Difficulty: Medium · Puzzle 6 of 6 · Topic: Debugging Riddles
A constraint block fails every randomize call. Find the logical contradiction.
class pkt;
rand bit [3:0] addr;
constraint c {
addr inside {[0:15]};
addr[0] == 1'b1; // odd
addr % 4 == 0; // multiple of 4
}
endclassCheck parity implied by modulo condition.
1) addr % 4 == 0 implies addr is divisible by 4, therefore LSB addr[0] must be 0.
2) Another constraint requires addr[0] == 1.
3) These cannot both hold, so solver has no legal solution.
4) Fix by relaxing one condition, e.g., use addr % 4 == 1 for odd values with fixed stride.Answer: Constraint set is unsatisfiable: odd-number requirement conflicts with multiple-of-4 requirement.
Debugging seed/tool settings before checking satisfiability algebraically.
How would you encode legal addresses as a weighted distribution instead?