RT11 distribution comes with (uncommented) sources.
Johnny
On 2020-07-21 00:31, Paul Moore wrote:
At the moment my ambitions are very lightweight. A pdp 11/20 with a cassette
drive (why that? cos CAPS11 is the first sw listed on the simh sw kit page).
And next is an RK11 with rk05. So I can run RT11 (the second thing on that
page).
The point that I am hearing is that, in general , the PDP11 sw doesn’t rely on
timing , there are a few corner cases tho. Contrast this with other systems
where precise knowledge of video flyback times are built into the core of the
OS for example. Or timing is achieved by looping instruction x n times to
produce an exact delay.
BTW - does anybody have the source of the RT11 on the simh kit site? I got the
source of CAPS11 from Lou Ernst and it was a life saver. I could not have
progressed without it.
-----Original Message-----
From: Simh <simh-boun...@trailing-edge.com> On Behalf Of s...@swabhawat.com
Sent: Monday, July 20, 2020 3:14 PM
To: Simh@trailing-edge.com
Subject: [Simh] FW: pdp 11 timing -->anf10 workstation on pdp11 with throttling
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-boun...@trailing-edge.com] On Behalf Of Johnny Billquist
Sent: Monday, 20 July, 2020 23:20
To: Paul Moore <paulmoore...@hotmail.com>; simh@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: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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