Suppose there is a class A like below:
class A;
rand logic [3:0] a;
rand logic [3:0] b;
rand logic [3:0] c;
constraint a_const{
a<'h4;
}
constraint b_const{
b<'h4;
}
endclass
When I use :
A at = new();
at.b_const.constraint_mode(0);
assert(at.randomize());
b is also randomized. But, I don't want that.
Is there a way I can only randomize a without randomizing b and c?
Because there can be many logics in a class, sometimes I just want to rand some of them. Put some of the logics in one class like A while some in other class B is one of the solutions, but it is too complicated.
If you only want one of the rand variables in a class to be randomized, then you can pass the variable to the randomize function:
assert(at.randomize(a));
Alternately, as you mentioned in the title to your question, you can use rand_mode to disable randomization of individual class variables:
at.b.rand_mode(0);
at.c.rand_mode(0);
assert(at.randomize());
Refer to IEEE Std 1800-2017, section 18.8 Disabling random variables with rand_mode().
With either of the above approaches, only a will be randomized.
I suspect you expected b_const.constraint_mode(0) to disable randomization of variable b. That line simply disables the named constraint, leaving b unconstrained. This means that b will be randomized in your original code (which is what you observed).