Difficulty: Medium · Puzzle 3 of 6 · Topic: Datatype and Bit Puzzles
Predict `a`, `b`, and `c`. Why are `b` and `c` different from `a`?
module p3;
logic [7:0] u8 = 8'h80;
logic signed [15:0] a, b, c;
initial begin
a = u8;
b = signed'(u8);
c = $signed(u8);
$display("a=%0d b=%0d c=%0d", a, b, c);
end
endmoduleAssignment from unsigned extends with zeros. `signed'(...)` and `$signed(...)` reinterpret source sign first, then assignment extends.
1) `a = u8` zero-extends 8'h80 into 16'h0080, which is +128.
2) `signed'(u8)` treats the 8-bit pattern as signed before assignment, so 8'h80 is -128 and sign-extends to 16'hFF80.
3) `$signed(u8)` has the same signed reinterpretation effect here, so `c` is also -128.Answer: a=128, b=-128, c=-128
Many candidates assume destination signedness alone decides extension behavior.
What value would `b` have if `u8` were `8'h7F` instead?