Constraint #46

Significance of new() for a RANDC Variable.

When using the randc keyword in SystemVerilog, a variable is declared to have cyclic randomization behavior. This means the variable will not repeat a value until it has cycled through all possible values within its range. SystemVerilog internally keeps track of the values that have been generated for randc variables to ensure uniqueness.

To maintain this behavior, SystemVerilog requires the variable to preserve its state across randomizations. This is typically achieved by ensuring that the variable is not assigned a new memory location before it has completed generating all possible values. If a new memory location is assigned prematurely, the variable loses track of its previous randomizations, leading to repeated values.

In contrast, variables declared with rand do not exhibit this behavior. They can be assigned new memory locations at any time without affecting their randomization behavior, as they do not rely on maintaining a history of previous values.

To illustrate this, consider two objects of the same class, each with a randc variable. If memory is allocated for one object only once, while memory is allocated for the other object on every randomization, the latter object will not exhibit the cyclic behavior of the randc variable. This is because the variable in the second object loses its history of previous randomizations each time memory is allocated, causing it to potentially repeat values prematurely.

In summary, careful memory allocation is crucial when working with randc variables in SystemVerilog to ensure they maintain their cyclic randomization behavior and do not repeat values prematurely.

class sample;
  randc bit [2:0] data;
endclass
                             
module top;
  sample S1;
  sample S2;
  
  initial begin
    $display("######## OUTPUT ########");
    $display("for S1 memory allocated only in initial time");
    $display("for S2 memory allocation done on every randomization");
    S1=new();
    repeat(10) begin
      S2=new();
      assert(S1.randomize());
      assert(S2.randomize());
      $display("S1.data = %0d	S2.data = %0d",S1.data,S2.data);
    end
    $display("######## END ########");
  end
endmodule        

In the provided example, two objects of the same class are created: S1 and S2. For S1, memory is allocated only once, while for S2, memory is allocated on every randomization. This difference in memory allocation has a significant impact on the behavior of the randc variable within each object.

In S1, where memory is allocated only once, the randc variable retains its cyclic behavior. This is because the variable maintains its state across randomizations since the memory location remains the same.

In contrast, in S2, where memory is allocated on every randomization, the randc variable loses its cyclic behavior. This is because each time memory is allocated, the variable essentially starts with a clean slate and does not retain the memory of its previous randomizations. As a result, S2 may generate values that have already been used, violating the cyclic behavior of randc.

This example demonstrates the importance of memory management when working with randc variables in SystemVerilog. Ensuring that memory is allocated appropriately is crucial for maintaining the desired behavior of randc variables.

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