I understand MS-DOS to be backwards-compatible, so is there a reason to run a version earlier than the last (6.22) on an old computer? The only reason I can think of would be for period-accuracy, but is that it?
Later versions of DOS tended to use more memory. On 386 systems, or systems with EMS memory, this wasn't a huge issue because dos could be configured to use memory outside of the standard 640K region for a large proportion of its use, but on 286 machines (which couldn't run EMM386 to emulate EMS memory, but generally didn't have any real EMS) this wasn't possible.
That said, I ran DOS 6.2 on my 286 happily for many years, just keeping back a boot disk with 3.3 on it for the rare occasion it was necessary.
I know you said "MS-DOS" specifically, but PC-DOS was MS-DOS up until PC-DOS 6.1 in 1993. (MS-DOS 6.0 came out 3 months earlier.) With the split, IBM dropped QBasic (later to be replaced in v7.0 with REXX) and replaced the MS-DOS editor with the superior programmer's editor E. For me, that was reason enough to stop using MS-DOS and become a PC-DOS user. I know that is not much earlier than MS-DOS 6.22, but it is earlier.
You might be interested in Wikipedia's Timeline of DOS operating systems. It includes a large amount of detail, and covers many decades, beginning even before MS-DOS 1.0.
If the old computer is '100% IBM PC Compatible'^tm, then you should be able to run basically any version of MS/PC-DOS on them. However, there were lots of old computers that ran MS-DOS (especially when the IBM PC was young) that had non-compatible hardware variations that would limit that. For example:
- If it uses 8" floppies, support for that disappeared in MS-DOS 2.11. More common than you might think at one time.
- Anything with an 80188/80186 rather than an 8088/8086 (e.g. Tandy 2000, Mindset) is probably going to have issues with later DOS.
- If it's an early PC-9801 series (the original 8086 or V30 only sorta PC compatible versions of the hugely popular Japanese PC-98 platform), then I'm pretty sure you're stuck at MS-DOS 3.3, but certainly no later than 6.20. NEC had a similar machine called the APC-III that I recall ran up to 3.11.
- Didja know TI made an IBM PC competitor? Neither did anyone else. If you have a TI 'Professional Computer' the not-quite-compatible design and TI's quick exit from the market leaves you at DOS 2.1 or so. Lots of orphans fall into this category: Dec Rainbow, Victor 9000, Sanyo MBC-55, Wang Professional, Zenith Z-100 series.
You get the idea.
Earlier versions of MS-DOS 6 included DoubleSpace - a compression utility that was important in the days of smaller hard drives and high costs per megabyte.
Later versions of MS-DOS 6 were downgrades which did not include DoubleSpace as the Stac Corporation had successfully sued Microsoft for infringement of data compression patents. https://en.wikipedia.org/wiki/Stac_Electronics.
If you intend to use the original hard disk this may be a relevant consideration.
6.2 had a brilliant file transfer utility which you could work through either the parallel port or serial port and a crossover cable. On earlier versions, you had to use a Norton based tool. On the later versions (W95 = DOS7), this facility was missing.
The great thing about it is it can be used for transferring the contents of a WinXP CD to an old laptop without a CD drive and then installing it. It is also great for transferring stuff from Linux CDs for Linux installations: as long as they keep to the 8.3 naming convention, there is no problem with the transfer.
The only problem is that the disks are limited to 2Gb.
There are some instances when having an older version of MS-DOS around could be helpful. Realistically, I would use the latest and greatest unless you have come across a reason otherwise.
When dealing with installs of old programs, I have come across ones which will only work on a certain version of DOS. For instance, MS-DOS 5.00 was one that I remember a couple programs needed.
Keep in mind this was doing installs of programs made in the late 1980s early 1990s.
I don't see much sense in using DOS older than the latest one, as in DOS7.1 from WIN98, not as in official DOS6.22. It had a lot of improvements over earlier versions, for example long file names and better optimized writes and reads. The only downside is probably the compatibility - I personally didn't get into any problems when using 7.1, but if you need to run very (probably pre-1985 tier) software, you'll be safer using 6.22.
One exception to this is when you have a legal copy of DOS that you just want to use on a retro computer - in this situation, it's completely understandable to use older versions.