What was the most popular operating system on the PDP-11 computers?

And is it known what percentage of PDP-11 computers used this operating system?

  • 1
    Even if we knew how these systems were configured on first purchase, these kinds of systems had second, third lives at universities, etc..
    – Erik Eidt
    Commented Jan 16, 2021 at 16:20
  • 2
    Does the question include PDP-11 systems that were used inside other systems? GenRad had a PDP-11 in several versions of their testing equipment they sold. Nothing on the outside of those said PDP-11 or Digital Equipment. I am sure that others did the same.
    – UncleBod
    Commented Jan 16, 2021 at 16:45
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    Howto define 'Most Popular'? Sales numbers? Also, to give a percentage, is there a clear definition what constitutes 100% of PDP-11. Are LSI-11 included? What about compatible machines?
    – Raffzahn
    Commented Jan 16, 2021 at 17:06
  • 1
    How do you define popularity? And in what year? Early -11 systems such as PTS and DOS/BATCH gave way to RSTS and RSX.
    – dave
    Commented Jan 16, 2021 at 17:28
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    Why closed as opinion-based? It seems to me that, for DEC-supplied systems at least, there is an objective answer based on number of licenses sold. Whether we can know that answer is a different matter.
    – dave
    Commented Jan 16, 2021 at 19:37

2 Answers 2


I'm only guessing, but I would guess in the late '70s and early '80s either

  • RSTS/E for traditional timesharing use

  • RSX-11M for real-time application use; maybe this transitioned to RSX-11M-Plus as larger memories became common

I have no data for this, just an impression of what was going on in DEC while I was there. I'd be interested in numbers if anyone can come up with them. The answer should in principle be knowable for DEC systems, since software came with a price tag.

  • I think this can't be properly answered without considering a timeline. RSX-11 systems probably have survived much longer than anything else (and they still survive, for example in lots of power plants), because they tended to be tangled into much more expensive hardware.
    – tofro
    Commented Jan 8, 2022 at 10:49

In the USSR there were produced a lot of clones of PDP-11, some of them more exact and some of them much less.

As should be expected, the most of them ran variants of RT-11 operating system. Since those computers were widely used in schools, RT-11 was perhaps the most popular operating system for PDP-11 clones.

But if we consider home computers, the situation was different.

The most of operating systems were created for the computers of the BK series. I would claim it is the home computer with the greatest number of various operating systems available for.

Initially, BK-0010 was released without any disk drive support with only external storage being a tape recorder. But very soon many amateurs and cooperative firms found ways to attach disk drives to these computers. Below, is a table of some operating systems for BK series computers. The list is not exhaustive. I for instance, saw a ROM-cartridge-embeeded RT-11 variant for BK-0010.

enter image description here

As you can see from the table, the most of operating systems used the MicroDOS file system, which was named after an early OS of the same name.

BK had very limited amount of RAM. On BK-0010 the disk drive controller expansion cards usually added 4 to 16, or, very rarely, 32 (with paging) KB of memory, so the operating system had to fit this area, otherwise the usual programs for BK would not even work.

Thus, the operating systems had to choose, whether to prioritize command-line interface, which usually would leave the user without any file manager but allow batch files, or integrate the file manager into the kernel, providing virtually no command line interface.

The more advanced operating systems, such as ANDOS and MKDOS choose the modular approach, with resident kernel that would provide basic file operations (and in later ANDOS versions, a basic file chooser dialog that could be invoked from other apps), and an optional file manager, sometimes even with option to chose one.

Since the tape file format of BK allowed for 16-character long file names, and many programs and games would have the names of overlay files hardcoded, and the files often had names ending with their number like "01", "02", etc, the RT-11 6.3 file name format was not very suitable out of compatibility reasons.

Thus, more widespread became the MicroDOS file system with its 14-character file length and FAT12 with 8.3 format and complicated algorithms used by operating systems, like ANDOS to convert the file names, keeping their last characters that often contained file number.

So, a file named "НЕПОБЕДИМЫЙ" would become "НЕПОБЕДЙ" on the disk drive and when a program would open a file "НЕПОБЕДИМЫЙ", its name would be automatically shortened.

That said, I think, the most popular OS (at least, according to this recent lecture by the creator of the OS himself, Alexey Nadezhin) on home computer BKs was ANDOS. He also claims he had sold about 5000 copies officially but the majority, in excess of 100000 copies were pirated.

Andos 3.30 with Disk Master file manager:

enter image description here

Andos 2.23 with BK SHELL file manager:

enter image description here

The later versions of ANDOS could support up to 63 drives: hard drive partitions, floppies, tape and a RAM drive (on BK-0011(M)).

The file manager Disk Master included viewer, editor, calculator, music player, disk formatting tool, tree view, customizable user menus, backup and restore and other functions, even a screensaver.

enter image description here

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It should be noted that all this was possible in just 16 KB (the shell occupied even less) due to the efficiency of the PDP-11 architecture.

The second most popular operating system on BK series was operating system MK-DOS (although according to Nadezhin, the creator of MK-DOS Mikhail Korolev thinks his system was more popular). It was the main competitor to ANDOS. It allowed to work with volumes up to 32 MB (unlike ANDOS which only allowed floppy-sized volumes) with hard drives up to 2 GB. Like ANDOS, it supported tape access and a RAM drive on BK-0011(M). MK-DOS supported true subdirectories, unlike ANDOS that only supported root directory on a drive but could visibly make illusion of different sub-folders. The distribution included Forth programming language.

The MK-DOS default file manager was a NC-like Micro Commander. enter image description here

Other notable operating systems included

  • AO-DOS, that supported up to 23 disk drives, including RAMdrive and supported BAT-files:

AO-DOS with DOS-SHELL file manager

AO-DOS with DOS-SHELL file manager

  • CSI-DOS, an operating system exclusively for the BK-0011(M) model that utilized the advanced features of that hardware. It was developed and popular in Samara:

enter image description here

CSI-DOS with X-SHELL file manager

  • OS BK-11, the only official OS for BK-11, a clone of RT-11, it was shipped with the computer:

enter image description here

You can find more info about operating systems for BK series on this page in Russian Wikipedia.

  • 1
    This does not answer the question but uses the space for a a lengthy praise of the BK010 series.
    – Raffzahn
    Commented Jan 8, 2022 at 14:20
  • @Raffzahn it is an answer to the question pouring light on certain (not small) sector of PDP-class machines. In the 1990s, I think, ANDOS was THE most popular OS for this family, so this is THE answer.
    – Anixx
    Commented Jan 8, 2022 at 14:21
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    But that's not asked for,nor is the question about file formats or file managers. I can understand your love for this machines, and they are cool (got one myself, although only the 'SCHool'), but that way you're not really helping either. As far as I can tell, none of the OS mentioned will run on a PDP-11, let allone due the missing video buffer.
    – Raffzahn
    Commented Jan 8, 2022 at 14:24
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    By listing 18 different operating systems for the BK, it would seem that at least 17 of them are not the answer to the question of "most popular operating system".
    – dave
    Commented Jan 8, 2022 at 22:32
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    I'm surprised people down-voted this. He's sharing knowledge from outside the US and Western Europe, and it seems quite responsive to me.
    – RichF
    Commented Jan 8, 2022 at 23:17

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