Arca Noae is modernizing OS/2 Warp for 21st century PCs Although the creator of OS/2 now owns Red Hat and has other fish to try, OS/2 lives on. The Reg spoke with Arca Noae's Lewis Rosenthal about the issues of updating OS/2 Warp for modern PCs in 2023 – and beyond. OS/2 has had a long and sometimes troubled history, which The Register looked at in depth when the OS turned 25. …
This post has been deleted by its author
[Author/interviewer here]
> ctrl alt del reboot
Arca Noae has supplied me with an evaluation licence, and I have got it on a machine already, but I am in the process of moving house and country, so I have not had much time to investigate it yet.
I think, but I am not sure about this, that it's an option now.
Yes. This was fixed by Stardock during the Warp 3 lifetime with Process Commander and a very similar product is bundled with Arca Noae. Upon pressing CAD it's possible to either reboot, kill a process, or switch to a full screen command prompt - it's very useful.
The main use for Process Commander at the time was the delights of the synchronous input queue hanging input to otherwise happily running applications, so killing the offending program enabled the system to carry on running. Once this was largely worked around in Warp 3 Fixpack 16 (*) the demand for the product went away.
(*) First public fixpack that fixed the issue. Actually first fixed in a special Fixpack 12 for Developer Connection.
"but no 64-bit Edition of Windows can run 16 bit code anymore"
Well, sorta true, at least out of the box.
I'm quite happily using winevdm to run a couple 16-bit Win3 era software on my 64-bit Windows laptop. YMMV.
Based on my hazy memories of OS/2 2.1 up to Warp 4 - DOS programs are still likely to run far better under DOSBox. If the business case is about running only DOS program(s) in bare metal, there's always FreeDOS.
If I needed to run 16-bit Windows code in a business environment, I'd more probably install a 32-bit Windows 10 than go with OS/2. Or just use the winevdm module.
If I needed to run OS/2 software, then this Arcaos would be an obvious candidate along with eComstation or Warp itself in a VM.
If you install "32-bit Windows 10" and disable all network access, will the OS be automatically deactivate after a number of months/years, or will it happily function until powered down during a few yearly factory downtime maintenance windows ? Basically will it be able to provide decent uptime if it is prevented from phoning home to upload its collected telemetry, while downloading patches, and automatically reboot at least once a month instigated by Microsoft. Will the storage slowly fill up with telemetry that will never be upload ?
Would you trust a machine running a Windows OS to fully control production machines in a factory ? Where downtime in the middle of a production run could end up costing you major contract penalties ?
Your post is full of FUD.
"If you install "32-bit Windows 10" and disable all network access, will the OS be automatically deactivate after a number of months/years"
Easy to test. I powered down a PC, moved the date 10 years forward and booted Windows 10 up with no network. No errors or anything. Still says it is activated.
In my past I have deployed perhaps a few hundred POS systems with Windows 10 and never connected to internet. No issues.
"Would you trust a machine running a Windows OS to fully control production machines in a factory ?"
I do work for a multinational pharmaceutical with a lot of Windows computers running their machinery. Not an issue.
I do work for a multinational pharmaceutical with a lot of Windows computers running their machinery. Not an issue.
That's really scary! I have never seen any Windows installation of anything that wasn't "an issue"! I remember being pointed to the EULA when I complained to MS about the fundamental lack of stability of any version of Windows: "it's your problem, not ours" is the gist. We solved the problem entirely with a move to FOSS software that was easy to maintain, and entirely reliable.
Definitely seen Windows over 10 years old still working solidly.
On one occasion, somebody connected a keyboard / monitor switch to a windows machine and it was still left signed in by an account of somebody who left over 10 years ago.
I know of a Novell Netware server from the 90s , still doing its job. There is currently some headscratching going on about what to do about it.
[Author/interviewer here]
> I'm quite happily using winevdm
I did write about this stuff quite recently:
https://www.theregister.com/2022/06/28/friday_foss_fest_running_dos/
I did encounter some problems TBH. Some things are not of clear legality, some seem to have possibly virus-infected distribution files, and the ones that are 100% legit and work are not very stable, not very compatible, or feature-poor.
It is fairly plain that while the compatibility was a standard feature -- e.g. in 32-bit Windows, and DOSemu in Linux -- it was well-tested and robust.
