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Randomization

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Overview

Randomization is the ablity in systemverilog to generate psuerandom values using differnt constructs and keywords. Randomization when used along with OOPs cocenpts give the engineer the strong ability to create random stimulus. Given below are the most important keywords which one must be familiar with when it comes to the SystemVerilog randomization topic.

Important Keywords

rand
randc
constraint
solve-before
soft
disable soft
:=
:/
->
inside {}
dist {}
randomize
randomize with {}
randcase
randsequence

Theory Questions
    This keyword is used to generate random numbers in a cyclic manner and only after having covered all possible values of the datatype with randc keyword, will old values start repeating again. For example if a variable is declared as rand bit [3:0] num, then on the first 16 calls of randomize() we will see 16 unique value for "num" variable. From the 17th iteration onwards, we will see the values repeat.
    Solve Before is used to control which constraints are to be solved first. This keyword is mostly used to reduce the randomization space over which the constraint solver has to work on.
    In case of a constraint conflict, we can either resolve the conflict by fixing the issue, or we can set one of the conflicting constrains as soft to give higher priority to the other non-soft declared constraint.
    The non-soft declared constraints are called hard constraints 
    // Constraint can be disabled by
// a. Calling constraint_mode on the specific constraint
// b. Extending the parent class and overriding the original constraint by declaring it again in the child class
    pre_randomize & post_randomize
    The order in which a solver prioritizes the constraint block to be solved is implementation specific. Certain simulators follow a top dow uproach, while some follow a bottom up 
    // Constraint randomization has the following advantages over directed randomization
// 1. Its easier to write the value ranges in which we want a randim value
// 2. Easier to manage constraints through the OOPs concepts
// 3. Its possible to write distribution based rand value generation
// 4. More input stimulus combinations can be covered with almost no manual effort
// 5. Makes testing far more easier and quicker
    In a simple expression like ( a == b ) -> c > 100;, the way it's actually understood by the constraint solver is "if a == b, then make sure c > 100" 
    // Systemverilog randomization has the following advantages over verilog randomization
// 1. Its possible to write constraints/rules to define the range of values which the solve can psuerandomly pick
// 2. Its possible to generate a n-bit random number unlike verilog, where atmost 32bit can be generated using $random
// 3. Constraint solvers now are much more efficient and easily cover a large range of random value more efficiently
// 4. Its easier to add new rules while making sure older restrictions are intact unlike in verilog where everything has to be rechecked
// 5. Combined with the OOPs concepts, systemverilog randomization is more robust, easy to code and future proof
Coding Questions 
    class cons;
    rand bit [7:0]array1[10];
    constraint y{foreach(array1[i])
        array1[i]%3==0 && array1[i] inside {[1:100]};}
    constraint z{unique{array1};}
endclass
cons c;
module test;
    initial
    begin
        c=new;
        assert(c.randomize);
        $display("%0p",c.array1);
    end
endmodule

    class cons;
    rand int a[10];
    real b[10];
    constraint x{foreach(a[i])
        a[i] inside {[990:1000]};}
    constraint z{foreach(a[i])
        foreach(a[j])
            if(i!=j)
                a[i]!=a[j];}
    function void post_randomize();
        foreach(b[i])
        b[i]=a[i]/10.0;
        $display("10 unique numbers between 99 and 100 are:%0p",b);
    endfunction
endclass
cons c;
module test;
    initial	
    begin
        c=new;
        assert(c.randomize);
    end
endmodule

    class cons;
    rand bit [7:0]a;
    constraint x{a dist {[0:100]:/70,[101:255]:/30};}
    function void post_randomize();
        $display(a);
    endfunction
endclass
cons c;
module top;
    int count;
    initial
    begin
        c=new;
        repeat(100)
        begin
            c.randomize;
            if(c.a<101)
                count=count+1;
        end
        $display("No of entries less than 100 are %0d",count);
        $display("No of entries less than 100 are %0d",100-count);
    end
endmodule

