[HECnet] Simplest PDP-11

[Paul Koning]

> On Feb 3, 2019, at 4:02 PM, Brian Hechinger <wonko at 4amlunch.net> wrote:
> 
> What is the simplest PDP-11 CPU/peripheral design that is capable of running RSX-11, RSTS/E or BSD?
> 
> Also, what is the minimum disk space needed to install those operating systems? I have a plan. Probably not a good one but I'll likely learn a lot in the the process so it won't be a waste of time.
> 
> -brian

I'm still wondering "simplest" in what sense?  Are you assembling an actual PDP-11 system?  Or building a simulated one in FPGAs?  Or in SIMH?  Are the peripherals involved in the question as well?

A minimal RSTS V6 or thereabouts system would be an 11/40 with EIS and at least 64 kW of memory.  For V10, I'm not sure if an 11/40 is still supported but an 11/45 would be ok, and minimal memory is 124 kW at this point.  For V6, you can fit a reasonable system on an RK05; for V10, you'll need a bit more than that I suspect but not a whole lot.  For terminals, you could attach a couple of DL-11s, or a DZ11 mux.  (DH11 also works but that is not a simple device.)  Later releases give you more options, like the DHU11 or even LAT.

Simple peripherals: a DL11 is as simple a terminal interface as you'll find.  An RK05 is a very simple disk; RK06 is similar.  RL01 is a bit more complex conceptually because of its packet interface, though not a whole lot.  MSCP disks are very nice but trying to emulate them would be a fairly large job.  As seen by software, Massbus disks are also simple; the extra bus interfaces are hardware complexity that doesn't make much difference to the software.  So if you wanted a "classic" (CSR style) disk, but large, you could emulate an RP06 or even (not officially supported but works fine at least as of V7) an RP07.

The above is how I would approach the question if it is "simple in an FPGA".  For SIMH, none of this matters, there I would just pick MSCP because it doesn't worry about device sizes.

For real hardware for RSTS, the answer would be "anything you can find that is supported".  That's pretty much any PDP-11 so long as it has EIS (MUL and DIV), memory management, and enough memory.  And you could use an 11/20 or 11/10 with RSTS V4 (but not RSTS/E).

	paul