What Is the Difference Between Verilog! and ~?
So It Ended up That the Bug That Had Kept Me on for Days, Was a Section of Code That Should Have Evaluated to False Evaluating to True. My Initial Code Went...
So it ended up that the bug that had kept me on for days, was a section of code that should have evaluated to False evaluating to True. My initial code went something like:
if(~x && ~y) begin
//do stuff
end
i.e. If x is NOT ONE and y is NOT ONE then do stuff. Stepping through the debugger, I realized even though x was 1 the expression in the if-statement still resulted into TRUE and the subsequent code was executed.
However, when I changed the statement to:
if(x == 0 && y == 0) begin
//do stuff
end
and also tried:
if(!x && !y) begin
//do stuff
end
the code within the if-statement was not evaluated which was the expected behaviour. I understand that ~ is a bitwise negation and ! a logical negation, but shouldn't (~x && ~y) and (!x && !y) evaluate to the same thing? I'm afraid the codebase is too large, so I can't paste it here, but this was the only alteration I made to make the code to work as I intended. Thanks.
In response, to one of the comments below, I have created a test-case to test this behaviour:
`timescale 10ns/1ns
module test_negation();
integer x, y;
initial begin
x = 1; y = 0;
if(~x && ~y) begin
$display("%s", "First case executed");
end
if(!x && !y) begin
$display("%s", "Second case executed");
end
if(x == 0 && y == 0) begin
$display("%s", "Third case executed");
end
end endmodule
And strangely enough, "First case executed" is printed to confirm the original behaviour I observed.
2 Answers
I see. The variable "x" in the above code was a Verilog integer (integer x;). However, an integer variable is represented by Verilog as a 32-bit integer number. So even though x was "1" as I had observed, ~x will not result in "0" but in "11111111111111111111111111111110"! And so it's no surprise that the First Case was executed. My fault. Thanks for all the answers.
~ is a bit-wise operator and returns the invert of the argument.
! is a logical operator and returns a single bit.
Example:
reg [7:0] bit_wise, logic_op;
initial begin
bit_wise = ~8'hA1; // bit_wise == 8'h5E
logic_op = !8'hA1; // logic_op == 8'b00
$display("bit_wise:%h logic_op:%h", bit_wise, logic_op); // bit_wise:5e logic_op:00
end
For your example:
if(~x && ~y) begin
//do stuff
end
Is effectively the same as:
if(x!='1 && y!='1) begin // '1 means the with of x all 1s and the with of y all 1s
//do stuff
end
Generally the best coding style is to use logical operators inside if statements. Only use bit-wise operators with data assignment manipulations.
Marcus Vance is a cybersecurity auditor and technology writer dedicated to educating the public about online safety, data privacy regulations, enterprise security, and emerging cyber threats.