    class cons;
    rand int a[];
    constraint x{a.size inside {[10:20]};}
    constraint y{foreach(a[i])
        if(i>1)
            (a[i]+a[i-1]+a[i-2])%2==0;}
    constraint z{foreach(a[i])
        a[i] inside {[0:10]};}
    function void post_randomize;
        $display("array is %0p",a);
    endfunction
endclass
cons c;
module top;
    initial
    begin
        c=new;
        c.randomize;
    end
endmodule

    class cons;
    rand bit [8:0]a;
    rand bit [8:0]b;
    constraint not_equal{a!=b;}
    constraint count{$countones(a)==5;}
    function void post_randomize;
        $display("a is %0b\n b is %0b",a,b);
    endfunction
endclass
cons c;
module top;
    initial
    begin
        c=new;
        c.randomize;
    end
endmodule

    class cons;
    rand bit [3:0]a;
    int que[$];
    constraint x{!(a inside {que});}
    
    function void post_randomize;
        que.push_front(a);
        if(que.size==6)
            que.pop_back;
        $display("a is %0d",a);
    endfunction
endclass
cons c;
module top;
    initial
    begin
        c=new;
        repeat(8)
            c.randomize;
    end
endmodule

    class cons;
    rand bit [1:0] a;
    int que[$];
    constraint x{!(a inside {que});}
    
    function void post_randomize();
        que.push_front(a);
        
        if(que.size==2**$size(a))
        begin
            que={};
        end
        $display("a is %0d",a);
    endfunction
endclass
cons c;
module top;
    initial
    begin
        c=new;
        repeat(8)
            c.randomize;
    end
endmodule

    class ex;

    rand bit [31:0] rand_var;

    constraint odd_num {
        $countones( rand_var ) & 1'b1 == 1;
    }

    constraint limit {
        rand_var < 100;
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %d ( %b )", R.rand_var, R.rand_var );
        end
    end

endmodule

    class sv_class;
    rand int rand_var;

    constraint odd_num_lt_1000 {
        rand_var % 2 == 1;
        rand_var < 1000;
    }
endclass

module top;
    sv_class sv;

    initial 
    begin
        sv = new;

        repeat( 10 )
        begin
            sv.randomize();
            $display( "rand_var : %h ( %d ) ( %b )", sv.rand_var, sv.rand_var, sv.rand_var );
        end    
    end
endmodule

    class decimal_palindrome;
    rand bit [31:0] rand_num;
    rand int palin_arr[];
    rand int palin_arr_pow_10s[];
    
    constraint digit_count {
        palin_arr.size() <= 9; // 32b can generate a maximum of 9 digit decimal
        palin_arr.size() > 0;
        palin_arr_pow_10s.size() == palin_arr.size();
    }
    
    constraint array_val {
        foreach( palin_arr[i] ) {
            palin_arr[i] < 10 && palin_arr[i] >= 0;
            if( i == 0 ) {
                palin_arr[i] != 0; 
            }
        }
    }
    
    constraint palindrome_condition {
        foreach( palin_arr[i] ) {
            if( !( ( 1 == palin_arr.size()%2 ) && ( palin_arr.size()/2 == i ) ) ) {
                palin_arr[i] == palin_arr[palin_arr.size()-i-1];
            }
        }
    }
    
    constraint load_palin_arr_pow_10s_with_10s_powers {
        foreach( palin_arr_pow_10s[i] ) {
            if( i == 0 ) {
                palin_arr_pow_10s[i] == 1;
            } else {
                palin_arr_pow_10s[i] == palin_arr_pow_10s[i-1]*10; 
            }
        }
    }
    
    constraint convert_array_to_num {
        foreach( palin_arr[i] ) {
            (rand_num/palin_arr_pow_10s[i])%10 == palin_arr[i];
        }
        rand_num/(palin_arr_pow_10s[palin_arr_pow_10s.size()-1]*10) == 0;
    }
endclass

