ch2-Queues

2019-10-24

SystemVerilog Queues

​ A SystemVerilog queue is a First In First Out scheme which can have a variable size to store elements of the same data type.

Queue Syntax

A queue is distinguished by it’s specification of the size using $ operator.

SystemVerilog Queue Example

module tb;
    // Create a queue that can store "string" values
    string   fruits[$] =  { "orange", "apple", "kiwi" };
 
  initial begin
       // Iterate and access each queue element
      foreach (fruits[i]) 
        $display ("fruits[%0d] = %s", i, fruits[i]);
 
      // Display elements in a queue
      $display ("fruits = %p", fruits);
 
        // Delete all elements in the queue
        fruits = {};
        $display ("After deletion, fruits = %p", fruits);
  end
endmodule

What are queue slice expressions ?

A slice expression selects a subset of the existing variable. Queue elements can be selected using slice expressions as shown in the example below.

Some simulators provide different results, hence using queue methods is recommended.

module tb;
  // Create a queue that can store "string" values
  string   fruits[$] =  { "orange", "apple", "lemon", "kiwi" };
 
  initial begin
    // Select a subset of the queue
    $display ("citrus fruits = %p", fruits[1:2]);
 
    // Get elements from index 1 to end of queue
    $display ("fruits = %p", fruits[1:$]);
 
    // Add element to the end of queue
    fruits[$+1] = "pineapple";
    $display ("fruits = %p", fruits);
 
    // Delete first element
    $display ("Remove orange, fruits = %p", fruits[1:$]);
  end
endmodule
 

Queue Methods Example

In addition to array operators, queues provide several built-in methods.

Function	Description
function int size ();	Returns the number of items in the queue, 0 if empty
function void insert (input integer index, input element_t item);	Inserts the given item at the specified index position
function void delete ( [input integer index] );	Deletes the element at the specified index, and if not provided all elements will be deleted
function element_t pop_front ();	Removes and returns the first element of the queue
function element_t pop_back ();	Removes and returns the last element of the queue
function void push_front (input element_t item);	Inserts the given element at the front of the queue
function void push_back (input element_t item);	Inserts the given element at the end of the queues

How to create a queue of dynamic arrays in SystemVerilog ?

// Declare a dynamic array to store strings as a datatype
typedef string str_da [];
 
module tb;
  // This is a queue of dynamic arrays
  str_da list [$];
 
  initial begin
    // Initialize separate dynamic arrays with some values
    str_da marvel = '{"Spiderman", "Hulk", "Captain America", "Iron Man"};
    str_da dcWorld = '{"Batman", "Superman" };
 
    // Push the previously created dynamic arrays to queue
    list.push_back (marvel);
    list.push_back (dcWorld);
 
    // Iterate through the queue and access dynamic array elements
    foreach (list[i])
      foreach (list[i][j])
        $display ("list[%0d][%0d] = %s", i, j, list[i][j]);
 
    // Simply print the queue
    $display ("list = %p", list);
  end