[TUHS] Unix on an 11/23 (Was: Work I've done with a PDP-11 simulator)

So I was idly looking through main.c for the Shoppa Unix (because it printed some unusual messages when it started, and I wanted to see that code), and I noticed it had some fancy code for dealing with the clock, and that tickled a very dim memory that LSI-11's had some unusual clock thing. So I decided I had better check up on that... I got out an LSI-11 manual, and it looked like the 23 should work, even for the 'vanilla' V6 from the Bell distro. But I decided I had better check it to be sure, so I fired up the simulator, mounted a Bell disk, set the cpu type to '23', and booted 'rkunix'. Which promptly halted! After a bit of digging, it turned out that the problem is that the 11/23 doesn't have a switch register! It hit a kernel NXM trying to touch it - and then another trying to read it in the putchar() routine trying to do a panic(), at which point it died a horrible death. So I added a SR (you can create all sorts of bizarre hybrids like that with Ersatz-11, like 11/40's with 11/45 type floating point :-), and then it booted fine. The clock even worked! So you will have to use the Shoppa disk to boot (but see below), or we'll have to spin you a special vanilla V6 Unix that doesn't try to touch the SR - that shouldn't be much work, I only found two place in the code that touch it. I did try the Shoppa 'unix', and it booted fine on an 11/23. Two things to check for, though: first, your 11/23 _has_ to have the MMU chip (that's the large DIP package with one chip on it nearest the edge of the card), so if yours looks like this: http://www.psych.usyd.edu.au/pdp-11/Images/23.jpeg you're OK. Without the MMU chip, most variants of Unix will not run on the 23 (although there's something called MiniUnix, IIRC, which runs on an LSI-11, which would probably run on a /23 without an MMU). Here's the part that might be a problem: To run any of the Unixes on the Shoppa disk, you also have to have the FPP chip too (that's the second large DIP package with two chips on it - the image above does not include that chip, so if yours looks like that, you have a minor problem, and I will have to build you a Unix or something). All of the Unixes on the Shoppa disk have to have the FPP, except one - and that one wants an RX floppy as the root/swap device! The others will all crash (I tried one, to make sure) if you try and boot them on an 11/23 without the FPP. I could try patching the binary on the one that doesn't expect to use the FPP to use the RL as the root, or either i) build you a vanilla V6 for a 23 (above), or ii) figure out how to build systems on the Shoppa disk, and build you a Unix there which i) uses the RL as the root/swap, and ii) does not expect to have the FPP. But let's first find out exactly what you have... Noel