The Oldest Known Surviving PC Operating System

You’ll all be familiar with the PC, the ubiquitous x86-powered workhorse of desktop and portable computing. All modern PCs are descendants of the original from IBM, the model 5150 which made its debut in August 1981. This 8088-CPU-driven machine was expensive and arguably not as accomplished as its competitors, yet became an instant commercial success.

The genesis of its principal operating system is famous in providing the foundation of Microsoft’s huge success. They had bought Seattle Computer Products’ 86-DOS, which they then fashioned into the first release version of IBM’s PC-DOS. And for those interested in these early PC operating systems there is a new insight to be found, in the form of a pre-release version of PC-DOS 1.0 that has found its way into the hands of OS/2 Museum.

Sadly they don’t show us the diskette itself, but we are told it is the single-sided 160K 5.25″ variety that would have been the standard on these early PCs. We say “the standard” rather than “standard” because a floppy drive was an optional extra on a 5150, the most basic model would have used cassette tape as a storage medium.

The disk is bootable, and indeed we can all have a play with its contents due to the magic of emulation. The dates on the files reveal a date of June 1981, so this is definitely a pre-release version and several months older than the previous oldest known PC-DOS version. They detail an array of differences between this disk and the DOS we might recognise, perhaps the most surprising of which is that even at this late stage it lacks support for .EXE executables.

You will probably never choose to run this DOS version on your PC, but it is an extremely interesting and important missing link between surviving 86-DOS and PC-DOS versions. It also has the interesting feature of being the oldest so-far-found operating system created specifically for the PC.

If you are interested in early PC hardware, take a look at this project using an AVR processor to emulate a PC’s 8088.

Header image: (CC BY-SA 3.0 DE).

67 thoughts on “The Oldest Known Surviving PC Operating System

  1. It wasn’t expensive compared to contemporary CP/M machines. I can’t believe anyone bought one without one floppy drive. Floppy drives were not that expensive and if you think cassette tape was acceptable, you’ve never used one as a storage medium.

    I’ll have to see if my old Snake game runs on the emulator. I bet I have a listing around here somewhere…

    1. Depends on the platform. TRS80 color computer Cassette drive was FASTER than the commodore 64 Floppy drive. and the old EBCDIC reel to reel drives I had would kick the crud out of early PC floppies. Tape is absolutely acceptable storage medium when it’s on a controlled deck. My coco Deck actually had controls to rewind stop and fast forward. I wrote an early database that used cassettes. Tape was used by a LOT of businesses well into the late 80’s for nearline storage as hard drives and floppy carousels were horridly overpriced.

      1. As someone who owned a couple TRS-80 CoCo for a number of years (a 1e and a 3), that doesn’t seem to be true. Tape was slow, and “seeking” to the right spot made tape quite a pain. I can remember how much frustration tapes were, and how slow CLOADM was vs LOADM… And the nice disk controllers had spot for 4x 27128 EPROMs too, which could contain a lot of useful stuff.

        1. I got a CoCo because it was “inexpensive” with a floppy drive. In quotes because maybe by the time I bought it all, other options could have been viable. I was ready to do more than program, and I wasn’t going to use cassette toes for word processing. The CoCo did mean being able to buy it all over a period of time, which was easier than buying it at one time. It was 1984.

          I bought a commercial word processor for it, but I also got OS-9, the multitasking OS for the 6809 from Microware. I definitely needed a floppy drive for that.

          I suspect, though, that in the UK and Europe thy made with cassettes as a storage medium longer than in North America.


    2. In 1980, an RT-11 compatible 8″ floppy drive was about £1500! A PDP-11 cost around £50K, and that was the option price. Sony had an option for one of their video editing computers with 2 x 8″ floppies for £3,170. A 5.25″ floppy for a BBC Micro was around £200. I know because I bought one. Cassette tape may not have been acceptable, but if it was the only game in town, then you had no choice. Paper tape was still the standard in 1980 on PDPs and I still have both a boot loader tape, and data tape in my archive box.

    1. Well technically they said the “oldest known surviving” OS, which based on the article I take to mean original, working install media, rather than just chronologically oldest.

