Re: fractional PCs

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 ??

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.

Also note: this is "completion" interrupt model used in 010, 020, 030
{Not sure about 040}.
It caused:
a) a bunch of bugs
b) a lot of strange stack state
c) which could be attacked by any thread with privilege.
Although it "sounds" like it saves {time, energy, forward progress}
the state saving/restoring on the stack pretty much consumes all of
that.