System Verilog: Random Number System Functions

System Verilog provides system functions – $urandom(),$urandom_range() and $srandom() for generating random numbers.

The $random verilog system function has only one random number generator shared between all threads, but each thread in simulation has its own random number generator for $urandom and $urandom_range. Separate random number generators for each thread helps to improve random stability

$urandom()
The $urandom() system function provides a mechanism for generating pseudo-random numbers. The function returns a new unsigned 32-bit random number each time it is called.

function int unsigned $urandom [ (int seed ) ] ; 

The seed is an optional argument that determines the sequence of random numbers generated.
This generates 32 bit unsigned number, but 2 bits, 4 bits etc. random numbers can be generated as shown below.

class rand_sysfn_class;

rand bit[31:0] addr1;
rand bit[63:0] addr2;
rand bit [1:0] selb;
rand bit [3:0] msel;

function new();
for(int i =0; i<5;i++) begin
  addr1 = $urandom();
  $display ("Addr1 = %h ",addr1);
  
  addr2 ={$urandom(),$urandom()};
  $display ("Addr2 = %h ",addr2);

  selb = $urandom & 7;  //2 bit random number
  $display ("Selb = %b ",selb);
  
  msel = $urandom & 15; //4 bit random number 
  $display ("msel = %b ",msel);
  
end
endfunction
endclass

program p1;
rand_sysfn_class c1 = new();
endprogram

/*
///////////////////
Addr1 = d7ab96af 
Addr2 = 20186340ff61aafe 
Selb = 00 
msel = 1011 
Addr1 = ebbca2d7 
Addr2 = e7f59ecffb84c2f7 
Selb = 01 
msel = 1111 
Addr1 = 96baee2d 
Addr2 = f0a8dce11e18993c 
Selb = 01 
msel = 0000 
Addr1 = 8921cc12 
Addr2 = 124f7d245177c3a2 
Selb = 01 
msel = 1100 
Addr1 = 6d4439da 
Addr2 = 925968242e00b55c 
Selb = 10 
msel = 1100 

*/

$urandom_range()
The $urandom_range() function returns an unsigned 32 integer within a specified range. Range would be specified using max and min values as shown below

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

In the below code addr1 range is from 32'hFFFF_FF00 to 32'hFFFF_FFFE. So it generated following values in 5 iterations.

 Addr1 = ffffff24 -> Addr1 = ffffffeb -> Addr1 = ffffff59 -> Addr1 = ffffff4c -> Addr1 = ffffffbf 

class rand_range_sysfn_class;
rand bit[31:0] addr1;
rand bit[63:0] addr2;

function new();
for(int i =0; i<5;i++) begin
  addr1 = $urandom_range(32'hFFFF_FFFE,32'hFFFF_FF00);
  $display ("Addr1 = %h ",addr1);
  
  addr2 ={$urandom_range(32'h0000_FFFF,32'h0000_0000),$urandom_range(32'h0000_000F,32'h0000_0001)};
  $display ("Addr2 = %h ",addr2);
  
end
endfunction
endclass
///////////////////
program p2;
rand_range_sysfn_class c2 = new();
endprogram
/*
Addr1 = ffffff24 
Addr2 = 0000fabd00000001 
Addr1 = ffffffeb 
Addr2 = 0000b7a10000000c 
Addr1 = ffffff59 
Addr2 = 00005e060000000f 
Addr1 = ffffff4c 
Addr2 = 000088430000000e 
Addr1 = ffffffbf 
Addr2 = 0000ba0b0000000a 
*/

srandom()
The srandom() method allows manually seeding the RNG of objects or threads.

function void srandom( int seed );

The srandom() method initializes an object’s RNG using the value of the given seed.

In the below code 5 iterations are done without setting a seed using $srandom(). So different random numbers are generated. But once we set srandom(1), RNG will generate same random numbers. This will change once we change the seed.

class srand_sysfn_class;
rand bit [31:0] addr;

function new();
for(int i =0; i<5;i++) begin
  addr = $urandom();
  $display ("addr = %h ",addr);
end

for(int i =0; i<5;i++) begin
  $srandom(1);
  addr = $urandom();
  $display ("addr - with srandom = %h ",addr);
end
endfunction
endclass

program p1;
 srand_sysfn_class c3 = new();
endprogram

/*
Without setting a seed using $srandom() 
addr = d7ab96af 
addr = ff61aafe 
addr = 20186340 
addr = 5075bfa0 
addr = 25afd34b 

Setting a seed using $srandom()
addr - with srandom = 80010e00 
addr - with srandom = 80010e00 
addr - with srandom = 80010e00 
addr - with srandom = 80010e00 
addr - with srandom = 80010e00 
*/