Using custom instruction from uCLinux user app on nios2mmu

To correct myself ... (if the whole thing works the way I suspect it does)

You can't let a combinatorial custom set state (it can return state) as the logic is 'executed' for every instruction fetched (there is no 'enable' line to the instruction). The custom opcode (etc) only affect the value selected and written back to the register file.

Every time I think about the custom instructions, I get more and more convinced that the 'rA' and 'rB' bits are ignored by the nios cpu core.