Difference between rand and randc keyword in system verilog

Hi Everyone !

 In this section dealing about rand,randc keywords and usage.

Rand Method:

                  By using this method we we can generate a random number. Their values are uniformly distributed. So the generated number can be repeated.

                   Rand can be used in all constraint.

            Real time example for rand keyword:

                   Think of rolling dice or coin. For every roll there is a equal probability of getting new value of repeated current one. There is a chance of  equal probability  of every occurrence.

Randc Method: 

            Randc keyword are random-cyclic. The basic idea is that randc iterated over all the values in the range and no value is repeated within an iteration. Or in other words solver does not repeat a random value until every possible value generation.Once the all the iteration is finished, a new iteration is automatically generated. For efficient memory usage maximum 8 bit is used. 

            Some constraints not allowed to declare a variable as randc. Usually solver first solve the randc keyword. Randc keyword is not supported in Distribution and solve before constraint 

             Real time example of randc keyword:

                     Think of dealing cards from a deck: you deal out every card in the deck in random order, then shuffle the deck, and deal out the cards in a different order. Note that the cyclic pattern is for a single variable. 

    Example:

            class Practice;

               rand bit[2:0] data1;

               randc bit[2:0] data2;     

            endclass

             module TB;

                  Practice p;

                  initial begin

                   p=new();

                    repeat (5) begin

                           p.randomize();

                            $display(" Data1=%0d Data2=%0d",p.data1,p.data2);

                    end

                    end

                endmodule

Simulator output:

                      Data1=3    Data2=  4

                      Data1=.4   Data2=  3

                      Data1= 2   Data2=  0

                      Data1= 3   Data2=  6 

                      Data1= 1   Data2 = 7

                

Data1 is declared as rand so it may produce a repeated value.(3 value is repeated twice)

Data2 is declares as randc so it won't produce a repeated value until every cycle completion.

TRICKY Interview question:

 How to implement randc functionality without using randc keyword? or Generate a unique numbers without using unique keyword ?. Here I am generating unique random values upto 15. 

                

class unique_value;

  rand int data[];

  constraint c1{foreach (data[i]) data[i] inside {[0:15]};}

  constraint c2{ foreach(data[i])

                {foreach(data[j])

                {if(i!=j)

                data[i]!=data[j];}}}

 

 endclass

 

module test();

  unique_value uq;

  initial begin

    uq = new();

    uq.data=new[15];

    uq.randomize();

    $display("Array = %p",uq.data);

  end

endmodule

 

OUTPUT: -       # Array = '{0, 14, 7, 15, 5, 9, 1, 8, 11, 12, 6, 3, 4, 10, 13}

Hope this code will be useful to everyone !!!

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