What is the difference between:
if (dataoutput[7:0] == 8'bx) begin
and
if (dataoutput[7:0] === 8'bx) begin
After executing dataoutput = 52'bx
, the second gives 1, but the first gives 0. Why? (0 or 1 is the comparison result.)
Some data types in Verilog, such as reg
, are 4-state. This means that each bit can be one of 4 values: 0,1,x,z.
With the "case equality" operator, ===
, x's are compared, and the result is 1.
With ==
, the result of the comparison is not 0, as you stated; rather, the result is x, according to the IEEE Std (1800-2009), section 11.4.5 "Equality operators":
For the logical equality and logical inequality operators (== and !=), if, due to unknown or high-impedance bits in the operands, the relation is ambiguous, then the result shall be a 1-bit unknown value (x).