2.1.1. Combinational Custom Instructions
A combinational custom instruction must not have side effects. In particular, a combinational custom instruction cannot have an external interface. This restriction exists because the Nios II processor issues combinational custom instructions speculatively, to optimize execution. It issues the instruction before knowing whether it is necessary, and ignores the result if it is not required.
A basic combinational custom instruction block, with the required ports shown in "Custom Instruction Types", implements a single custom operation. This operation has a selection index determined when the instruction is instantiated in the system using Platform Designer.
You can further optimize combinational custom instructions by implementing the extended custom instruction. Refer to “Extended Custom Instructions”.