Re: fractional PCs

Representing the PC as a fixed-point number because it records which micro-op of the micro-op stream for an instruction got interrupted. It was easier to restart the micro-op stream than to defer interrupts to the next instruction.
The lead micro-ops on a interrupt return are just NOP'd out. ATM there is no micro-op state that needs to be saved and restored through context switches, other than the index into the stream which can be saved as part of the PC.