      That said, I wouldn’t be surprised if there’s some diskettes or tapes floating around out there undiscovered, containing a working installation of Apple DOS or Atari DOS, or even CP/M. Like paleontology, it’s only the oldest dinosaur until you dig up its ancestor!

    2. In the first sentence, I believe he’s using “PC” to mean the original IBM PC. If I recall correctly, PC was not really a generic term back then, although ‘personal computer’ might have been.

    3. Multi-user OS for a minicomputer – obviously an example of a Personal Computer system! :D

      How about the Wang 2200? It was intended to be used as a personal workstation. Didn’t really have a separate OS though.

  2. I still have an old MS-DOS 1.2 manual lying around in an old box. What has always wondered me is that according to the manual, the DIR command was only introduced in version 1.2. So the earliest version did not even have DIR?

    1. It was pretty primitive to begin with, no subdirectories and I forget what else. I suspect there must have been something to view what files there were, though maybe thy just forgot, hence the next release came very soon after.


      1. I hear it was MS-DOS 2.0 which introduced the concept of directories… and AFAIK that was thanks to Xenix, which was Microsoft’s other OS.

        I still wonder to this day what the IT world would look like had Microsoft just offered Xenix up as the OS for the IBM/PC instead of buying DOS…

        1. Why do you assume they didn’t? IBM wanted an inexpensive operating system for their “inexpensive” personal computer. Xenix wasn’t inexpensive, it had higher hardware requirements than MSDOS and it added the Unix licence frees.

    2. It helps to remember that MS-DOS/PC-DOS — being derived from Seattle Computer Products’ 86-DOS — was itself a poorly-executed clone of CP/M. Which absolutely _DID_ have a DIR command. I think some folks here are confusing DIR (which means “show me a list of the files on this drive) with the mkdir and chdir commands that supported multiple directories.

  3. Well I am confused!

    The PC Junior (PC Jr) predates the XT by a year and a half (Released August 1981) and had PC DOS 2.1 and a Floppy Drive. I never saw any Cassette player on IBMs or Clones though cassette players abounded on home (game) computers. The IBM was different in that it was aimed as a business computer. The PC Jr was also an 8088.

    The genuine IBM XT has a 10MB Hard drive by default. For most of the clones, a hard drive was optional and in the very early days it was a MFM Seagate 20MB. A short time later a RLL controller was released that would get 30MB from the same drive. Shortly after that there was an avalanche of drives/manufacturers/ sizes. The brand ‘CCS’ was the most common clone that I saw. I had a CCS that had two floppy drives and no hard drive. I later added 2 RRL Seagate drives. Though this says noting of standards as I built my XT from parts. I was a computer maintenance engineer at the time.

    I never had a PC Jr. My first if the IBM (or clones) was an XT and if my memory serves me, I was using MS DOS 2.1

    1. Another thing that doesn’t make sense. This disk is 160kB which is 8 sectors per track and that format was very common in earlier home (game) computers that had nothing to do with IBM or clones. All the early IBMs (Jr, XT AT) had a 360kB Floppy drive and that is 9 sectors per track.

      1. The original PC supported 35 track drives but IBM never shipped them with anything other than 40 track drives. If you wanted to use a 35 track drive from another vendor, you could.

      1. What are you calling an “IBM PC”. The only model I remember with PC in the name was PCJr.

        OK, the XT and PCJr were the other way around but what does that change.

        What did the PC-DOS run on ? PC-DOS is for 8088 (IBM XT and PCjr). MS-DOS was for 8086 (IBM AT onwards).

        1. OK lol. I confused the model *numbers* 5150 and 5160. I didn’t see the 5150 because I entered that field after it was created. I saw hundreds of XT’s (5160) and only one PCJr

    2. I have a friend who worked for IBM at the time the original PC was released; employees could get a good deal on them; and his had a cassette tape interface. Ironically he was working for their storage division at the time as an engineer, helping make better disk packs for their mainframes….

  4. The original PC had single sided 160K floppies. DOS 1.x had no sub directories. DIR was a command in the first versions of PC-DOS or MS-DOS if you prefer. With a later version of DOS they changed it to 180K and supported double sided disks and sub-directories. You could have written your own drivers if you wanted. At one point, I tried to write a driver that would read Apple ][ floppies but was in over my head.

