[Simh] FW: pdp 11 timing -->anf10 workstation on pdp11 with throttling

[Johnny Billquist]

Well, then the first question that needs to be answered, which model of 
PDP-11 was that code expected to run on, because the results will differ 
depending on that. Also, what kind of memory? (I would guess some old, 
small core memory boards.)
The PDP-11 execution speed really does vary based on many factors, on 
real hardware...

   Johnny

On 2020-07-21 00:13, simh at swabhawat.com wrote:
> 
> 
> L.S.
> 
> Actually where this is important, is when using Pdp11 based ANF10 workstations in the Tops10 realm.
> 
> When starting up, the Anf10 software on the pdp11 sim test various devices for functionality thereby using instruction count based loops etc.
> When all the devices necessary (paper tape reader/punch, incremental plotter interface, DZ and DH multiplexors, DMS and DUP/KDP devices and DL11 interfaces) are properly verified, it cranks up the communication configuration with  scanning the network for active Pdp10 Tops10 host systems.
> The throttling of the pdp11 should be carefully selected to let this function.
> 
> 
> Reindert
> 
> 
> -----Original Message-----
> From: Simh [mailto:simh-bounces at trailing-edge.com] On Behalf Of Johnny Billquist
> Sent: Monday, 20 July, 2020 23:20
> To: Paul Moore <paulmoore100 at hotmail.com>; simh at trailing-edge.com
> Subject: Re: [Simh] pdp 11 timing
> 
> 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