Which Dec Emulation is the MOST useful and Versatile?

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Oct 26 14:16:11 CDT 2017

    > From: Kip Koon

    > I was initially thinking of a strictly software only solution

Whatever you eventually do in the way of hardware, it might be a good idea to
start with this. You can get familiar with whatever OS you decide to go with,
and get used to its tools, get to know the instruction set of that
machine, etc, etc.

So then, if you do do a hardware project, it won't be such a big gulp, and
you'll have the knowledge base covering all the above already there to draw

    > which still presents a problem for me and that is which PDP do I teach
    > myself and set up.

Probably the way to answer that is, if you're going to build hardware at some
point, a combination of 'what's out there that I can get to talk to', and
'how complicated a beast are we talking about'.

For the first, there's a lot of QBUS stuff around, some UNIBUS, and basically
zilch on the PDP-10 or PDP-15 front. For the second, most -11's (both QBUS
and UNIBUS) are relatively simple and straightforward. Any kind of PDP-10 is
pretty complex (depending on if you emulate the original busses, or not).

    > 3rd, and this is a big factor in the choice of DEC PDP computer to pick
    > for simulation or emulation and that is the small cash flow and itty
    > bitty storage space I have available to me.


    > The choice so far it seems is the PDP-11/70.

If all you're doing is simulation (software), the -11/70 would be fine. It's
no more work to set up than one of the other timesharing-capable models; it's
only slightly more complicated than say, an -11/45, _from the programmer's
point of view_ (there's a UNIBUS map as well as the usual memory mapping
hardware), but if you're running an existing OS, that should not affect you.

    > Remember I still have no idea ... what boards and peripherals
    > a PDP-11/70 consists of.

Hardware-wise, the -11/70 could be a complex project - it depends on exactly
how much you try and emulate, a full emulation could be a very complex
undertaking indeed.

The thing is that while the /70 looks to the programmer a lot like one of the
simpler models, the hardware is quite a lot more complicated: there is a
cache, a separate memory bus, high-speed I/I controllers with their own
special bus to the devices (MASSBUS), etc. It's basically an -11/45 with a
bunch of extra stuff glued onto the sides of it to boost the performance; the
board count went from 10 (w/o floating point, which adds an extra 4) to a
minimum of of 16 (w/o FP), plus 4 for each high-speed I/O controller (up to

Now, if all you're doing is emulating the system, _without_ providing any of
the busses, no problem; all that complexity is hidden inside the simulator.
But once you start emulating real busses (i.e. to be able to plug in real
hardware) - whole different kettle of fish.


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