Now, it's a niche edge interest, and it shows. :-(
DOSbox is an emulator. If that's OK for you, good, but it has the same limitations as VMs. It can't talk to hardware, such as an ISA expansion card.
DOSemu is the real OS on the real CPU, so I know people using it to write EPROMs in original Centronics-port EPROM blowers, for instance. That kind of thing probably won't work in VMs or emulators.
FreeDOS works very well but it can't boot on a UEFI machine without BIOS emulation. It can't handle >2TB hard disks, which need GPT partitioning.
The use cases here are niche but they are real.
eComStation is still out there. There is a copy on the Internet Archive, I've installed it, and it works. It's extremely limited, though. It does include Firefox but a very elderly version that can't open any modern websites, and updating it is a massive and very complicated undertaking.
After 2 days of work I got it from Firefox 4 to Firefox 10, via about 25 separate dependencies... and F/f 10 is no better and still can't open any HTTPS sites.
So, it's there, and you are welcome to try, but it's not trivial. It will require solid 32-bit OS/2 knowledge from the 1990s. DOS, Windows and Linux skills mostly will not help you here.
"I did encounter some problems TBH. Some things are not of clear legality, some seem to have possibly virus-infected distribution files, and the ones that are 100% legit and work are not very stable, not very compatible, or feature-poor."
The stuff is in Github, should be easy to compile your own, if you don't trust the pre-compiled releases there. As i said, "YMMV".
Then there's the 25-year-old issue of Win32s. OS/2 used to support only up to v1.25 but Win32s spec 1.3 wasn't supported. Some software was known to use the latter API and didn't work with OS/2. Kudos to Arca Noae if they have overcome this obstruction!
"DOSbox is an emulator. If that's OK for you, good, but it has the same limitations as VMs. It can't talk to hardware, such as an ISA expansion card. DOSemu is the real OS on the real CPU, so I know people using it to write EPROMs in original Centronics-port EPROM blowers, for instance. That kind of thing probably won't work in VMs or emulators."
Yes, DOSBox is an emulator, but it can talk to serial ports and some non-vanilla forks also have had decent enough parallel port support to work with HASP dongles. If your use case is to run DOS era hw diagnostics thru serial, I'd go with DOSBox first. Expansion cards are a no-go, of course.
"FreeDOS works very well but it can't boot on a UEFI machine without BIOS emulation. It can't handle >2TB hard disks, which need GPT partitioning."
While true, those are issue (or a hindrance) only if you're dual booting with another OS. My 10th gen Lenovo T14 still has legacy boot option.
The latter can be averted if you boot FreeDOS from USB, so even an old 2 GB stick could be perfectly serviceable and is still spectacularly fast for old DOS stuff. I used to have a 16MB DOS stick (USB 1.1) back in early 00's for easy and fast BIOS & FW upgrades. (until the updates grew too large)
If you have a UEFI machine or GPT disks, you most likely won't have ISA or PCI slots. If it's an industrial PC with such slots, it will absolutely have legacy boot option as well.
Thank you Liam for the retro computing OS series. Some of it is almost an "alternate universe" kind of thing. What if Apple choose BeOS and not NEXT in 1997? Now the new version of Haiku is almost ready. Same with OS/2 and Arca Noae. Imagine in 1985 a free version of DOS. What would Microsoft have said and done? Good to see these "old technologies" being updated and being able to meet the developers. It would be interesting to know about some of the developers of some of the older open source products such as SeaMonkey and maybe OpenOffice are doing and what there plans for teh future are. Very interesting reading about things I remember from the 1980s and 1990s and how they still continue but at a slower base and often by a dedicated small team of developers.
One former co-worked stated that the world would have been a much better place had IBM went with the 68000 running the 68K version of OS-9. Don;t think he was too far off.
I have been reading through the Volume 0 (1975-76) issues of BYTE and was intrigued by the issues of working with bare metal hardware. One writer was wondering about how nice it would be if there was some standard software that would handle the chores of doing I/O - which sounds like an OS to me.
[Author here]
> the world would have been a much better place had IBM went with the 68000
Would we really want a _more_ powerful IBM in the 1980s? I am not at all sure about that!
There were lots of successful 68000 machines. I think it got its decade in the sun -- and indeed in the SUN. ;-)
Yes, I was just thinking that a blanket "thank you" was in order.
It has been an eye-opener for at least some if us ancient diehards, that so much of our youth is still chugging along too.