module top;

    decimal_palindrome p;
    
    initial
    begin
        p = new;
        
        repeat( 10 )
        begin
            p.randomize();
            //$display( "Array : %p", p.palin_arr );
            $display( "Val : %d", p.rand_num );
        end
    end

endmodule

    class sv_class;
    rand int rand_var;
    rand int fib_arr[];

    constraint populate_fib_arr {
        fib_arr.size != 0;
        fib_arr.size < 1000; // Issues with leaving the upper boundary unconstrained

        foreach( fib_arr[i] ) {
            if( i == 0 || i == 1 ) {
                fib_arr[i] == 1;
            } else {
                fib_arr[i] == fib_arr[i-1] + fib_arr[i-2];
            }
        }
    }

    constraint rand_var_value {
        rand_var inside {fib_arr};
    }
endclass

module top;
    sv_class sv;

    initial 
    begin
        sv = new;

        repeat( 10 )
        begin
            sv.randomize();
            $display( "rand_var : %h ( %d ) ( %b )", sv.rand_var, sv.rand_var, sv.rand_var );
        end    
    end
endmodule

    class sv_class;
    rand int rand_var;

    constraint odd_num_lt_1000 {
        rand_var % 2 == 1;
        rand_var < 1000;
    }
endclass

module top;
    sv_class sv;

    initial 
    begin
        sv = new;

        repeat( 10 )
        begin
            sv.randomize();
            $display( "rand_var : %h ( %d ) ( %b )", sv.rand_var, sv.rand_var, sv.rand_var );
        end    
    end
endmodule

    class practice;
    rand bit[31:0]a;
    bit[2:0] pattern=3'b110;
    rand int b[];

    constraint b_size {
        b.size ==3;  //for 3 counts
    }

    constraint b_cond_1 {
        foreach(b[i])
        {
            b[i] > 0;
            b[i] <30;
        }
    }

    constraint b_cond_2{ //condition for the indices not to overlap
        foreach(b[i])
        {
            foreach(b[j])
            {
                if(i!=j)
                {
                    unsigned'(b[i]-b[j]) >='d3 ;
                }
            }
        }
    }

    constraint a_set {
        foreach(a[i])
        {
            if(i inside {b})
            {
                a[i+:3] == pattern;
            }
            else
            {
                (i<29)->a[i+:3] != pattern ;
            }
        }
    }
endclass

module test;
    practice p;
    
    initial
    begin
        p=new();
        assert(p.randomize);
        $display("%b  :var:%p",p.a,p.b);
    end
endmodule

    class ex;

    rand bit [31:0] rand_var;

    constraint x11 {
        rand_var % 11 == 0;
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %d ( %b )", R.rand_var, R.rand_var );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_var;

    constraint power_of_2 {
        $countones( rand_var ) == 1;
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %d ( %b )", R.rand_var, R.rand_var );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_var;
    rand int digits;
    rand bit [3:0] ele[];

    constraint solve_bf {
        solve digits before ele;
    }

    constraint digit {
        ele.size == digits;

        digits <= 32/4;
        digits > 0;
    }

    constraint hex_palindrome {
        soft rand_var == 32'd0;

        foreach( ele[i] ) {
            if( i < ele.size/2 ) {
                ele[i] == ele[ele.size-i-1];
            }
        }
        ele[0] != 0;
    }

    function void post_randomize();
        foreach( ele[i] )
        begin
            rand_var[4*i+:4] = ele[i];
        end
    endfunction

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %d ( %h )", R.rand_var, R.rand_var );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_var;
    rand int digits [];

    constraint digit_count {
        digits.size > 0 && digits.size < 11;
    }

    constraint sum_17 {
        foreach( digits[i] ) {
            digits[i] >= 0 && digits[i] < 10;
        }

        digits.sum == 17;
    }

    function void post_randomize();
        rand_var = 0;

        foreach( digits[i] )
        begin
            rand_var += (10**i)*digits[i];
        end
    endfunction

