Difficulty: Easy · Puzzle 2 of 6 · Topic: randomize() with Puzzles
What len values are possible and with what probabilities under tuple counting?
class pkt;
rand bit is_write;
rand bit [3:0] len;
constraint c_base { len inside {[1:8]}; }
endclass
pkt p = new();
assert(p.randomize() with {
is_write == 0;
if (is_write) len inside {[1:2]};
else len inside {[7:8]};
});Given is_write==0, only else branch constrains len.
1) Inline fixes is_write to 0.
2) Therefore only else branch applies: len in {7,8}.
3) Base len 1..8 intersects to {7,8}.
4) Two legal tuples remain, so tuple-count gives 50/50 for len 7 and 8.Answer: Only len=7 or 8 is legal, each at 50% under equal tuple weighting.
Candidates sometimes enforce both if and else branches, which is incorrect.
If is_write were unconstrained, what is P(is_write==1) under tuple counting?