Randomize Variable in SystemVerilog

How to randomize a variable in SystemVerilog?

SystemVerilog provides multiple methods to generate random data. By using any of these methods a variable can be randomized.

Systemverilog randomization methods

  • $urandom( ) and $random( )
  • $urandom_range( )
  • std::randomize():
  • randomize():

$urandom( ) and $random( )

The $urandom( ) function returns a new 32-bit random number

variable = $urandom(seed); //seed is an optional argument

$random() is same as $urandom() but it generates signed numbers

$urandom_range( )

The $urandom_range() function returns an unsigned integer within a specified range.

variable = $urandom_range( int unsigned maxval, int unsigned minval = 0 );

Below example shows the usage of urandom and urandom_range.

module system_funcations;
  bit [31:0] addr1;
  bit [31:0] addr2;
  bit [64:0] addr3;
  bit [31:0] data;
  initial begin
    addr1 = $urandom();
    addr2 = $urandom(89);
    addr3 = {$urandom(),$urandom()};
    data  = $urandom * 6;

    $display("addr1=%0d, addr2=%0d, addr3=%0d, data=%0d",addr1,addr2,addr3,data);
    addr1 = $urandom_range(30,20);
    addr2 = $urandom_range(20); //takes max value as '0'
    addr3 = $urandom_range(20,30); //considers max value as '30' and min value as '20'
    $display("addr1=%0d, addr2=%0d, addr3=%0d",addr1,addr2,addr3);

Simulator Output

addr1=303379748, addr2=2153631232, addr3=423959822444962108, data=546103870
addr1=27, addr2=6, addr3=25

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Variables can be randomized by using std::randomize method. It can accept the inline constraints using the “with” clause.

std::randomize (variable);

Can also be used as below,

std::randomize (variable);
std::randomize (variable) with { constraint's; };

std::randomize (variable-1, variable-2 ... variable-n);
std::randomize (variable-1, variable-2 ... variable-n) with { constraint's; };

Below example shows the usage of std::randomize().

program std_randomize;
  bit [07:0] addr;
  bit [31:0] data;
  bit [63:0] data_x_4;

  initial begin
    $display("Value of addr is %h",addr);
    std::randomize(data,data_x_4) with { data     == addr * 8; 
                                         data_x_4 == data * 4;   
    $display("Value of data is %0d",data);
    $display("Value of data_x_4 is %0d",data_x_4);

Simulator Output

Value of addr is 36
Value of data is 432
Value of data_x_4 is 1728

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This method is used to randomize class fields declared with rand/randc. It can accept inline constraints using the “with” clause in addition to the constraints defined in a class context.

refer Randomization and Constraints for detailed description and examples of randomize() method.

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