Instruction set extension is a common way to improve the performance of embedded processors. During my work the Microblaze synthesizable processor was used. It has a special high-speed bus called fast simplex link (FSL) enabling to develop multiprocessor systems and to integrate customized IP cores into the design. In the Xilinx usage scenario the operands of the special instruction are transmitted through the FSL link. To achieve higher data transfer rate the FSL was used only to control the function of the custom IP while the operands required for the operation read directly from the on-chip data memory.