Building my own emulator. I only have the basic , no memory management, model going (11/20?). I started with caps11. What’s my best choice for a disk based os.

EDIT: and of course if anybody has the source code that would be awesome. In no way could I have got this far without the caps11 source from Lou Ernst

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    No PDP-11 is "dumb" :-) – another-dave Jul 3 at 15:19
  • It might be better to specify the PDP-11 model in the title, instead of saying "dumb". – wizzwizz4 Jul 3 at 16:27
  • Might be the case that the emulator isn't an exact match for any real PDP-11. – another-dave Jul 3 at 16:30
  • @wizzwizz4 going back to the spec doc - I believe I have a 11/20 – pm100 Jul 3 at 21:50
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    @another-dave my intention is that it is an 11/20 (now that I have looked up the details of what I have built). I am sure I have bugs, hence the need to run various OS on it – pm100 Jul 3 at 21:51

RSTS-11 (not RSTS/E) ran on the 11/20. It offered multiuser timesharing in BASIC. I think that in addition to the base 11/20 hardware, you needed a clock. You needed to max out the memory (28 Kwords).

V4 was the last RSTS-11 release before it became RSTS/E.

System manager's manual for RSTS-11.

I have not checked to see if there are software kits available.

For a single user disk system, I'd suggest RT-11. You might need a relatively older version for a very small system, I don't know.

You can definitely run RSX-11M unmapped (i.e., no memory management), but I don't know whether it supports the 11/20. My experience was with an 11/40. If you try, I'd suggest V3.1; as I recall 11M got a little fatter after that, at least when running mapped.

On the other hand, configuring an unmapped 11M system is not something I'd recommend to the raw beginner. Better to get familiar with 11M on a larger system first. The unmapped system requires determining during sysgen the physical addresses at which tasks will be loaded.

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  • I have an 11/20 that will take up to 28kw (which I know is not legal), ie I have maxed out the address space – pm100 Jul 3 at 21:52
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    I'm pretty sure you can configure 28KW. Two 4K MM11-E in the base box and another five in an expansion chassis. – another-dave Jul 3 at 22:11

Manchester MUSS ran on a PDP11/10, but I can't offer a working PDP11 image for it at the moment as it is lost in passage of time....

It would have been able to do its own memory management and swap, multitasking etc.

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  • The diagrams of the MU5 complex I have seen show the -11 as a front end processor to the MU5 itself. Was significant end-user computing done on the -11? Was MUSS used on PDP-11 outside of its use in MU5? (Maybe I need to write up some MUSS/MU5 questions!) – another-dave Jul 3 at 16:29
  • @another-dave Yes: It run MUSS as the OS on the PDP11/10 to enable it to run as the terminal handler front end. The ICL1905E also ran MUSS as the exchangeable disk and peripheral front end, the VAX11/750 next door also ran MUSS and together they were the MU5 complex. The PDP11/34 in the student lab also ran MUSS. The source of MUSS is available but the bootable binaries are lost.... – Brian Tompsett - 汤莱恩 Jul 3 at 16:58

There was LSX - a small, single-user UNIX for non-MMU PDP-11's based on UNIX V6. It also ran on the LSI 11/03. It was "interesting" to hear the RX01 floppies clunk and chatter when running piped commands!

LSX Unix Restoration Page http://www.mailcom.com/lsx/

... LSX needed at least 40 kbytes. That's 20 kilowords: 8 kilowords ... for the system and 12 kilowords ... for the user programs.

The Bell System Technical Journal paper UNIX on a Microprocessor by H. Lycklama from July 1978: https://www.tuhs.org/Archive/Documentation/Papers/BSTJ/bstj57-6-2087.pdf

There's Mini-Unix for the PDP-11/20, but it wants to see an RK05 disk: http://www.tavi.co.uk/unixhistory/mini-unix.html

And as @anotherDave mentioned, RT-11 was popular, and not bad to use. Here's one place: https://www.pcjs.org/software/dec/pdp11/disks/rk03/rt11v4/ And the Trailing Edge site is good: http://simh.trailing-edge.com/software.html

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My primary experience on a PDP-11 class machine was using Forth. We were using it for high-end (at the time) image processing and display algorithms. The advantage is that it is very compact and quick. If you need quicker, you can write "words" (that's Forth for subroutines) incorporating assembly. From a modern perspective, Forth might not look very advanced. OTOH, how much more advanced can you be than having 100% control of everything all of the time? It fully supports multiple users running multiple processes simultaneously. (My next job was using Unix machines, which sort of gave me whiplash. Now I had partial control of some things when the OS allowed.)

The downside is that if you have no experience with Forth, everything will seem backwards. The language itself uses a sort of "Reverse Polish" logic for everything -- "operand operand action". Forth is not just a computer language in the PDP-11 environment. It is also the operating system, device drivers, development environment, and applications. You aren't writing Forth programs; instead you are making Forth bigger. If this sounds intriguing, give it a shot. If not, run.

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  • I know forth. Is there a specific pdp 11 forth somewhere – pm100 Jul 4 at 14:14
  • @pm100 I know of two possible options, both pretty old (... he says on the retro board ...) The Forth Interest Group (FIG) had Forths for many machines, including PDP-11. See: stackosaurus.com/figforth.html and also www.forth.org . You might also contact [Forth Inc.}(www.forth.com) and see if they would supply you a copy of their commercial PDP-11 Forth for cheap. I'm sure there isn't any sustainable market for it any more. – RichF Jul 4 at 14:25

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