First Sixth Edition does not have support for either the 11/23 or a floppy.  So your issues might be found in either place.  Where did you get the modifications? Maybe: https://www.hamartun.priv.no/v6unix.html  Your guess WRT to floppy being the source, would be where I started to poke. 

Frankly, early UNIX when ported to a system with a floppy has a lot of issues [we did this at Tektronix in the early late 1970s for the magnolia and some of its siblings).   Take a look at the physical floppy - you will notice a lack of magnetic material near the center of the diskette.  This is the i-list and v6 (and v7) hits it pretty hard.   Caching of the inodes was added later, but was thought to do on the 11s due to lack of address space [you need to BSD 2.X later versions and generally need support for separate I/D 11 and a lot more physical memory].

I thought I remember that the 11/23 is a 11/40 class processor, not a 45 class so it lacks the '17th' bit as it were, although I think it can support more the 256K bytes of memory, but again it takes hacking V6 to enable and use it.

One thing you could do is put the kernel image on simh [https://github.com/simh/simh] or Ersatz11 [http://www.dbit.com/] and try it with an 11/40 and an RK05 emulator instead of the 11/23  and floppy since 11/40 with an RK05 and RP03/04 were the native distribution.    Then try to change processors, and finally disks on the emulation.    BTW: it if works fine with an emulated floppy [I would not be surprised if it does] I would look at the driver.   I would think that it is possible for the floppy driver is causing the 11/23 to have missed interrupts with how the spl()/splx() calls are working [which can get tricky and easy to be 'almost right'].

One other thought, I'm pretty sure that Noel's V6+ system from MIT can support a 23 but I don't know about floppies, you might try his image.  Check out gunkies - https://gunkies.org/wiki/UNIX_Sixth_Edition, but Noel might reply here directly with more info. 

Have fun.


On Mon, Dec 20, 2021 at 10:45 AM Dave Plonka <dave@plonka.us> wrote:
Hi Paul

We talked about this a bit more in the comments on the video, with
Gavin doing some experiment afterwards.

While at the show, one experiment we did was to redirection the
bas(1)ic program's output to a file and what we found was that (a)
characters would still sometimes be lost (which is how we determined
it's not due to a serial communications, directly and (b) the dropped
characters would coalesce together into a small run.

Gavin's speculation at the time was that the floppy driver
interfered/interrupted the bas' output somehow, since we noticed there
was also disk activity coinciding roughly in time with a bout of
dropped characters.

What is unusual about Gavin's machine (at the time of this video) is
that everything was on floppies - including root, tmp, etc. because he
did not have a functioning fixed disk yet. It may be a configuration
that was rarely used, e.g., scratch space for bas on floppy. (We found
bas seems to read the source code from the file system even at

Any insights welcome - esp. if you can put them in the comments there.


On Sun, Dec 19, 2021 at 11:25 AM Paul Ruizendaal <pnr@planet.nl> wrote:
> While doing some end of year retrocomputing revisiting, I thought some
> of you might enjoy this - there is hope for the next generation(s)! ;)
> https://www.youtube.com/watch?v=_Zyng5Ob-e8
> Thanks for that video link!
> I noticed the bit at the end about V6 and  the occasional dropped character and that this was not a serial line issue. I have the same issue in my V6 port to the TI-990 and always assumed that it was a bug I introduced myself when hacking the tty driver.
> Does anyone remember, was this a real life bug back in 6th edition back in the 1970’s? Maybe only showing at higher baud rates?
> Paul


dave@plonka.us  http://www.cs.wisc.edu/~plonka/