SystemVerilog Disable Randomization

Randomization of variables in a class can be disabled using rand_mode method call.

This is very similar to the constraint_mode() method used to Disable Constraints. So a disabled random variable is treated the same as if they had not been declared rand or randc.

rand_mode can be called both as a function and task. Current state of the variable will be returned if it is called as a function.

	// Disables randomization of variable [variable_name] inside [class_object] class
	[class_object].[variable_name].rand_mode (0);   
	
	// Enables randomization of variable [variable_name] inside [class_object] class
	[class_object].[variable_name].rand_mode (1);   

Example

Let's first try without rand_mode to compare with the one that disables constraints.

Without using rand_mode()

By default, variables declared as rand and randc are enabled as visible in the example shown below.

// Create a class that contains random variables
class Fruits;
  rand bit [3:0] var1;
  rand bit [1:0] var2;
endclass

module tb;
  initial begin
  	// Instantiate an object of the class
    Fruits f = new(); 
    
    // Print values of those variables before randomization
    $display ("Before randomization var1=%0d var2=%0d", f.var1, f.var2);
    
    // rand_mode() is called as a function which returns the state of the given variable
    // If it is enabled, then print a statement
    if (f.var1.rand_mode())
    	if (f.var2.rand_mode())
      		$display ("Randomization of all variables enabled");
    
    // Randomize the class object which in turn randomizes all internal variables
    // declared using rand/randc keywords
    f.randomize();
    
    // Print the value of these variables after randomization
    $display ("After randomization var1=%0d var2=%0d", f.var1, f.var2);
  end
endmodule

ncsim> run
Before randomization var1=0 var2=0
Randomization of all variables enabled
After randomization var1=15 var2=3
ncsim: *W,RNQUIE: Simulation is complete.

After using rand_mode()

Next, we'll disable var1 and see how it affects randomization of that variable.

// Create a class that contains random variables
class Fruits;
  rand bit [3:0] var1;
  rand bit [1:0] var2;
endclass

module tb;
  initial begin
    Fruits f = new(); 
    $display ("Before randomization var1=%0d var2=%0d", f.var1, f.var2);
    
    // Turn off randomization for var1
    f.var1.rand_mode (0);
    
    // Print if var1 has randomization enabled/disabled
    if (f.var1.rand_mode())
      $display ("Randomization of var1 enabled");
    else
      $display ("Randomization of var1 disabled");
    
    f.randomize();
    
    $display ("After randomization var1=%0d var2=%0d", f.var1, f.var2);
  end
endmodule

ncsim> run
Before randomization var1=0 var2=0
Randomization of var1 disabled
After randomization var1=0 var2=3
ncsim: *W,RNQUIE: Simulation is complete.

Next, we'll skip mentioning a variable name while invoking rand_mode().

// Create a class that contains random variables
class Fruits;
  rand bit [3:0] var1;
  rand bit [1:0] var2;
endclass

module tb;

  initial begin
    Fruits f = new(); 
    $display ("Before randomization var1=%0d var2=%0d", f.var1, f.var2);
    
    // Turns off randomization for all variables
    f.rand_mode (0);    
    
    if (! f.var1.rand_mode())
      if (! f.var2.rand_mode())
        $display ("Randomization of all variables disabled");
    
    f.randomize();
    
    $display ("After randomization var1=%0d var2=%0d", f.var1, f.var2);
  end
endmodule

ncsim> run
Before randomization var1=0 var2=0
Randomization of all variables disabled
After randomization var1=0 var2=0
ncsim: *W,RNQUIE: Simulation is complete.