SystemVerilog Randomize Array

Table of Contents

Most of the array usage application needs randomization of an array. randomization is possible for array size as well as for array elements.

constrained randomization of array

It is possible to get the specific value on randomization, this can be achieved by writing/specifying the constraints.
During randomization, constraints of size are solved first, and then the elements constraints.

Array randomization is applicable to all the array types, The below section describes examples on array randomization and using array methods in constrained randomization.

Fixed Size Array Randomization

In a fixed size array, randomization is possible only for the array elements. as the size is fixed, it is not possible to change.

Generating random value for array elements.

In the below example, random values will be generated for array elements.

Declare array as rand
On randomization, the array will get random values

class fs_array;
  rand bit [3:0] array1[4];
  rand bit [7:0] array2[6];
  
  function void display();
    $display("array1 = %p",array1);
    $display("array2 = %p",array2);
  endfunction
endclass

program fixedsize_array_randomization;
  fs_array pkt;

  initial begin
    pkt = new();
    pkt.randomize();
    pkt.display();   
  end
  
endprogram

Simulator Output

array1 = '{'he, 'h4, 'h4, 'h8} 
array2 = '{'h9b, 'h9a, 'h10, 'h5f, 'hde, 'h84}

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Generate unique elements in an array

In the above example, we have seen randomization with random values. The below example shows the randomization with unique values by using the shuffle array method.

Constrain array with element value same as an index value
In post randomization shuffle the array, so that array will not have incremental value.

class fs_array;
  rand bit [7:0] array1[6];
  
  constraint array_c { foreach(array1[i]) array1[i] == i;}
  
  function void post_randomize();
    array1.shuffle();  
  endfunction
  
  function void display();
    $display("array1 = %p",array1);
  endfunction
  
endclass

program fixedsize_array_randomization;
  fs_array pkt;

  initial begin
    pkt = new();
    pkt.randomize();
    pkt.display();   
  end
endprogram

Simulator Output

array1 = '{'h2, 'h1, 'h4, 'h0, 'h5, 'h3}

Click to execute on

array sum constraint

In the below example, the array is randomized in such a way that the sum of all the elements equals 30.

declare an array with rand.
Constraint sum of an array using the array method sum().

class fs_array;
  rand bit [7:0] array1[6];
  
  constraint array_c { array1.sum() == 30;}
  
  function void display();
    $display("array1 = %p",array1);
  endfunction
  
endclass

program fixedsize_array_randomization;
  fs_array pkt;

  initial begin
    pkt = new();
    pkt.randomize();
    pkt.display();   
  end
endprogram

Simulator Output

array1 = '{'h7, 'h0, 'h17, 'h0, 'h0, 'h0}

Click to execute on

Array sum constraint with array elements

In the previous example, only the sum of array elements is considered, array elements can take any value. But in the below example array sum and also the value of each element is constrained.

declare an array with rand
Constraint sum of an array using the array method sum().
Constraint array elements value.

class fs_array;
  rand bit [7:0] array1[6];
  
  constraint array_c { array1.sum() == 30;}
  constraint array_c_e { foreach(array1[i]) array1[i] > 2;}
  
  function void display();
    $display("array1 = %p",array1);
  endfunction
  
endclass

program fixedsize_array_randomization;
  fs_array pkt;

  initial begin
    pkt = new();
    pkt.randomize();
    pkt.display();   
  end
endprogram

Simulator Output

array1 = '{'h7, 'h3, 'hb, 'h3, 'h3, 'h3}

Click to execute on

Multidimensional

The below example shows the declaration and randomization of a multidimensional array with constraints.

cclass fs_array;
  rand bit [7:0] array1[3][4];
  
  constraint array_c { foreach(array1[i,j]) array1[i][j] == i*j;}
  
  function void display();
    $display("array1 = %p",array1);
  endfunction
  
endclass

program fixedsize_array_randomization;
  fs_array pkt;

  initial begin
    pkt = new();
    pkt.randomize();
    pkt.display();   
  end 
endprogram

Simulator Output

array1 = '{'{'h0, 'h0, 'h0, 'h0}, '{'h0, 'h1, 'h2, 'h3}, '{'h0, 'h2, 'h4, 'h6}}

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A multidimensional array with sum method

In below example sum of an array, elements is constrained.

cclass fs_array;
  rand bit [7:0] array1[3][4];
  
  constraint array_c { array1.sum() == 30;}
  
  function void display();
    $display("array1 = %p",array1);
  endfunction
  
endclass

program fixedsize_array_randomization;
  fs_array pkt;

  initial begin
    pkt = new();
    pkt.randomize();
    pkt.display();   
  end 
endprogram

Simulator Output

array1 = '{'{'h0, 'h0, 'h16, 'h0}, '{'h0, 'h0, 'h0, 'h0}, '{'h0, 'h0, 'h8, 'h0}}

Click to execute on

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