[Simh] EXT : Questions regarding future simulator development

[Christian Gauger-Cosgrove]

On 10 April 2013 13:48, Hittner, David T (IS) <david.hittner at ngc.com> wrote:
> To answer your questions:
>
> 1> Yes, you could detach the command channel from the simulation loop by putting the command channel in a different thread. The problem then becomes one of synchronization between the two threads to do the [limited] amount of things that should be allowed while the simulation loop is still running. You would also need to put in options to enable/disable command-channel threading - some people do not want to change the removable media "on the fly", and some older host systems do not support pthreads. Someone may have already submitted this code, or it may be in the next version of SIMH (4.0), it would be best to check with Mark.
>
That makes a lot of sense, and I did know that you would have to put
the core simulator and simulator-command system on separate threads, I
was just wondering would that be something that requires massive
rewrites to the entire simulator codebase?


> 2> Assuming that the MASSBUS hardware devices actually supported the extended configuration, you could certainly add the extra components into the configuration. The question then becomes do any of the operating systems that ran on the original hardware support the extended configuration? If the operation systems don't support the extensions, then adding the extra simulated components is a waste of time. Otherwise, you just have to ask yourself "Is it worth the effort?" Note that any new components added to a simulation should be disabled by default for compatibility with older scripts.
>
Yes, you could have four RH-whatever MASSBUS controllers on a single
system, just go look at the PDP-11/70, it can quite easily support
four RH70s, or a third party controller in the place of the RH70, as
apparently there are some third-party boardsets that can slot into the
RH70's slots on the 11/70's backplane.

I think, though I am not completely sure, that one of the various UNIX
systems supports multiple MASSBUS channels (I want to say it is V7M,
which could be built in such a way as to have it's "/" drive on one
channel, while using a second MASSBUS disk controller channel for a
swap disk, with yet another channel as tape).


> 3> Yes. There is an unreleased VAX simulator variant in the wild with 32 processor support. It would certainly be possible to run additional CPU simulator loops on other threads - you just have to add synchronization support for the system shared resources - I/O bus, system bus, memory, etc. However, in the case of the PDP 11-11/74, you have to ask yourself the question "Is it worth the effort to add multi-cpu support to more "correctly" simulate a PDP-11/74?" since the current PDP-11/70 simulation already outperforms the fastest hardware ever made. :-) Not knowing what the PDP-15 UNICHANNEL-15 is, I can't comment on it.
>
You'd also have to worry about the Interprocessor Interrupt and Sanity
Timer for the 11/74; and in terms of the question of whether it is
worth it, while yes a simulated 11/70 is signficantly more capable
then the fastest '11, but how else can one "experiment" with an 11/74?
(Yes, yes, we know E-11 exists, *AND* it supports two different
methods of multiple processor PDP-11, both the 11/74, and the "let's
bridge the memory (FASTBUS) and normal (UNIBUS) buses on one PDP-11/45
and plug it into the UNIBUS of a second 11/45" style machine. But E-11
is expensive, if I wanted to experiment with systems on very large
storage (read: RM05).)

Also, UNICHANNEL-15 which you can find on BitSavers was a small PDP-11
(11/05 if I remember correctly) "glued" to a PDP-15, letting the
PDP-15 make use of RK05 disks and LP11 printers. (I think the UC15
system also offered other peripherals to be accessible to the PDP-15's
operating systems, but I only remember the RK05 and LP11.) XVM/DOS and
XVM/RSX support the UNICHANNEL devices, going by the XVM/RSX SYSGEN
prompts.


Thanks for taking the time to answer my mail Mr. Hittner!


Cheers,
Christian