[Simh] pdp 11 timing

[Johnny Billquist]

Instruction timing as such is not relevant. Different implementations 
had very different timings, not to mention that speed of memory also 
makes a difference.

Devices basically do not have a strict timing either, but yes, there is 
plenty of software that assumes that an interrupt does not happen before 
a single instruction have been executed after the previous interrupt, 
from the same device, for example.
On real hardware that was just an absurd case that lots of code never 
considered, since it wasn't really physically possible for it to happen.

The throttling in simh is because some people want the emulation to 
somewhat mimic the real thing. For some people, that experience of 
slowness is desirable.

   Johnny

On 2020-07-20 23:10, Paul Moore 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).
> 
> 
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-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol