1. A method for scheduling instructions for clustered digital signal processors, the signal processors being comprised of a plurality of clusters, each cluster having at least two functional units and a first register file having a first unit, a second unit and a single set of access ports shared by the functional units, the method comprising steps of: checking whether executing an instruction needs to data to be read from the first unit and the second unit of the first register file; generating a copying instruction to transfer data from the first unit to the second unit of the first register file if the checking result is true; and scheduling the instruction after the copying instruction.