FP Custom Instructions - exp() won't get any faster

I've not seen any 'useful' docs ...

The only ones I found just explained how to tick the boxes to enable the FP instructions.

I've written a couple of combinatorial custom instructions, what I realised is that the nios doesn't really have an instruction 'decoder', just a great big mux that selects the required result. The A and B register values are read for every instruction, even tightly coupled data memory is read every clock - the value is discarded unless the instruction is a memory read for the required address range.

So a combinatorial instuction creates a result value every clock that can be based on all 32bits of the instruction word and the the 32bit values read from the register file.