If the expression on the left-hand side of an assignment is wider than the expression on the right-hand side, the left-hand side is padded to the left according to the following rules:
•If the right-hand expression is signed, the left-hand expression is padded with the sign bit.
•If the right-hand expression is unsigned, the left-hand expression is padded with 0 (zero).
•For unsized x or z constants only, the following rule applies:
If the value of the right-hand expression's leftmost bit is z (high impedance) or x (unknown), regardless of whether the right-hand expression is signed or unsigned, the left-hand expression is padded with that value (z or x, respectively).