Difficulty: Medium · Puzzle 1 of 6 · Topic: Race Condition Puzzles
Predict post-NBA values of x and y on each edge. Is behavior deterministic across simulators?
module tb;
bit clk = 0;
int x = 0;
int y = 0;
always #5 clk = ~clk;
always @(posedge clk) begin
x = x + 1;
y <= x;
end
always @(posedge clk) begin
x = x + 10;
end
initial begin
repeat (2) begin
@(posedge clk);
$strobe("[%0t] x=%0d y=%0d", $time, x, y);
end
$finish;
end
endmoduleBoth blocks write x in active with no ordering guarantee.
1) x ends each edge as old_x + 11 regardless of order.
2) y<=x samples x in whichever intermediate active state Process A sees.
3) If A runs first on first edge, y becomes 1; if B runs first, y becomes 11.
4) Same ambiguity repeats each edge, so y is race-dependent.Answer: x is deterministic per edge (+11), but y is nondeterministic because it samples race-dependent active-state x before NBA update.
Using NBA on y does not sanitize the active-region race on x.
How would you refactor to a single writer while preserving intended math?