PDP-11 questions for possible novel/story

What I (think) I know:
The PDP-11 is the computer that the first version of Unix that was written in C and went by the unix name rather than Unics or whatever the multics offshoot was originally called. This was made by DEC originally using Q-Bus but later switching to Unibos architecture wehre everything was a card on a backplane and as a hardware platform in the west it had a thirty year run with advances and upgrades but overal architecturally consistant.

Mini-computers often ran a time-share system where you had a bunch of users logged in over dumb terminals either locally or via modem. The original/older iterations of the PDP11 were refridgerator sized but had ended up shrinking down to something not too dissimilar from a desktop form factor, but most OS’s expected several users working concurrently as a sort of ancestor to cloud computing.

When the soviets turned from their own architecture research to cloning, the PDP11 ended up being probably one of the most cloned platforms for… a variety of reasons.

And that is itself a fairly oversimplified thing that I’m sure details are wrong on but it’s a sort of ‘ground level myth’ of the era of computing just before I was born.

i know there are several PDP11 tagged posts here o I figure maybe here is a good place to ask even if I fear the questions may be a bit dumb sounding:

as near as i can tell the hardware does not care what the terminal is so long as it conforms to connection standards and make the handshake. Whether it’s a line printer, a VT100, VT350, a PC, or another computer is irrelevant.

What was actual use like as compared to using a modern desktop?

I suspect getting data in or out would be by todays standards excruciatingly slow even for locally avalible material.

I’m on a pubnix server and that server allows me to browse other user’s home directories and the system at large. This has proved helpful in that I was able to go ‘Hey guys anyone interested in a thing make a dot file with this name’ and thanks to another user whipping up a script iwas able to quickly get a list of everyone who had made said file so Icould go thorugh each one as i liked (though in hindsight would’ve probably been able to make a script that hoovers everything into a compiled file in my home directory with some sort of character lines or other teltale break points.

I bring that up because my mind goes to this pubnix as about as good as i can reasonably expect approximation of what using a minicomp would’ve been like. Full text enviroment, just poking away, and occasionally glancing at something else as refrence while you do things. I also very much doubt anything like tmux or screen existed that let you multiplex/have more than one application up at a time.

I feel confident enough with the keg-in-a-box style hard drives to not need to ask overly about those. Ditto the eight inch floppies, but have there been any interesting bits of storage media that just arne’t used today or in the past twenty years outside of maybe tape?

I know there is a late iteration of the PDP-11 that effectivly shrunk a lot of the core system down to… a silver/white looking dip package with two gold looking chips on it? Has anyone, hobbiest, clone, or otherwise tried to make a portable out of the architecture? IE laptop, luggable, or even something like the apple IIC in formfactor?

Likewise has anyone, probably the soviets, made extensions to the architecture or even tried extending it to 32bit? I want to say the original had a 16bit bus, but there was like a 22 or 24 bit bus variant?

I’m mainly just poking about and trying t ofindm ore about an architecture just before I was born. I have this screwy idea of an old mall that was built in the 70’s and closed in the mid 2000’s when the recession hit that had a PDP11 as the thing a lot of its automated systems were controlled by (lights, rolling shutters, door locks, alarms, etc) and nobody ever swapped it out for something newer because it worked thus why bother, getting something to replace it would equite redoing a lot of the things that plugged into it, and I just like the concept.

A group of teenagers doing the youtuber thing of exploring the local dead mall, and shenanagins happen to get one of them to want to try finding a way to make it Not Dead.

Would the hard drives or tape systems have any recoverable data? assuming the data closet still was weather proofed and rats hadn’t decided to nest in the cabling. There’s not a chance in hell of the thing powering on but any chance there isn’t cap juice and corrosion eating the thing?

How easy would getting a modern workalike be? I know the PiDP11 is a thing but that’s more a hobbiest exhebit rather than ‘here you can plug things into it.’

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The modern work a like was the VAX. The catch with the PDP 11 it is still a commerial product
as far as I know, so software and hardware can be expensive.
Ebay often has bare metal PDP 11 chipsets, and if you know Russian, you may find a clone
like DEC pdp-11 clone Elektronika MS-0511.

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I thought production had stopped in the 90’s.

Still. The hardware vintage does put this well past the main cast’s ability to do more than go ‘we need someone who actually knows this stuff’ since bunch of teenagers and none of them are especially into old computers.

