The first thing in designing application-specific instruction set processor is having instruction set closely matching hardware characteristics. This instruction set design problem can be more complicated when cobined with implementation method selection problem of each instruction. Our processor model supports two kinds of instructions-primitive or special instructions. Primitive instructions are implemented using common multifunctional hardware such as ALU. Special instructions require a set of dedicated hardware, which actually functions as a coprocessor to the main processor. In this case, special instructions and primitive instructions can be executed independently. In this paper, we present novel algorithm for genrating special instructions for given application. Parallelism between special instructions and primitive instructions is also considered during the performance estimation stage of generated special instructions.