Queue

What is a Queue? #

A queue in SystemVerilog is a variable-size, ordered collection of homogeneous elements (i.e., all of the same type). It is a single-dimensional unpacked array that can grow or shrink dynamically during simulation.

Unlike fixed-size arrays, queues do not require a predefined upper limit, making them highly useful for scenarios where data sizes vary.

Syntax:

data_type queue_name[$];         // Unbounded queue

Why Use Queues? #

Queues offer two major advantages in verification:

1. Dynamic Size #

• A queue can hold zero or more elements at any time.
• You don’t need to define an artificial limit like in fixed-size arrays.
• Ideal for dynamic data storage such as:
  • Transaction lists
  • Packets of unknown size
  • Temporary buffers

2. Ordered Access & Flexible Behavior #

• Can emulate both:
  • FIFO behavior using push_back() and pop_front()
  • LIFO behavior using push_back() and pop_back()
• Still allows direct index access like a regular array.

Example: Declaring and Using a Queue

module test;
  int q[$];  // Declare an unbounded queue
  int val;   
  initial begin
    q.push_back(10);        // q = {10}
    q.push_front(5);        // q = {5, 10}
    q.push_back(20);        // q = {5, 10, 20}

    $display("Queue: %p", q);               
    $display("First: %0d", q[0]);         
    $display("Last:  %0d", q[$]);           

    val = q.pop_front();                    // Removes 5
    $display("After pop_front: %p", q);     

    q.delete();                             // Delete entire queue
    $display("Size after delete: %0d", q.size());  
  end
endmodule

Output:

Queue: '{5, 10, 20} 
First: 5
Last:  20
After pop_front: '{10, 20} 
Size after delete: 0

Built In Methods

When to Use Queues #

Use a queue when:

• You need flexible data structures that grow/shrink during simulation
• You’re implementing FIFO/LIFO transaction handling
• You require random access with constant time performance
• You want to simulate hardware buffers, transaction pools, or event queues