52. Instruction Decoder with Don’t-Care

module opcode_decoder (
    input  [3:0] opcode,
    output reg [3:0] alu_op
);
    always @* begin
        casez (opcode)
            4'b1???: alu_op = 4'b0001; // ADD
            4'b01??: alu_op = 4'b0010; // SUB
            default: alu_op = 4'b0000; // NOP
        endcase
    end
endmodule

💡Remember

casez treats z and ? bits as don’t-cares → perfect for opcodes with wildcards.
casex also treats x as don’t-care, but ⚠️ this can mask real unknowns in simulation.
Always include default to avoid latches.
Use casez for pattern-based decoders (efficient + safe).
Avoid long if–else chains — casez expresses intent directly and synthesizes cleanly.