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, [email protected] 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:[email protected]] On Behalf Of Johnny Billquist
Sent: Monday, 20 July, 2020 23:20
To: Paul Moore <[email protected]>; [email protected]
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).
_______________________________________________
Simh mailing list
[email protected]
http://mailman.trailing-edge.com/mailman/listinfo/simh
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: [email protected] || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
_______________________________________________
Simh mailing list
[email protected]
http://mailman.trailing-edge.com/mailman/listinfo/simh
