Custom instructions are user-added hardware blocks that augment the arithmetic logic unit (ALU) of a CPU. Nios II processors fully support the use of custom instructions, allowing you to fine-tune your system hardware to meet performance goals. You can create up to 256 custom instructions per Nios II processor core used in the system. Similar to native Nios II instructions, custom instruction logic can take values from up to two source registers and optionally write back a result to a destination register.