The first PDP-11s were Unibus, later PDP-11s were QBus. The big difference is that QBus is multiplexed. They are electrically incompatible, but use very similar bus transactions, so there are Unibus-to-QBus adapters and vice-versa. I suspect that many Unibus cards will not work on later QBus systems because the bus cycle time is too fast for them.

This is correct. The original PDP-11s would be tough or impossible to fit in a single rack, but later PDP-11s easily fit in a half rack with storage, and toward the end there were desk-side tower systems. There were LSI-11 desktop systems, but most of them were not multi-user systems.

Correct! The standard line cards for early PDP-11s will connect either current loop terminals (such as printing Teletypes) or EIA-232 (RS-232) terminals, at speeds ranging from under 100 baud up to about 9600 baud. Later machines and interfaces may not handle current loops and may run at faster speeds. The PDP-11 was infamous for having rather meager and inefficient first-party terminal interfaces in its early years (primitive or lacking interrupt support, and minimal, typically one character, buffering), but third-party companies made rather fast and efficient multi-terminal adapters.

A “fast” PDP-11 running BSD 2 with only a few users feels enough like a modern Unix system that over a serial line you might think it seems more or less like a modern system. At some point, though, you’ll do something like try to compile a moderate sized program and realize that it is NOT. :wink: In particular, disk access is abominably slow by modern standards, and of course the processor is under 1 MIPS.

I think a pubnix is a good example of what using a Unix PDP-11 “back in the day” would feel like, although on older systems you had to be much more cognizant of things like how much disk space or CPU you’re using. Files were much smaller, but total storage was likely in the tens of megabytes, so with many users it was often still pretty tight.

I think the very first PDP-11s were indeed a 16 bit bus, but virtually all of the shipping systems had at least a nominal 18 bit address with an MMU. The two very common schemes were an 18 bit bus and a 22 bit bus, for a maximum of 256 kB (128 kW) or 4 MB (2 kW) of RAM.

If there’s a PDP-11 that was running in the mid-2000s … it might just power on. I have brought up several machines that were shut down in the early 90s with relatively little trouble. I also have quite a few bits from the same time that don’t work, however. Jerry Walker has some good videos on YouTube going through an 11/34, and there are probably other good resources out there. Go slowly, check the power first, and be very careful with the disk systems (dust and debris can kill them quickly!). While there may be bad capacitors, they probably haven’t killed anything, because they were pretty high quality units; they may just not have good capacitance.

The Unibone or Q-Bone from retrocmp are electrically compatible with Unibus or QBus systems (respectively) and emulate functionality ranging from RAM to the CPU to disk storage systems in a variety of configurations. With one of those and a backplane, you could hook up a lot of devices.

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Thank you for the clarification.

Useage is the big boogyman for me in writing, because too much has changed to just blindly go ‘oh yea computer i know how that works.’ I’m not even getting into word v Byte storage thing, littleendian etc. Not yet.


Oh that’s gonna be a fun chapter to write, especially s everyone else on scene gets impatiant and just wants ot hit the big chonkin power switch

Wonder if anyone’s homebrewed up a solid state drive replacement. Won’t help for archiving an existing drive but it would give a more stable home for the data.

Glad to know modern solutions exist for card replacements that aren’t ‘hack it together’ since, while that is believable to me knowing the hobby, believability to the reader may be diffrent.

I greatly appreciate those that either took time to learn these things, or just as likely were there with these old beasts when they were relevant.

Unrelated/simi related. Pubnix is a genuinely fun thing assuming you find the right server.

Look at some pictures of PDP-11 peripherals for the kinds of disks that were used. In particular, the pizza sized removable hard disks (normally around 5MB) were popular.

Early PDP-11s used core memory while later models used semiconductor memories. With core, even if the machine has been off for a few decades it is very likely that whatever it was last running is still in there and with a little care it could be restarted like a modern laptop coming out of sleep mode.

The 32 bit VAX literally means “Virtual Address eXtension of the PDP-11”, though only the first model had a mode to run old PDP-11 binaries and its own instruction set was more complicated (twice the registers, more address modes, variable length instructions and so on).

One Soviet clone of the PDP-11 came in a “computer in the keyboard” form factor like the Commodore 64 and so many others. That is pretty close to a laptop if you connected it to a small LCD monitor and don’t mind plugging it into an electrical outlet.