    I never saw anyone use cassette with an IBM PC, though if I remember correctly it was an option. I had one of the first 5150s in the Bay Area. I believe my dad bought it in 1981. I don’t remember what eventually happened to it. I know the Altair that I had before it was traded for a dot matrix printer in the mid-80s.

    1. PC DOS was for the 8088 and MS DOS was for the 8086.

      MS DOS supported many different Floppy formats and the utility even had switches for Tracks (cylinders) and sectors –

      format a: /t:40 /n:8 would format 320kB – 40 tracks, 8 sectors or single sided 160kB
      format a: /t:40 /n:9 would format 360kB – 40 tracks, 9 sectors or single sided 180kB

      1. IBM never released a PC model with the 8086 processor. Compaq didn’t either. Your history is a little off.

        The IBM PC and original Compaq computer had the 8088. The IBM AT had the 80286.

          1. @ROB

            I think Buddy’s ‘point’ was this:

            PC DOS was intended to be sold with genuine IBM machines (like the Model 5150/5160 series etc.), while MS DOS (as agreed between MS and IBM) was to be sold to non-IBM x86 systems (call these PC clones, if you will). With VERY few exceptions, nearly ALL of those early PCs (IBM and clones) used the 8088 almost exclusively for their CPU. Note that I am also including clones of the 8088 CPU here as well ie. NEC V20 :-)

            The i8086 only ever registered a tiny blip on the radar of mainstream PCs, if at all (ie. Tandy 2000 computer, an interesting machine that went nowhere IIRC..).

    2. Apple II drives were nearly all software and self calibrating. They could use a wide speed variation and still move disks between systems. You have to know all the Woz tricks – which were available in the Red Book and other sources – to write a driver.

  5. There’s some weird story about a time traveler coming back in time to retrieve one of these computers. Something about it being special, so they could rebuild a computer infrastructure with it without having to start from scratch… It was weird.

  6. A weird thing about the PC was its five expansion slots were spaced farther apart than what became the standard spacing on the XT – still used to this day. What’s extra weird is from the start, all the expansion cards had the backplate shape used throughout the entire history of PC’s – when on the PC that offset wasn’t needed. The plates could have been wider, with straight sides.

    Was IBM already planning to narrow the spacing and go up to 8 slots before the first 5150 ever shipped?

    The wider spacing and the presence of a cassette drive port next to the keyboard port made it impossible to use any motherboard in a 5150 case other than a 5150 motherboard. The first PC was also the first with a proprietary physical design.

    Wayyyyy back when, I hadn’t given that any thought – until the time I got another board and went to install it in a 5150. “What the? The slots are farther apart?! I wonder if I can get any money out of this old PC to use towards a new case?”

  7. The first 5150 IBM PC had no floppy drives in its base configuration. The motherboard had 5 slots and 16k RAM expandable to 64k with chips. It also had the cassette interface. No hard drive was available. I`m guessing IBM created such a basic model to compete with the Apple II in the home market.
    The XT, though, did away with the cassette interface, had 8 slots and up to 256K RAM onboard. It also had a hard drive.
    MS-DOS was the name of the original CP/M-inspired OS by SCP. It ran on 8086 or 8088. IBM bought a license and called it PC-DOS. 1.0 was released with the original floppy only PC; 2.0 came with the XT and had subdirectories, a most useful feature when you had to manage the (then) huge 10M of storage!

  8. According to Wikipedia, the original IBM PC had an 8088 CPU, a whopping >> 16K << of RAM, and _NO_ floppy disks. Given that, I'd say that a cassette drive would have been of very much interest.

    1. While it is true the original PC was offered in the configuration you mentioned, almost nobody was going to pay nearly $2,000 for such a system (including monitor) to run BASIC from ROM and use.. a cassette drive for storage (ugh :-). IBM’s potential competitors were already selling systems to business customers with disk drives installed/included (like hotcakes!) in 1981 so..