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %d ( %h ) ( digits : %p )", R.rand_var, R.rand_var, R.digits );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_var;
    rand int bit_count;

    constraint bit_cnt_solve {
        solve bit_count before rand_var;
    }

    constraint bit_cnt {
        bit_count < 32;
        bit_count > 0;
        bit_count % 3 == 0;
    }

    constraint bit_msb_x3 {
        foreach( rand_var[i] ) {
            if( i < bit_count ) {
                rand_var[32-1-i] == 1'b1;
            } else if( i == bit_count ) {
                rand_var[32-1-i] == 1'b0;
            }
        }
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %b ( %h ) ( bit_count : %d )", R.rand_var, R.rand_var, R.bit_count );
        end
    end

endmodule

    // Important Note : This code is only solvable efficiently by Xcelieum & VCS simulators.
//                  On Questasim the solver will fail in many instances but on some instances correct output will be generated.

class prime;

    rand int prime_num;
    rand bit prime_size[];

    constraint prime_check {    
                                prime_num % 2 != 0; // Must not be an even number

                                //( prime_num == 2 | prime_num == 3 | ( prime_num > 3 ) ) == 1; // Can generate 2,3 or any number more than 3
                                foreach( prime_size[i] ){
                                    if( i > 1 ){
                                        i % 2 != 0 -> prime_num % i != 0; // Makes sure no value of i is a divisor of prime_num
                                    }
                                }
    };
    
    constraint a { 
                    prime_num < 20000;
                    prime_size.size() == prime_num/2;
                    prime_num > 3;
    }

    function void post_randomize();
        if( prime_num inside { 1,2,3 } )
        begin
            // Special prime
            $display( "Special prime generated" );
        end
        else
        begin
            for( int i = 2; i <= prime_num/2; i++ )
            begin
                if( prime_num % i == 0 )
                begin
                    $display( "Not a prime. Number is %d", prime_num );
                    $finish;
                end
            end
        end
    endfunction

endclass

module top;

    prime p;

    initial
    begin
        p = new();

        repeat( 30 )
        begin
            p.randomize();
            $display( "Randomized Prime Number : %0d", p.prime_num );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_var;
    rand int num [];
    rand int digits;

    constraint num_size {
        digits < 10;
        digits > 0;
        digits[0] == 0;

        num.size == digits/2;
    }

    constraint num_val {
        foreach( num[i] ) {
            if( i > 0 ) {
                num[i] == num[i-1] + 1;
            }
            num[i] < 100 && num[i] > 9;
        }
    }

    function void post_randomize();
        rand_var = 0;

        foreach( num[i] )
        begin
            rand_var += ((100**(i))*(num[num.size-1-i]));
        end
    endfunction
    
endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %d ( %h ) ( %p )", R.rand_var, R.rand_var, R.num );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_var;
    rand int digits;
    rand int num [];

    constraint sol_b {
        solve digits before num;
        solve digits before rand_var;
        solve rand_var before num;
    }

    constraint digits_len {
        digits < 6;
        digits > 1;

        num.size == digits;
    }

    constraint arm_sum {
        foreach( num[i] ) {
            num[i] == ((rand_var/(10**i))%10)**digits;
        }

        num.sum == rand_var;
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_var : %d ( %h ) ( %p )", R.rand_var, R.rand_var, R.num );
        end
    end

endmodule

    class ex;

    rand int rand_arr [];

    constraint uniq_element {
        rand_arr.size < 10;

        foreach( rand_arr[i] ) {
            rand_arr[i] > 0 && rand_arr[i] < 10000;
            foreach( rand_arr[j] ) {
                if( j > i ) {
                    rand_arr[i] != rand_arr[j];
                }
            }
        }
    }

