51. Opcode Decoder

module alu_decoder (
    input  [1:0] opcode,
    output reg [3:0] alu_op
);
    always @* begin
        case (opcode)
            2'b00: alu_op = 4'b0001;
            2'b01: alu_op = 4'b0010;
            2'b10: alu_op = 4'b0100;
            2'b11: alu_op = 4'b1000;
            default: alu_op = 4'b0000;
        endcase
    end
endmodule

💡Remember

if–else = priority (first true wins). case = parallel decode (exact match).
Always include a default (or final else) to avoid latches.
For decoders/opcodes/FSM state → prefer case (flatter, faster).
case is more readable than long if–else chains, especially when checking multiple exclusive values.
Synthesis tools map case directly to efficient decoder hardware, while if–else infers cascaded multiplexers (slower and larger).
Using case makes intent clear: you want one-hot decode, not priority logic.