OK, looks like this isn't so trivial to solve. The code really likes having something in chan->engn[] so that it can get the address. The old code just stuck a "4" in if it was a non-GR class. I wonder if the current code should check for ->engn[] != null, and if it's null, use a 4 there for the inst address. This will need consultation with Ben, as this is well outside my knowledge area.