      1. It’s true. By 1978 my TRS-80 was running two 5.25″ floppies (albeit not very well — TRS-DOS 1.0 was full of bugs). More importantly, S-100 systems had been running CP/M with 64K of RAM and four 8″ floppies since 1977. _AND_ running serious business software like Peachtree’s business suite.

        But those, of course, were not IBM PC’s, which I think was the original topic of this thread.

        1. My school had a TRS-80 Model 1 with an expansion unit (64kB RAM), cassette, 4 x 5.25″ single sided floppy drives (we used to notch the floppy to use the second side), and a foil printer and then a DMP1 or was it DMP1000?

          Once we had NewDOS I never went back to TRSDOS.

    2. I think by that time, few would have used it without a floppy or two. Why pay extra money for a “business computer” if you had about the same capability as a Commodore 64?

      What definitely happened was that some did buy the basic system, then buy ram and floppy disk drives elsewhere, and upgrade. Done right, it was cheaper than buying the system more loaded from IBM.

      And once the clone business opened up, even if you went with IBM for the computer, the upgrades became even cheaper.


      1. >> I think by that time, few would have used it without a floppy or two. Why pay extra money for a “business computer” if you had about the same capability as a Commodore 64?

        I don’t think even IBM would have seriously claimed that the PC was a “business computer.” In 1981, that niche was thoroughly filled by high-end S-100 systems running CP/M on 8080’s and Z80’s.

        As I recall, the clone business didn’t come along until much later. I got mine in 1986, Admittedly, I was a late-comer to the PC clone world. I was dragged kicking and screaming from the world of CP/M, simply because my ’83 Kaypro Z80 system would kick the PC’s butt in any performance comparison.

        A little-known fact: When Intel came out with the 8086, they realized that they’d have to sell it to people and groups that had a huge investment in CP/M-based software. So they developed a translator that would take in 8080 object code, and spew out 8086/8088-compatible code. It worked, but with a performance penalty.

        Which explains why the 16K Microsoft BASIC for CP/M turned out over 100K on the PC.

  9. Best I can do is MS-DOS 2.11 in ROM, 1987. Still working.

    In deference to Ms. List’s apparent intent, I will not enter my 8″ CP/M floppy disc, nor my 5¼” Atari DOS 3.0 floppy disc into this highly technical version of “I Can Name That Tune in…Notes”.

    Sill have your copy of Lifeboat Associates’ catalog? (err, catalogue?)

  10. Speaking of hacks, the original IBM PC used single-sided floppy disk drives. I hacked mine to use double-sided drives (though, the OS treated them as two separate drives–I gave up trying before finding a workaround for that), which amazed my compatriots at the ComputerLand where I worked. I also had fun hacking the code to display various wacky messages (mostly Star Trek references) during the morning startup. Neither is the slightest bit impressive by today’s standards, but I like to think they were cutting edge at the time. Oh, and I also hacked my Exidy Sorcerer to have true bitmapped graphics instead of the character-based graphics that were the standard. The biggest challenge there was figuring out how dynamic RAM worked so that I could expand it.

  11. “The Commodore PET (Personal Electronic Transactor) is a line of home/personal computers produced starting in 1977 by Commodore International.” – didn’t use anything by microspasm.

    1. >> didn’t use anything by microspasm.

      Which brings up an interesting point of philosophy. Twice in my life, I’ve managed to achieve my own personal goal of having a computer with nothing inside from either Intel or Microsoft. Nothing against Intel, actually. My beef with them is only in their x86 architecture.

      My first was the Kaypro IV “luggable” running CP/M on a Z-80. Yes, the Kaypro came with MBASIC, but I replaced it with SBASIC (which I used, like NEVER!). Microsoft also offered MASM and Pascal, but nobody forced you to take them, and I did just fine with Turbo Pascal, BDS C, two flavors of Modula 2, and SLR’s very excellent assembler suite.

      The second was with a PC clone with a Processor Technology PT68/K2 motherboard running a Moto 68000 and SK*DOS operating system. No worries about running Microsoft software on THAT!!!

      My dream is to realize the original goal at lease one more time, this time using a TI MSP430 and TI’s Code Composer IDE.

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