UVM Sequence item Methods

One of the key components in UVM is the sequence item (uvm_sequence_item), which represents transactions in a testbench. UVM provides several built-in methods for sequence items to facilitate debugging, comparison, and data manipulation.

In this article, we will explore the most commonly used UVM sequence item methods with examples:

create()
print()
copy() and clone()
compare()
pack() and unpack()

1. `create()` Method

The create() method is used to instantiate an object dynamically using the UVM factory. It ensures that objects are created correctly and allows for factory overrides.

Example:

class my_transaction extends uvm_sequence_item;
    `uvm_object_utils(my_transaction)  // Register with the factory

    rand bit [7:0] addr;
    rand bit [31:0] data;

    function new(string name = "my_transaction");
        super.new(name);
    endfunction
endclass

// Using the create() method
my_transaction tr;
tr = my_transaction::type_id::create("tr");

Why use create()?

Supports factory overrides
Allows for flexible object instantiation
Essential for reusable and configurable testbenches

2. `print()` Method

The print() method displays the contents of a sequence item, making it useful for debugging.

Example:

class my_transaction extends uvm_object;

  rand bit [7:0] addr;  // Randomizable field
  rand bit [31:0] data; // Randomizable field
  
  //utility and field macros
  `uvm_object_utils_begin(my_transaction)
  `uvm_field_int(addr,UVM_ALL_ON)
  `uvm_field_int(data,UVM_ALL_ON)
  `uvm_object_utils_end

  // Constructor
  function new(string name = "my_transaction");
    super.new(name);
  endfunction

endclass

module testbench;

  initial begin
    my_transaction txn;

    txn = my_transaction::type_id::create("txn"); // Factory-based creation
    txn.randomize(); // Randomize the transaction fields
    txn.print(); // Print the object details
  end
endmodule

Output:

----------------------------------------
Name    Type            Size  Value     
----------------------------------------
txn     my_transaction  -     @336      
  addr  integral        8     'hb9      
  data  integral        32    'ha1098c6c
----------------------------------------
           V C S   S i m u l a t i o n   R e p o r t 
Time: 0 ns

Execute on

3. `copy()` and `clone()` Methods

`copy()`

The copy() method copies the values from one sequence item to another.

`clone()`

The clone() method creates a deep copy of the sequence item.

clone() = create() + copy()

Example:

class my_transaction extends uvm_object;

  rand bit [7:0] addr;  // Randomizable field
  rand bit [31:0] data; // Randomizable field
  
  //utility and field macros
  `uvm_object_utils_begin(my_transaction)
  `uvm_field_int(addr,UVM_ALL_ON)
  `uvm_field_int(data,UVM_ALL_ON)
  `uvm_object_utils_end

  // Constructor
  function new(string name = "my_transaction");
    super.new(name);
  endfunction

endclass

module testbench;

  initial begin
    my_transaction txn_0;
    my_transaction txn_1;
    my_transaction txn_2;

    txn_0 = my_transaction::type_id::create("txn_0"); // Factory-based creation
    txn_1 = my_transaction::type_id::create("txn_1"); // Factory-based creation
    txn_0.randomize(); // Randomize the transaction fields
    txn_0.print(); // Print the object details
    
    txn_1.copy(txn_0); //copy txn_0 to txn_1
    txn_1.print(); //printing the txn_1
    
    `uvm_info("","clone process",UVM_LOW)
    $cast(txn_2,txn_0.clone()); //creating txn_2 object and copy txn_0 to txn_1
    txn_2.print(); //printing the txn_2
  end
endmodule

Output:

----------------------------------------
Name    Type            Size  Value     
----------------------------------------
txn_0   my_transaction  -     @336      
  addr  integral        8     'hb9      
  data  integral        32    'ha1098c6c
----------------------------------------
----------------------------------------
Name    Type            Size  Value     
----------------------------------------
txn_1   my_transaction  -     @337      
  addr  integral        8     'hb9      
  data  integral        32    'ha1098c6c
----------------------------------------
UVM_INFO testbench.sv(36) @ 0: reporter [] clone process
----------------------------------------
Name    Type            Size  Value     
----------------------------------------
txn_0   my_transaction  -     @340      
  addr  integral        8     'hb9      
  data  integral        32    'ha1098c6c
----------------------------------------
           V C S   S i m u l a t i o n   R e p o r t 
Time: 0 ns

Name of txn_2 item will be printed as txn_0 because in cloning process, while creating the object, there is no option of passing argument.

4. `compare()` Method

The compare() method compares two sequence items and returns a boolean indicating whether they match.

