126.96.36.199. Select Index Field (N)
The Nios II processor supports up to 256 distinct custom instructions through the custom opcode. A custom instruction component can implement a single instruction, or multiple instructions.
In the case of a simple (non-extended) custom instruction, the select index is a simple 8-bit value, assigned to the custom instruction block when it is instantiated in Platform Designer.
Components that implement multiple instructions possess an n port, as described in "Extended Custom Instructions". The n port implements an extension index, which is a subfield of the select index, as shown in the following figure.