Booting Unixv4

Overview of the timeline of booting UnixV4 on the PDP-11/40 for the first time in 40 years. Most of this work takes place on the ClassicCMP Discord.

20/12/2025 - Unix 4 discussion

Around 9pm (NZD), news about the new Unixv4 image started to make its way onto the Discord.

20/12/2025 6:14 pm - Me: So what hardware is this image intended to run on?

20/12/2025 6:24 pm - Zoot: PDP 11. You can apparently install the system under SIMH via PDP-11/45

20/12/2025 6:40 pm - Me: Looks to need an 11/45, was hoping to run it on my 11/40

Soon after, the file system was extracted, and people began to look over the files, myself included. Others and I noticed there was a single flag in one of the files:

.45 = 1			/ 0 = 11/40  1 = 11/45

21/12/2025 9:08 am - Kernel Rebuild

21/12/2025 9:08 am - aap: nice nice. as i wrote you in my email i'm currently deep in the unix kernel. i just got warren toomey's nsys-kernel restoration to build and boot on top of a v4 filesystem. the next step would be to fully implement pipes to make a fully functional system. and then i'll see about what changes he made that i may want to reverse. something about the filesystem format

21/12/2025 9:08 am - aap: and rebuilding the v4 kernel on itself works fine too

21/12/2025 9:11 am - Me: Can you build it for an 11/40?

21/12/2025 9:24 am - aap: probably! what do you need? just an RK image? let's see!

21/12/2025 9:29 am - Me: Yeah, I am just going to run it on the unibone. No promises my 11/40 is up to the task. It's a little iffy with Unix.

21/12/2025 9:32 am: aap provided me with an image of UnixV4 with a kernel built for the PDP-11/40.

Initial tests were unsuccessful and I was not able to get the kernel to boot. Switching back to testing with UnixV6 I found that there was a fault with the FIS and EIS boards.

Over the next few minutes, I debugged the FIS and EIS boards and identified the faulty board. I temporarily switched over to a spear set. I have since fixed the FIS and EIS boards.

21/12/2025 10:33 am - UnixV4 Booted?

21/12/2025 10:33 am - Me: Well I think unixv4 booted, but my uart has issues. Let me swap a card and see

It goes through the motions of booting but no console output. It look likes it running. Might be RTS/CTS let me try the DECWritter...
Nothing.

Despite trying all sorts of different things, we did not have any success at getting UnixV4 to correctly boot. From the above video, you can clearly see that there's quite a lot of activity on the bus. The system acts as if Unix has booted, but there is just no console output.

aap, bitsavers and I spent the next few hours debbing the UART, trying to work out why it would not show the login prompt... ultimately without success.

In the meantime, Schpliphy Samjones was diligently working to get their PDP-11/45 running...

23/12/2025 12:21 pm - 11/45 Running XXDP

Schpliphy Samjones: OMG the ‘45 is up on XXDP for the first time since I got it in 1997. I think I can write this V4-1/2 image via vtserver now

Schpliphy Samjones, app and others spent the next few days working on producing a real RK05 disk pack and getting the 11/45 running.

24/12/2025 7:25 pm - UnixV4 Booted on 11/45!

After a few days of work, Schpliphy Samjones got UnixV4 booted on his 11/45. Making him the first to have a fully functional and running install of UnixV4.

26/12/2025 9:07 pm - UnixV4 Booted on 11/40!

Having no luck getting it to boot from the UniBone, I moved on to setting my RK05 emulator and disk controller. With a real DK11 and an emulated RK05, it runs. But with just the UniBone, it's not happy. To this day, I still do not know the issue with the UniBone.