> On Jul 20, 2020, at 5:10 PM, Paul Moore <paulmoore...@hotmail.com> wrote:
>
> (I am writing my own emulator just because I have never done that before, and
> the PDP 11 is such a pivotal system in the history of modern computing it
> seemed worth learning about, and what better way to learn than to emulate it )
>
> So how important is timing of instruction execution and device response?
>
> The PDP 11 docs go to great length giving instruction timing. But the fact
> that there is a % throttle in simh suggest that’s not important. I assume
> that turning that throttle up and down makes the emulated CPU go faster and
> slower. I have seen code using simple counters as delays but I assume that if
> you want precision you use the Kw11.
>
> With regards device responses I have found that going ’too fast’ upsets code.
> If they do something that triggers an interrupt (set ‘go’ for example) and
> the interrupt arrives too soon (like before the next instruction) they get
> surprised and can misbehave (you could argue that’s a bug, but that’s
> irrelevant). So always wait a few beats. But I assume there is no reason to
> try to precisely emulate the timing of , say, a disk drive. (The early
> handbooks state how awesome the async nature of the IO subsystem is cos you
> can swap out old for new and things just go faster).
For the most part that should work fine. The one exception I can think of is
DECtape. Driving one of those involves doing RNUM (read block number)
operations to look for the desired block, then switching to the read or write
data operation to do the actual I/O. If a block goes by too fast, that won't
work. Related: RT-11 has contiguous files, and DECtape I/O should be able to
see the consecutive blocks without overshooting, i.e., after block completion
the next action is another RNUM (I believe) which should see the next block.
I don't think any PDP-11 OS does timing based block position prediction
("overlapped seek") on DECtape, the way TOPS-10 and (reported) VMS do. For
that to work the timing has to be rather more accurately emulated.
Lastly, I don't know if anyone expects RT-11 FILEX reading of TOPS-10 tapes to
work in emulation. If I remember right, that does a rather strange thing: DMA
the block to get the bottom 16 bits of each word, while watching the extended
data register to get the upper 2 bits as the words fly by. It doesn't use RALL
which would have been a cleaner solution. I think Anton said he didn't think
of that, which seems hard to believe.
paul
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