How to check unknown logic in Verilog?

问题

I'm checking primality of a number in a form of 6n+1 or 6n-1. I have the below code, but it doesn't seem to be generated correct result.

    module prime(clk, rst, start, A, ready, P);

    input clk, rst, start;
    input [7:0] A;

    output ready, P;

    reg ready, P;

    reg [7:0] divisor;
    reg val;


    always @ (posedge clk or negedge rst) begin
        if (!rst) begin
            P <= 1'bx;
        end
        else if (start) 
        begin
            case (A)
               -1 : P <= 1;
                0 : P <= 1;
                1 : P <= 1;
                2 : P <= 1;
                3 : P <= 1;
                5 : P <= 1;
            endcase

            if (A%2 == 0 && A != 2) begin
                P <= 0;
                ready <= 1;
            end

            else if(A > 6) 
            begin
                for(divisor = 5; divisor <= A/divisor; divisor=divisor+6) begin
                    if (A%divisor == 0 || A%(divisor+2) == 0) begin
                        P <= 0;
                    end
                end 

                   if (P != 0 && P == 1'bx) begin // need to fix
                        P <= 1;
                    end
            end
        end
    end
endmodule

The point is that this part doesn't seem to be working:

if (P != 0 && P == 1'bx)

How can I check whether a variable includes unknown logic, meaning x. Checking as the above

P == 1'bx

doesn't seem to be working.

回答1:

You should rather use case equality operator (===), which tests 4-state logical equality, then logical equality operator (==).

For the == operator, the result is x, if either operand contains an x or a z. For the === operator bits with x and z are included in the comparison.

来源：https://stackoverflow.com/questions/24465776/how-to-check-unknown-logic-in-verilog