1.1.1. Custom Instruction Hardware Implementation
A Nios II custom instruction logic block interfaces with the Nios II processor through three ports: dataa, datab, and result.
The custom instruction logic provides a result based on the inputs provided by the Nios II processor. The Nios II custom instruction logic receives input on its dataa port, or on its dataa and datab ports, and drives the result to its result port.
The Nios II processor supports several types of custom instructions. The figure above shows all the ports required to accommodate all custom instruction types. Any particular custom instruction implementation requires only the ports specific to its custom instruction type.
The figure above also shows a conduit interface to external logic. The interface to external logic allows you to include a custom interface to system resources outside of the Nios II processor datapath.