an associative array grades to store the grade of each student. The initializeGrades function assigns random grades to each student in the associative array. The printGrades function displays the grade of each student.

In conclusion, associative arrays in SystemVerilog offer a powerful and versatile data structure for managing and organizing data in various applications. Through the use of practical examples, we have demonstrated the ease and effectiveness of associative arrays in real-life scenarios. By mastering the usage of associative arrays, you can greatly improve the efficiency, readability, and performance of your SystemVerilog code, enabling you to tackle more complex problems with ease.