    constraint decending_order {
        foreach( rand_arr[i] ) {
            if( i > 0 ) {
                rand_arr[i-1] > rand_arr[i];
            }
        }
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_arr : %p", R.rand_arr );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_arr [];

    constraint elements {
        rand_arr.size < 10;
    }

    constraint sum_100 {
        rand_arr.sum == 100;

        foreach( rand_arr[i] ) {
            rand_arr[i] < ( 2**32 - 1 );
            rand_arr[i] > 0;
        }
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_arr : %p", R.rand_arr );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_arr [];

    constraint elements {
        rand_arr.size < 10;
        rand_arr.size > 0;
    }

    constraint inc_pow_of_5 {
        foreach( rand_arr[i] ) {
            if( i == 0 ) {
                rand_arr[i] == 1;
            } else {
                rand_arr[i] == 5*rand_arr[i-1];
            }
        }
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_arr : %p", R.rand_arr );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_arr [];

    constraint elements {
        rand_arr.size < 100;
        rand_arr.size > 0;
    }

    constraint fib {
        foreach( rand_arr[i] ) {
            if( i == 0 || i == 1 ) {
                rand_arr[i] == 1;
            } else {
                rand_arr[i] == rand_arr[i-1]+rand_arr[i-2];
            }
        }
    }

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_arr : %p", R.rand_arr );
        end
    end

endmodule

    // Important Note : This code is only solvable efficiently by Xcelieum & VCS simulators.
//                  On Questasim the solver will fail in many instances but on some instances correct output will be generated.

class prime_num;

    rand int prime_num;
    rand bit prime_size[];

    constraint prime_check {    
                                prime_num % 2 != 0; // Must not be an even number

                                //( prime_num == 2 | prime_num == 3 | ( prime_num > 3 ) ) == 1; // Can generate 2,3 or any number more than 3
                                foreach( prime_size[i] ){
                                    if( i > 1 ){
                                        i % 2 != 0 -> prime_num % i != 0; // Makes sure no value of i is a divisor of prime_num
                                    }
                                }
    };
    
    constraint a { 
                    prime_num < 2000; // For faster solver solution
                    prime_size.size() == prime_num/2;
                    prime_num > 3;
    }

    function void post_randomize();
        if( prime_num inside { 1,2,3 } )
        begin
            // Special prime
            $display( "Special prime generated" );
        end
        else
        begin
            for( int i = 2; i <= prime_num/2; i++ )
            begin
                if( prime_num % i == 0 )
                begin
                    $display( "Not a prime. Number is %d", prime_num );
                    $finish;
                end
            end
        end
    endfunction

endclass

class ex;

    rand bit [31:0] rand_arr [];

    constraint elements {
        rand_arr.size < 10;
        rand_arr.size > 0;
    }

    function void post_randomize();
        prime_num p;
        p = new();

        foreach( rand_arr[i] )
        begin
            p.randomize();
            rand_arr[i] = p.prime_num;
        end
    endfunction

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_arr : %p", R.rand_arr );
        end
    end

endmodule

    class ex;

    rand bit [31:0] rand_arr [];
    rand int arr_size;

    constraint soft_default {
        rand_arr.size == 1;

        soft arr_size < 100; // Modifiable
        arr_size > 0;
    }

    function x3_x_1_next_lfsr_val( int start_seed ); // x3+x+1
        bit [2:0] ini;
        bit [2:0] ini_temp;

        ini = start_seed;
        ini_temp[2] = ini[2];
        ini_temp[1] = ini[1];
        ini_temp[0] = ini[0];
        
        ini[2] = ini_temp[0];
        ini[1] = ini_temp[2];
        ini[0] = ini_temp[1] ^ ini_temp[0];

        return ini;
    endfunction

    function void post_randomize();
        // Generate a sequence of x3+x+1 based on rand_arr[0]'s value
        rand_arr = new[arr_size](rand_arr);

