5.5 Support for values x and z
The value x may be used as a primary on the RHS of an assignment to indicate a don’t care value for synthesis.
The value x may be used in case item expressions (may be mixed with other expressions, such as 4’b01x0) in a casex statement to imply a don’t care value for synthesis.
The value x shall not be used with any operators or mixed with other expressions.
The value z may be used as a primary on the RHS of an assignment to infer a three-state driver as described in 5.4.
The value z (or ?) may be used in case item expressions (may be mixed with other expressions, such as 4’bz1z0) for casex and casez statements to imply a don’t care value for synthesis.
The value z shall not be used with any operators or mixed with other expressions.