Thalia Archibaldthalia
Aug 11, 2026, 4:37 PMI got telnet working with UNIX V4, multiplexed with my physical terminals so more Low Resource Computing attendees can play with it at once!
Replies
- Thalia ArchibaldthaliaAug 11, 2026, 5:50 PM
I've just used up a full roll of paper for the first time!
My new Silent 707/1200 is just so pleasant to develop with, as it types 4x faster than my previous favorite, the Silent 703, at 1200 baud instead of 300, and all keys work perfectly—no stubborn keys like the other.
The stack of paper is trimmings from previous sessions on this and other terminals. Literally cut/paste.
- Thalia ArchibaldthaliaAug 11, 2026, 8:55 PM
I got all 16 DC11 lines working now! That's a terminal multiplexer that allows multiple terminals to connect simultaneously. But the dl driver in this version of UNIX limits the number of terminals to 14. After some kernel hacking, I was able to raise that to 16.
I don't know whether those extra two connections will be useful for LRC participants, but now it can support 17 users.
Now, I ought to enable the second KL console for an 18th user…
- Thalia ArchibaldthaliaAug 11, 2026, 9:17 PM
VCF PNW next year is going to be so much fun. I hope to have a PDP-11/03 by then, the least powerful model of the whole line, which I will backport Mini-UNIX from a V6 base to V4 to put on it. My friend will be bringing her Data General Nova and PDP-11/23 too to collaborate on a mega booth. And, of course, I’ll be bringing my teletypes to connect to them. Minicomputers are back!
- Thalia ArchibaldthaliaAug 11, 2026, 9:35 PM
It’s going to be quite chaotic if we actually end up having this many users at LRC next week.
I should build a little disk backup utility that commits the SIMH disk image every minute, so I can rollback if someone does `rm -rf /` or otherwise breaks the system. I might even encourage breaking the system. I’d have to sacrifice a terminal connection to it, though, so it could send `sync` over telnet just before saving the disk image outside the emulator.
- Thalia ArchibaldthaliaAug 12, 2026, 5:50 AM
I now got UNIX V4 working with 32 terminal lines! I believe this is the highest number of terminals ever configured for any UNIX V4 installation. It required three kernel modifications to increase limits (in init and the dc and kl drivers).
Technically the KL11 hardware supported a 17th terminal line, but UNIX V4 is not compatible with this and it took me half of the day to figure out why. Turns out, UNIX uses a clever trick of saving the device minor number to the processor status word when invoking an interrupt handler, as a cheap way to pass a small argument. Only the low four PS bits are safe to repurpose in this way (they're the condition codes), but the fifth is the trace bit, which causes a kernel panic when set. Well, the 17th KL line has minor number 16, which fills 5 bits and sets the trace bit, causing a kernel panic!
So even though the hardware supports 33 terminal lines, I have to settle for 32, unless I rearchitect the interrupt vectors.
Ptrickyptricky@macsurf.orgAug 11, 2026, 4:48 PM@thalia Awesome!
Anthkanthk@neopaquita.esAug 11, 2026, 5:42 PM
Aug 11, 2026, 6:10 PM@thalia it is the early 80s and me and my brother are going back through the saved scrolls of our last Adventure run to improve our maps.
DougMerritt (log😅 = 💧log😄)dougmerritt@mathstodon.xyzAug 11, 2026, 8:59 PM@thalia
Around finals at UC Berkeley, our PDP 11/70 running Unix v7 with a 64 way multiplexer ground almost to a halt. Like a minute or more to get another shell prompt after just hitting return.So that's one possible ceiling, anyway!
Sounds like you're putting in good work, fighting the good fight. :)
- Thalia ArchibaldthaliaAug 12, 2026, 1:06 AM
@dougmerritt Sounds like you were using a DZ11 multiplexer, since it supports any number of lines, in multiples of 8. Going by SIMH's emulation of them, KL11/DL11 supports 1–17, DC11 supports 1–16, and DH11 supports 16. V4 didn't support DZ11 and my SIMH/REALCONS build doesn't include DH11, so I'm doing the best I can.
One advantage of emulators is that I can easily increase the machine specs. Once I work out the issues in my 17-line KL setup, I'll work on increasing system resources.
- DougMerritt (log😅 = 💧log😄)dougmerritt@mathstodon.xyzAug 12, 2026, 3:13 AM
@thalia
I wasn't involved, but I know it was heavily hacked (by Bob Kridle, IIRC). Could it have been a two-tier multiplexer, if it had been locally modded? That half rings a bell. 
MontyOnTheRunmontyontherun@bitbang.socialAug 12, 2026, 6:23 AM@thalia was it any slower than regular usage? Would a user notice any difference?
- Thalia ArchibaldthaliaAug 12, 2026, 4:24 PM
@nabijaczleweli Yes! I have a running document tracking my steps. Once I finish all the features I want for LRC, I’ll clean it up to be a reference guide for setting up UNIX for using physical terminals and telnet connections and tuning the system. The failed approaches will become side notes, instead of twists and backtracking in the narrative. This will fill the gap of the documentation I wanted, that I’ve instead had to learn through consulting UNIX and SIMH sources.