import struct


def addHeader(opcode, raw_instr):
    """Adds the protocol's header to the given instruction

    Args:
        opcode (numeric): instruction opcode ID
        raw_instr (string): binary data of the wanted instruction
    Return Value:
        string containing the complete serialized instruction
    """
    return struct.pack( "!HL", opcode, len( raw_instr )) + raw_instr