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My first experience of a PDP11 was a 11/40 with core memory running Unix v6. The senior lecturer in charge of it at uni wanted to upgrade it to semiconductor memory, but was hampered by the price, so took a gamble and cancelled one quarters (ie. 3 months) of maintenance and used that money to buy the DRAM board - which had double the capacity went at double the speed (no write-back needed) and maintenance was a fraction of the core memory boards…

Fun old time…


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A PDP-11 outside academia would have been more likely to run RSX-11 (or RT-11, if it was a small system) than any Unix variant. The user experience is quite different.

I still know some VMS-heads who describe Unix as “a means to make DEC hardware run slowly”.

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Oooo useful info! Have one of the group go ‘yea i run linux unix is basically old linux so-’

stares at screen

‘I have no idea what this is.’

I wonder if how true this is; in particular, that virtually all PDP-11s shipped had >16-bit busses. Perhaps its true because there were so many more later systems produced than earlier systems. But my take is that the PDP-11/05, with its 16-bit UNIBUS, sold in good quantities in its day (mostly to OEMs?).

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The 11/05 had an 18-bit Unibus, it simply wasn’t capable of managing any memory that had either of the two high bits set. Electrically, it had the extra two address bits, but there was no MMU to drive them.

I don’t have a citation right in front of me, but my recollection is that the original PDP-11 (that became the 11/20) only had a 16-bit Unibus, and that two bus pins were repurposed to be the 16th and 17th address bits when the MMU (KS-11?) was introduced at some later date. From that point forward, Unibus had 18 physical address bits.

Machines that had 22-bit address space and Unibus had the equivalent of an IO MMU to provide access to all 22 bits of memory through the Unibus 18-bit address space, and placed the actual RAM on a different bus (Extended Unibus, which is like MUD but for 22-bit memories, and includes a SPC slot, or the Memory Bus on the 11/70). Machines with a 22-bit address space on QBus simply used a 22-bit QBus; it was not quite as stable over time as Unibus was.

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Thank you for that! Indeed you are correct, as a quick re-read of the engineering drawings for the 11/05 reveals the presence of address lines A16 and A17, both driven in unison (ether 1 or 0) from the M7261 board.

I suppose the OP’s original question related more to how many systems were sold that were capable of addressing more than 16-bits of memory. I do still think that’s an interesting question.

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For RSX-11, that would absolutely be the result. :wink:

RT-11 actually seems reasonably familiar to anyone who knows MS-DOS (or even some parts of the modern cmd.exe Windows command prompt. DOS was modeled after CP/M, and CP/M was modeled after the DEC command interpreters that Gary Kildall was familiar with, which were in the same family of command languages as RT-11 used. In fact, a CP/M user might take a moment to figure out that RT-11 isn’t CP/M, before noticing that the filenames are shorter (6.3 instead of 8.3) and discovering that there’s no familiar editor in sight. Commands like dir, ren(ame), and pip will work just fine!


The days of old, when you paid a big price for a tiny OS. The Wofford Witch lets you play with all that.
Wofford Witch The price of memory the same as new car.

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This is interesting timing. I cut my teeth on PDP-11s running RSTS/E. I’m currently running the latest version of VMS on my Mac in a VM. It’s a native x86 port of OpenVMS.

I miss the PDP-11, and if I ever come across one of the later ones, I may jump on the chance to buy it…

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Note that the PDP-8 pre-dated the PDP-11 in the DEC machine timeline, and the Soviets also cloned the PDP-8 in large numbers at that point in history (although of course computers of all types were more numerous later, which included PDP-11s).

DEC launched the VAX ostensibly as a 32-bit address extension of the PDP-11. The “VAX” moniker stood for “Virtual Address EXtention”. The first model of VAX was the VAX-11/780 (incorporating the “11” name). The VAX had a compatibility mode which would run PDP-11 RSX-11/M programs unchanged. Subsequent models after the 7xx series dropped the “-11”.

The software on the core memory boards of any PDP-8 or PDP-11 from the 1970s should still be intact provided the boards are in good shape and not exposed to very strong magnetic fields.

You can experience the original UNIX systems first hand courtesy of the fine folks at SDF, but I suggest reading more here before you dive in.


Lots of info in the wiki over on Gunkies.org, for example:
RSTS/E - Computer History Wiki

There’s an online emulator here with a narrative and links to more notes about various OSe and applications:

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