MS-DOS 3.31: The DOS you couldn’t have

Last Updated on March 3, 2022 by Dave Farquhar

When it comes to the best operating system to use for a vintage PC, if it has a 286 or better processor, there is little reason to use anything older than MS-DOS 5 or PC-DOS 5. But with an XT, it’s a little different. The best MS-DOS version to use is one you may have never heard of. MS-DOS 3.31 was an upgrade you couldn’t buy.

What MS-DOS 3.31 was

MS-DOS 3.31
MS-DOS 3.31 originated at Compaq, so when it turns up, it may have Compaq branding. But other OEMs also supplied it, such as Emerson.

Like its name suggests, MS DOS 3.31 was a very minor update to the very common MS-DOS 3.3. Except that minor update is extremely important now.

MS-DOS 3.3 had a lot of limitations. No memory management, no decent text editor, no command line history, but it was small and light. It took minimal space on disk and minimal space and memory, which is important on XT class machines, especially if they don’t have 640k.

But there was one other limitation. It did not support hard drives bigger than 32 MB, regardless of what your BIOS may or may not support.

This is why a lot of MFM hard drive controllers had a virtual drive option. If you bought a 40 MB drive, you could split it into a pair of 20s. And then you didn’t waste any space. It was inconvenient, but throwing away 8 MB was even less convenient. That 8 megabytes was worth $40, at least.

Enter MS-DOS 4.0

If you needed to use monster hard drives, you were supposed to buy MS-DOS 4.0. Or PC DOS 4.0, the IBM equivalent, if you had an IBM. MS-DOS 4.0 was, shall we say, the Windows Vista or the Windows 8 of its day. It promised the moon, delivered a lot less, and what it did deliver was buggy.

Everyone who bought it regretted it, and the major PC makers quickly fell back to shipping machines with DOS 3.3. It wasn’t ideal because DOS 3.3 was showing its age, but it mostly worked.

In those days, the difference between MS-DOS and PC DOS was mostly marketing. IBM and Microsoft developed the product jointly, and IBM sold it under its brand name through its dealers, and Microsoft retained the right to license it to anyone else who wanted it.

Prior to version 4, Microsoft took the lead, and IBM mostly did QA work. With version 4, IBM took the lead. So when things went sideways, Microsoft naturally blamed IBM.

Compaq to the rescue

Microsoft was also tight with Compaq. Part of this was because Compaq wanted to build a PC based on a 386 processor, and IBM did not. Microsoft was naturally very interested in a 386-based PC. If nothing else, it would make their development go faster.

There is a reason why Windows 3.0 and Windows 3.1 ran better on Compaq PCs than any other brand. Microsoft wrote the product on those PCs.

Compaq had the bright idea to backport the disk support from MS-DOS 4 to MS-DOS 3.3. Which company did the work seems to be lost to history, and at that time, Microsoft did not sell MS-DOS at retail. You bought it from the company that sold you your PC.

MS-DOS 3.31 only shipped with select PCs, generally those that had large hard drives from the factory. Of course Compaq was one, but there were some other companies that sold it. I have a copy of MS-DOS 3.31 that originated at Emerson that doesn’t contain any OEM branding. The Compaq version did have OEM branding.

Digital Research saw an opening and pounced. It was during this same timeframe that DR DOS hit the market. DR DOS reported itself as DOS version 3.31.

What’s so great about MS-DOS 3.31?

If you have an XT class machine and you have an XT IDE card for it, you’re going to have a monster drive by XT standards. You will need to use MS-DOS 5 to be able to use more than 32 megabytes of the drive. The problem with that is the memory usage. Version 5 uses about 20k of memory more than version 3.3 did. That’s no big deal on a 286 or better, because you can use upper memory or the high memory area and gain more than you lose.

Unless your XT has upper memory blocks, you just lose 20k of memory. Not necessarily a deal breaker, but if you can avoid it, it would be nice.

If you don’t need the other features of MS-DOS 5, which you may not, especially on a hobby machine rather than a daily use machine, MS-DOS 3.31 is a nice compromise. You get 1987 functionality with larger drive support.

And on machines like a Tandy 1000, which officially did not support anything newer than MS-DOS 3.3, version 3.31 is a nice compromise. Yes, you can get version 5 or even 6.22 to work, or PC DOS 7 for that matter, but it may feel more authentic to use MS-DOS 3.31 and a copy over the Tandy modified utilities. It’s a hack, and not many people did it, but you can make it look like something that someone might have done.

And for all I know, there may have been people in St Louis who did such a thing. In the 1980s, the epicenter for computing in St Louis was very near the corporate headquarters for McDonnell Douglas and Emerson. Emerson sold PCs that used MS-DOS 3.31. Emerson employees therefore may have had access to this rare version of MS-DOS. They would have had friends working at McDonnell Douglas.

I don’t know that sharing went on. But it’s easy to imagine a scenario where an Emerson employee hacked a 40 MB hard card into an XT class machine at home and used a boot disk from work to make it into a single drive. And then they told their friend about it and shared the boot disk.

By the time I got a PC, MS-DOS 3.31 was obsolete. I can tell you in the MS-DOS 6 days, we traded boot disks like crazy. Circa 1988, the situation may not have been all that different. I just don’t know how many people were buying 40 MB hard drives and putting them in an XT class machine at that point.

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2 thoughts on “MS-DOS 3.31: The DOS you couldn’t have

  • January 19, 2022 at 2:44 pm

    As I recall, at the same time as the MS-DOD 4 failure, Digital Research introduced its excellent and compatible DR-DOS!

    • January 23, 2022 at 11:07 pm

      Yes, that was the same timeframe. And that is one whopper of a tangled story. I wonder if I’m up for that one…?

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