        $display( "Initial Value : %p", rand_arr );

        foreach( rand_arr[i] ) 
        begin
            if( i > 0 )
            begin
                rand_arr[i] = x3_x_1_next_lfsr_val( rand_arr[i-1] );
            end
        end
    endfunction

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_arr : %p", R.rand_arr );
        end
    end

endmodule

    class ex;

    rand bit rand_arr [8][8]; // Horizontal, Vertical

    rand bit [7:0] h [8];
    rand bit [7:0] v [8];

    constraint board_conditions {
        foreach( h[i] ) {
            $countones(h[i]) == 1;
            $countones(v[i]) == 1;
        }

        foreach( v[i] ) {
            foreach( v[j] ) {
                v[j][i] == h[i][j];
                rand_arr[i][j] == h[i][j];
            }
        }
    }

    function void print_board();
        int t;

        t = 0;
        
        foreach( rand_arr[i] )
        begin
            foreach( rand_arr[j] )
            begin
                if( t != i )
                begin
                    $display( "" );
                    t = i;
                end
                $write( "%1d ", rand_arr[i][j] );
            end
        end
        $display( "\n" );
    endfunction

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            R.print_board();
        end
    end

endmodule
    Locked Solution 
    class ex;

    rand bit [31:0] rand_arr [];
    rand int arr_size;

    constraint soft_default {
        rand_arr.size == 1;

        soft arr_size < 100; // Modifiable
        arr_size > 0;
    }

    function x3_x_1_next_lfsr_val( int start_seed ); // x3+x+1
        bit [2:0] ini;
        bit [2:0] ini_temp;

        ini = start_seed;
        ini_temp[2] = ini[2];
        ini_temp[1] = ini[1];
        ini_temp[0] = ini[0];
        
        ini[2] = ini_temp[0];
        ini[1] = ini_temp[2];
        ini[0] = ini_temp[1] ^ ini_temp[0];

        return ini;
    endfunction

    function void post_randomize();
        // Generate a sequence of x3+x+1 based on rand_arr[0]'s value
        rand_arr = new[arr_size](rand_arr);

        $display( "Initial Value : %p", rand_arr );

        foreach( rand_arr[i] ) 
        begin
            if( i > 0 )
            begin
                rand_arr[i] = x3_x_1_next_lfsr_val( rand_arr[i-1] );
            end
        end
    endfunction

endclass

module top;

    ex R;

    initial
    begin
        R = new();

        repeat( 10 )
        begin
            R.randomize();
            $display( "R.rand_arr : %p", R.rand_arr );
        end
    end

endmodule
    Locked Solution 
    class practice;
    rand bit[31:0] a;
    rand int count[32];
    rand int b;

    constraint set_count {
        foreach(count[i])
        {
        count[i]==a[i];
        }
    }

    constraint set_b {
        count.sum()%2==0 ;
        b == count.sum() ;
    }
endclass

module test;
    practice p;

    initial
    begin
        p=new();
        assert(p.randomize);
        $display("a:%0d b:%0d count:%p",p.a,p.b,p.count);
        //$display("a=%0d div=%0d",p.);
    end
endmodule

    class ran;
    rand int base_addr[];
    rand int offset[];

    /*
        Example:

        ---------- -> 1024KB ( 2^20 -> 20bits )
        |        |
        |        |
        |        |
        |        |
        |        |
        |        | -> offset[n+1]+base_addr[n+1]
        |        | -> base_addr[n+1] -> ( SEGMENT N+1 )
        |        |
        |        |
        |        |
        |        |
        |        | -> offset[n]+base_addr[n]
        |        |
        |        |
        |        | -> base_addr[n] -> ( SEGMENT N )
        .        .
        .        .
        .        .
        |        |
        ---------- -> 0B
    */

    constraint segments {
        base_addr.size == 8;
        offset.size == 8;
    }

