Randc behavior using dist operator

SystemVerilog
1

Hi All,

consider the following example:

rand bit [1:0] a;

  constraint c1 {
    a dist {
      0:=1,
      1:=1,
      2:=1,
      3:=1};
}

  constraint c2 {
    a dist {
      0:=25,
      1:=25,
      2:=25,
      3:=25};
}

I want to get all the possible values for a (0 1 2 3), but it was not successful with above constraints C1 or C2.

Is it possible to get a randc behavior, where I will get all posssible values when I do randomization 4 times by giving weightage?

Thank You,

2

The probability of getting all possible values (0,1,2,3 in any order) with just 4 calls to randomize() is 24/256 = 9.375%.

When you randomize a 2-bit variable 4 times, there are 256 outcomes. Only 24 of those outcomes give you randc behavior.

3

Thank you Dave for the reply, yes it worked.

Also, request you to please suggest the same for;

3 bit rand variable doing randomization for 8 times to get all possible values 0 to 7.
According to my calculation, total outcomes = 4096
80/4096 = 1.953125 %

But some values are getting repeated, I feel my probability/weightage calculation is incorrect. Could you please tell me what is the weightage to be used for all 8 values?

Thank You,

4

Randomizing a 3-bit number 8 times is 24 random bits: 224 = 16,777,216 possible outcomes. 8! (factorial) = 40,320 of those outcomes do not have numbers that repeat. That means you have 99.97% chance of having a repeating number.

You only get the distributions you specify with the dist constraint as the number of randomizations approach infinity.