A SystemVerilog queue is a First In First Out scheme which can have a variable size to store elements of the same data type.
It is similar to a one-dimensional unpacked array that grows and shrinks automatically. They can also be manipulated by indexing, concatenation and slicing operators. Queues can be passed to tasks/functions as ref or non-ref arguments.
Types of Queues
A bounded queue has a specific size and can hold a limited number of entries. Shown below is a bounded queue of depth N and is full with N items and cannot accept more.
[data_type] [name_of_queue] [$:N];
int bounded_queue [$:10]; // Depth 10
An unbounded queue can have an unlimited number of entries. Shown below is an unbounded queue which has 5 items and can accept more.
[data_type] [name_of_queue] [$];
int unbounded_queue [$]; // Unlimited entries
SystemVerilog Queue Usage
A queue is distinguished by it's specification of the size using $ operator.
string name_list [$]; // A queue of string elements
bit [3:0] data [$]; // A queue of 4-bit elements
logic [7:0] elements [$:127]; // A bounded queue of 8-bits with maximum size of 128 slots
int q1 [$] = { 1, 2, 3, 4, 5 }; // Integer queue, initialize elements
int q2 [$]; // Integer queue, empty
int tmp; // Temporary variable to store values
tmp = q1 [0]; // Get first item of q1 (index 0) and store in tmp
tmp = q1 [$]; // Get last item of q1 (index 4) and store in tmp
q2 = q1; // Copy all elements in q1 into q2
q1 = {}; // Empty the queue (delete all items)
q2[2] = 15; // Replace element at index 2 with 15
q2.insert (2, 15); // Inserts value 15 to index# 2
q2 = { q2, 22 }; // Append 22 to q2
q2 = { 99, q2 }; // Put 99 as the first element of q2
q2 = q2 [1:$]; // Delete first item
q2 = q2 [0:$-1]; // Delete last item
q2 = q2 [1:$-1]; // Delete first and last item
SystemVerilog Queue Example
module tb;
// Create a queue that can store "string" values
string fruits[$] = { "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 = {};
$display ("After deletion, fruits = %p", fruits);
end
endmodule
Simulation Log
ncsim> run
fruits[0] = orange
fruits[1] = apple
fruits[2] = kiwi
fruits = '{"orange", "apple", "kiwi"}
After deletion, fruits = '{}
ncsim: *W,RNQUIE: Simulation is complete.
What are queue slice expressions ?
A slice expression selects a subset of the existing variable. Queue elements can be selected using slice expressions as shown in the example below.
Some simulators provide different results, hence using queue methods is recommended.
module tb;
// Create a queue that can store "string" values
string fruits[$] = { "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
Simulation Log
Compiler version J-2014.12-SP1-1; Runtime version J-2014.12-SP1-1; May 15 16:21 2018
citrus fruits = '{"apple", "lemon"}
fruits = '{"apple", "lemon", "kiwi"}
fruits = '{"orange", "apple", "lemon", "kiwi", "pineapple"}
Remove orange, fruits = '{"apple", "lemon", "kiwi", "pineapple"}
V C S S i m u l a t i o n R e p o r t
SystemVerilog Queue Methods
SystemVerilog Queue methods are several built-in methods in addition to array operators to operate on the queue and are listed in the table below.
| Methods | 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 queue |
SystemVerilog Queue Methods Example
module tb;
string fruits[$] = {"apple", "pear", "mango", "banana"};
initial begin
// size() - Gets size of the given queue
$display ("Number of fruits=%0d fruits=%p", fruits.size(), fruits);
// insert() - Insert an element to the given index
fruits.insert (1, "peach");
$display ("Insert peach, size=%0d fruits=%p", fruits.size(), fruits);
// delete() - Delete element at given index
fruits.delete (3);
$display ("Delete mango, size=%0d fruits=%p", fruits.size(), fruits);
// pop_front() - Pop out element at the front
$display ("Pop %s, size=%0d fruits=%p", fruits.pop_front(), fruits.size(), fruits);
// push_front() - Push a new element to front of the queue
fruits.push_front("apricot");
$display ("Push apricot, size=%0d fruits=%p", fruits.size(), fruits);
// pop_back() - Pop out element from the back
$display ("Pop %s, size=%0d fruits=%p", fruits.pop_back(), fruits.size(), fruits);
// push_back() - Push element to the back
fruits.push_back("plum");
$display ("Push plum, size=%0d fruits=%p", fruits.size(), fruits);
end
endmodule
Simulation Log
ncsim> run
Number of fruits=4 fruits='{"apple", "pear", "mango", "banana"}
Insert peach, size=5 fruits='{"apple", "peach", "pear", "mango", "banana"}
Delete mango, size=4 fruits='{"apple", "peach", "pear", "banana"}
Pop apple, size=3 fruits='{"peach", "pear", "banana"}
Push apricot, size=4 fruits='{"apricot", "peach", "pear", "banana"}
Pop banana, size=3 fruits='{"apricot", "peach", "pear"}
Push plum, size=4 fruits='{"apricot", "peach", "pear", "plum"}
ncsim: *W,RNQUIE: Simulation is complete.
How to create a queue of classes in SystemVerilog ?
// Define a class with a single string member called "name"
class Fruit;
string name;
function new (string name="Unknown");
this.name = name;
endfunction
endclass
module tb;
// Create a queue that can hold values of data type "Fruit"
Fruit list [$];
initial begin
// Create a new class object and call it "Apple"
// and push into the queue
Fruit f = new ("Apple");
list.push_back (f);
// Create another class object and call it "Banana" and
// push into the queue
f = new ("Banana");
list.push_back (f);
// Iterate through queue and access each class object
foreach (list[i])
$display ("list[%0d] = %s", i, list[i].name);
// Simply print the whole queue, note that class handles are printed
// and not class object contents
$display ("list = %p", list);
end
endmodule
Simulation Log
ncsim> run
list[0] = Apple
list[1] = Banana
list = '{$unit_0x4ccdf83b::Fruit@2_1, $unit_0x4ccdf83b::Fruit@4_1}
ncsim: *W,RNQUIE: Simulation is complete.
How to create a queue of dynamic arrays in SystemVerilog ?
// Declare a dynamic array to store strings as a datatype
typedef string str_da [];
module tb;
// This is a queue of dynamic arrays
str_da list [$];
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
endmodule
Simulation Log
ncsim> run
list[0][0] = Spiderman
list[0][1] = Hulk
list[0][2] = Captain America
list[0][3] = Iron Man
list[1][0] = Batman
list[1][1] = Superman
list = '{'{"Spiderman", "Hulk", "Captain America", "Iron Man"}, '{"Batman", "Superman"}}
ncsim: *W,RNQUIE: Simulation is complete.