    constraint addr {
        foreach( base_addr[i] ) {
            if( i > 0 ) {
                base_addr[i] >= base_addr[i-1]+offset[i-1];
            }
            base_addr[i] >= 0 && base_addr[i] < 2**20;
            offset[i] >= 0 && offset[i] < 2**20;
        }
    }
endclass

    class ran;
    rand int base_addr[];
    rand int offset[];

    constraint segments {
        base_addr.size == 8;
        offset.size == 8;
    }

    constraint addr {
        foreach( base_addr[i] ) {

            base_addr[i][1:0] == 2'd0; // All generated addresses are 4Byte aligned
            offset[i] >= 16; // Minimum of 16Bytes

            if( i > 0 ) {
                base_addr[i] >= base_addr[i-1]+offset[i-1];
            }
            base_addr[i] >= 0 && base_addr[i] < 2**20;
            offset[i] >= 0 && offset[i] < 2**20;
        }
    }
endclass
    Locked Solution
    Locked Solution 
    class cons;
	rand int a[];
	constraint x{a.size==10;}
	constraint y{foreach(a[i])
					if(i%2==0)
						a[i]==0;
					else
						a[i]==1;}
	function void post_randomize();
		$display("Randomized data is %0p",a);
	endfunction
endclass

cons c;
module top;
	initial 
		begin
			c=new();
			assert(c.randomize());
		end
endmodule

    class cons;
	rand int a[];
	constraint size{a.size==6;}
	constraint x{foreach(a[i])
					if(i%2==0)
						a[i]==(i*5)+5;
					else
						a[i]==-5*(i+1);}
	function void post_randomize();
		$display("Required pattern is %0p",a);
	endfunction
endclass

cons c;

module top;
	initial
		begin
			c=new();
			assert(c.randomize());
		end
endmodule

    class cons;
	rand int a;
	real b;
	constraint size{a inside {[1350:2570]};}
	function void post_randomize();
		b=a/1000.0;
		$display("Random number in between 1.35 and 2.57 is %0f",b);
	endfunction
endclass

cons c;

module top;
	initial
		begin
			c=new();
			repeat(10)
			assert(c.randomize);
		end
endmodule

    class cons;
    rand bit [63:0]a;
	constraint no_of_ones{$countones(a)==12;}
	//We can use constraint no_of_ones{$countbits(a,1)==12;}
	constraint x{foreach(a[i])
					if(i>0&&a[i]==1)
						a[i]!=a[i-1];}
	function void post_randomize();
        $display("Randomized 64 bit variable having 12 ones is:%0b",a);
		$display("Displaying no of ones in the variable that we randomized is %0d",$countones(a));
	endfunction
endclass

cons c;

module test;
	initial
		begin
			c=new;
			c.randomize;
		end
endmodule

    class cons;
	rand int even[];
	rand int odd[];
	//function for doing the factorial
	function int fact(int i);
		if(i==0)
			fact=1;
		else
			fact=i*fact(i-1);
	endfunction
	//constraint 
	constraint size{even.size==5;}
	//for even numbers factorial
	constraint even_fact{foreach(even[j])
							even[j]==fact(2*(j+1));}
	//for odd numbers factorial
	constraint odd_fact{foreach(odd[k])
							odd[k]==fact((2*k)+1);}
	function void post_randomize();
		$display("Factorial of 1st five even numbers is %0p",even);
		//$display("Factorial of 1st five odd numbers is %0p",odd);
	endfunction
endclass

cons c;

module test;
	initial
		begin
			c=new;
			c.randomize;
		end
endmodule

    class cons;
	rand int eo[];
	constraint size{eo.size inside {[10:20]};}
	constraint range{foreach(eo[i])
						eo[i] inside {[1:100]};}
	constraint x{foreach(eo[i])
					if(i%2==0)
						eo[i]%2==1;
					else
						eo[i]%2==0;}
	function void post_randomize();
		$display("After randomizing data is %0p",eo);
	endfunction
endclass

cons c;

module test;
	initial
		begin
			c=new;
			c.randomize;
		end
endmodule