Example:

class my_transaction extends uvm_object;

  rand bit [7:0] addr;  // Randomizable field
  rand bit [31:0] data; // Randomizable field
  
  //utility and field macros
  `uvm_object_utils_begin(my_transaction)
  `uvm_field_int(addr,UVM_ALL_ON)
  `uvm_field_int(data,UVM_ALL_ON)
  `uvm_object_utils_end

  // Constructor
  function new(string name = "my_transaction");
    super.new(name);
  endfunction

endclass

module testbench;

  initial begin
    my_transaction txn_0;
    my_transaction txn_1;

    txn_0 = my_transaction::type_id::create("txn_0"); // Factory-based creation
    txn_1 = my_transaction::type_id::create("txn_1"); // Factory-based creation
    
    txn_0.addr = 8'hAA;
    txn_0.data = 32'h55555555;

    txn_1.addr = 8'hAA;
    txn_1.data = 32'h55555555;
    
    if (txn_0.compare(txn_1)) 
    `uvm_info("COMPARE", "Transactions match!", UVM_MEDIUM)
      else
    `uvm_info("COMPARE", "Transactions do not match!", UVM_MEDIUM)
        end
        
endmodule

Output:

UVM_INFO testbench.sv(37) @ 0: reporter [COMPARE] Transactions match!

5. `pack()` and `unpack()` Methods

`pack()`

The pack() method converts a sequence item into a bit-stream (array of bits).

`unpack()`

The unpack() method extracts values from a bit-stream into a sequence item.

Example:

class my_transaction extends uvm_object;

  rand bit [7:0] addr;  // Randomizable field
  rand bit [31:0] data; // Randomizable field
  
  //utility and field macros
  `uvm_object_utils_begin(my_transaction)
  `uvm_field_int(addr,UVM_ALL_ON)
  `uvm_field_int(data,UVM_ALL_ON)
  `uvm_object_utils_end

  // Constructor
  function new(string name = "my_transaction");
    super.new(name);
  endfunction

endclass

module testbench;

  initial begin
    my_transaction txn_0;
    my_transaction txn_1;
    bit bitstream[];

    txn_0 = my_transaction::type_id::create("txn_0"); // Factory-based creation
    txn_1 = my_transaction::type_id::create("txn_1"); // Factory-based creation
    
    txn_0.addr = 8'hAA;
    txn_0.data = 32'h55555555;
    
    // Pack transaction into a bitstream
    txn_0.pack(bitstream);
    foreach(bitstream[i]) 
      `uvm_info("PACK",$sformatf("bit_packed_data[%0d] = %b",i,bitstream[i]), UVM_LOW)
    
    // Unpack bitstream into another transaction
    txn_1.unpack(bitstream);
    
    // Check if both transactions are identical
    if (txn_0.compare(txn_1))
    `uvm_info("PACK_UNPACK", "Packing and unpacking successful", UVM_MEDIUM)
      else
    `uvm_info("PACK_UNPACK", "Mismatch after unpacking", UVM_MEDIUM)
        end
        
endmodule

Output:

UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[0] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[1] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[2] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[3] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[4] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[5] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[6] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[7] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[8] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[9] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[10] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[11] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[12] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[13] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[14] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[15] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[16] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[17] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[18] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[19] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[20] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[21] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[22] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[23] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[24] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[25] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[26] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[27] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[28] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[29] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[30] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[31] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[32] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[33] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[34] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[35] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[36] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[37] = 1
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[38] = 0
UVM_INFO testbench.sv(37) @ 0: reporter [PACK] bit_packed_data[39] = 1
UVM_INFO testbench.sv(44) @ 0: reporter [PACK_UNPACK] Packing and unpacking successful
           V C S   S i m u l a t i o n   R e p o r t

Conclusion

UVM sequence item methods make it easier to create, manipulate, and compare transactions in a testbench. Here’s a quick summary of their use:

Method	Purpose
`create()`	Instantiates objects using the UVM factory
`print()`	Displays sequence item data
`copy()`	Copies values from one object to another
`clone()`	Creates a deep copy of an object
`compare()`	Compares two transactions
`pack()`	Converts a transaction into a bit-stream
`unpack()`	Restores a transaction from a bit-stream

By leveraging these methods effectively, you can build a robust and efficient verification environment.

1. create() Method

Example:

2. print() Method

Example:

Output:

3. copy() and clone() Methods

copy()

clone()

Example:

Output:

4. compare() Method

Example:

Output:

5. pack() and unpack() Methods

pack()

unpack()

Example:

Output:

Conclusion

1. `create()` Method

2. `print()` Method

3. `copy()` and `clone()` Methods

`copy()`

`clone()`

4. `compare()` Method

5. `pack()` and `unpack()` Methods

`pack()`

`unpack()`