Re: fractional PCs

On Mon, 28 Apr 2025 2:32:52 +0000, Robert Finch wrote:

On 2025-04-27 4:53 p.m., MitchAlsup1 wrote:

On Sun, 27 Apr 2025 11:36:05 +0000, Robert Finch wrote:
>

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.

>
Why not just backup to the instruction boundary ??

>
I think I was worried about an instruction disabling interrupts or
causing an exception that should be processed before the interrupt
occurred. (keeping the interrupt precise). I did not want to need to
consider other exceptions that might have occurred before the interrupt.
>
Searching for an instruction boundary in either direction is I think
more logic than just recording the micro-op number.

You say your interrupt-PC is fixed point so it can point at the
micro-Op that raised the exception (or was interrupted). It
seems to me that simply wiping the fractional bits from the PC
should put you at the instruction boundary. That is:: Round Down.

                                                    It is more FFs to
record the number, but fewer LUTs. There is like 8 x10 bit comparators
plus muxes on the re-order buffer to backup to the instruction boundary
and mark an interrupts. Just recording the micro-op number is just
stuffing 3 bits into the PC, plus three bits propagated down the
pipeline (FFs). The PC has two zero bits available already.