NIOS Custom Instruction with more than two input

Up on the Altera documentation page go to the Nios II section and there should be a document called "custom instruction user guide"... or something along those lines. I suspect that docs like that might get sucked into the main software developer manual at some point so if someone reads this a year from now .... well now you know where it went :)

Like DSL and I mentioned in previous notes, if your hardware has a lot of inputs it probably makes more sense for those to live in memory and have your hardware master that memory instead. Custom instructions are good for quick computations with a small data set but they'll have diminishing returns of your software spends all it's time shoving operands to the custom instruction through a series of calls to the hardware.