SystemVerilog Disable Randomization

SystemVerilog lets you selectively turn randomization off for individual rand or randc class properties, or for an entire object, using the built-in rand_mode() method. A property with randomization disabled behaves exactly as if it had never been declared rand or randc in the first place — it simply keeps whatever value it already holds the next time randomize() is called.

6 min read | Intermediate Level

What You'll Learn

• How rand_mode() enables or disables randomization for one variable, or for a whole object
• How to check a variable's current rand_mode() state
• What happens to a disabled variable's value when randomize() is called
• How disabling a whole object differs from disabling one variable at a time

Syntax: rand_mode()

rand_mode() can be called either as a task, to set the state, or as a function, to read the current state back.

// 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);  

Parameters:

• 0 — disables randomization for the target (task form)
• 1 — enables randomization for the target (task form)
• no argument — returns the current state as 1 (enabled) or 0 (disabled) (function form)

This is very similar to the constraint_mode() method used to disable constraint blocks — the difference is that rand_mode() targets a variable, while constraint_mode() targets a constraint block.

Randomization Enabled by Default

By default, every variable declared rand or randc has randomization enabled, as this first example confirms before disabling anything:

// 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

Output

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.

Disabling a Single Variable

Next, we'll disable var1 and see how that affects randomization of that one 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

Output

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

var1 stays at its pre-randomization value of 0 even after f.randomize() runs, because rand_mode(0) made the solver treat it as if it were never rand in the first place. var2, left enabled, gets a fresh random value as usual.

Disabling All Variables in an Object

Calling rand_mode() directly on the object, without naming a specific variable, applies to every rand/randc property inside it at once:

// 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

Output

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.

With every property disabled, f.randomize() still returns successfully, but neither var1 nor var2 changes — there's nothing left for the solver to randomize.

Common Beginner Mistakes

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 var2 this time
    f.var2.rand_mode (0);

    f.randomize();

    $display ("After randomization var1=%0d var2=%0d", f.var1, f.var2);
  end
endmodule