[Simh] Questions regarding future simulator development

[Mark Pizzolato - Info Comm]

Hi Christian,

On April 10, 2013 at 9:28 AM, Christian Gauger-Cosgrove wrote:
> Hello,
> 
> I have a few quick questions about what could be added to SIMH. Not that I
> could add the features, my skills at C are not quite excellent, so in general, I
> wouldn't really be able to add anything to the project... or it would be better
> to say, I wouldn't be able to add anything remotely competently.
> 
> Any way on to the questions! How difficult, and better to ask, how possible
> would it be to add the following to SIMH at some point in the future?
> 
> 1. Some way of interacting with devices while the simulator is still running.
> What I mean would be some way of breaking to the "sim>"
> prompt without halting the simulated CPU. So one could do things like swap
> disk packs, floppy diskettes and magnetic tapes without halting the system.
> Kind of like the way the Hercules emulator let's you affect the system on the
> fly.

Anything is possible.  Someone could extend a particular simulator instance to implement a something (a GUI perhaps) which provided a way for a user to express the desire to perform one of these actions and to gather the needed details relating to the desired action.

Hooks exist in the codebase to allow a particular simulator to provide what can be reasonably described as arbitrary extensions the basic simh framework.  These extensions can include both simulator specific commands and/or arbitrarily anything else. 

> 2. Additional MASSBUS controllers and disks. What I mean is, since the actual
> PDP-11/70 could have four RH70 MASSBUS controllers installed in it, would it
> be possible to have another set of RP disks and TU tape drives added to
> SIMH? Also, would it be possible to have the RS03 and
> RS04 fixed head disks added to the RP device list? I ask only of the
> RS03 and RS04 because while they are not strictly necessary any more (being
> they were small logically, but rather quick, swap disks), it would be nice to
> have to be able to (re)create more "authentic"
> systems.

Once again, anything is possible, but this would be a significant amount of work.   If there was some formal documentation available which described the RS03 and RS04 disks, then adding them to the pdp11_rq.c code would probably not be too hard.  Looking in the 'usual places' on bitsavers.org doesn't provide any information on these disk devices though.

Meanwhile, there already exists support for many disks within the current simh codebase.  There is existing support for 4 RQDX controllers each of which can support 4 drives.  The RL controller can support up to 4 drives.  The RP can support up to 8 drives.  The HK (RK611) can support up to 8 drives.

Simulated systems can easily be configured which could have never exist in the real hardware space due to issues like 1) cost, 2) concurrency of technology, 3) power/bus loading constraints, etc.  Yes, you can't currently configure a system which could have arbitrarily been designed with real hardware, but you can also configure many system configurations which exceed many of the assumptions which existed when working with real hardware.

> 3. Would it be at all possible for SIMH to become capable of supporting a
> multiprocessor system? E.g. the PDP-15's UNICHANNEL-15 option, or the
> PDP-11/74.

Once again, anything is possible.  There are no magic bullets here.  Someone with sufficient motivation, skill and detailed information would have to make the effort to do these types of things.

- Mark Pizzolato