I begin to wonder what it does take to kill off an old OS.
[Author here]
> Some of it is almost an "alternate universe" kind of thing.
Well thank you! I am glad people are enjoying it. There will be more. :-)
Some of it is very illuminating, IMHO.
If Apple bought Be instead of NeXT, I think Apple would be long dead. Nobody discussed it then, but for me, the best course for Be back then would have been a deal with Acorn. Acorn could make SMP Arm boxes in the late 1990s, when nobody else could, and the price/performance and performance/Watt of Arm was unbeatable.
SMP Pentium boxes did exist but they cost a lot and needed serious power and cooling.
Acorn could have done a 4-ARM box that ran on less juice and needed less cooling, and Be could have done an OS that could have usefully exploited it, long before Linux kernel 2.0 (when SMP started working) or Linux on ARM, before NT ran on Arm, etc.
If IBM had let OS/2 1.x be 386-specific as Microsoft allegedly wanted, it could have been a big success. IBM would have made enough money to give every owner of 286-based PS/2s a free 386 upgrade.
But frankly, Windows 3 was quicker, easier, ran on low-end kit well, and was more use to more people.
Then, 2 years before the media hoopla over Windows 95, Windows NT was a very good solid functional OS out of the gate. A poor UI but stable, networked, portable, SMP-capable from the first version.
NT showed the benefits of MS' experience over IBM's lack of it. It was far far easier to install and configure.
OS/2 2 and later Warp were good OSes, but hard to install, hard to configure.
NT integrated all kinds of things IBM tried to reserve for servers or premium editions: SMP as standard, networking as standard, Internet support as standard, *and* it was easier.
OS/2 had the snazzy UI and ran on much lower-end kit, but in the fullness of time, NT caught up or surpassed it in these areas, and the hardware caught up with NT.
Apple made the right call with NeXT.
The PC industry made the right call with Windows.
Acorn and Be didn't spot the paths to possible success.
For me, the real big "what if" is probably Digital Research.
It had a better DOS, a better multitasking OS earlier than anyone, a better lighter GUI desktop.
But DR got screwed over by Intel removing features Concurrent DOS 286 needed from the final shipping step of the 80286, and while it adapted cleverly and survived, it never thrived as it deserved to.
DR could have given us a multitasking DOS-compatible OS with a GUI long before Microsoft and IBM did... but it never got the chance.
Firstly, thank you for this series - you just reminded me of a sliding doors moment between OS/2 and WNT.
From what I can vaguely remember we were an IBM shop with AS/400 and 5250 terminals. I was new and had only used Windows PCs up until then but it became my job to integrate the AS/400 with desktop WforWG 3.11. IBM were pushing for us to use OS/2 - boss ordered new server and copy of os/2 landed on my desk. Spent about 3 days trying to install OS/2 - called IBm support etc. no joy.
Happened to buy a pc mag that had NT3.51 CD on it. Thought what the hell let’s give it a go - almost straight away NT said that the network card was not functioning!
Swapped network card out for a known good one, and NT was up and running. SNA server soon went on and my career started!
Always wondered what would have happened had IBM put in more usable error messages!
Retro computing is a huge scene, tons of gamers have enjoyed retro gaming for decades due to the hard work of people pulling old hardware and software apart and seeing what makes it tick. One of the most rewarding things I've done is learn how to write a basic retro emulator, it's bloody hard work but you learn so much more about what makes coding and software tick when you have "think in two places at once".
It's a small team of die hards, some very young indeed, who are keeping all this going and despite what the likes of MS, Apple and the the cloudy corps want us to believe that we should never look back. Like most things in life, seeing where you've come from, spending time revelling in it sometimes, is a good thing as you appreciate how far we've come.
OS/2's excellent support for subsystems was eventually part of its problem: companies didn't need to bother writing native code and thus becoming invested in its success. The APIs were better, hardware support was fantastic (including driver virtualisation) but it was hobbled for many by the WPS locking up so easily. This felt like the OS had crashed and, unless you knew that it hadn't and how to close and restart the WPS, it was to all intents. But for decades airports and banks ran on OS/2 because it never crashed and provided fantastic support for all their mainframe systems.
In the meantime Microsoft continued to make Windows more acceptable and eventually even better. Though, even today, there are things that OS/2 could do that Windows still can't without running virtual machines.
And any good IT student could do worse than take a peek at OS/2, especially the internals to understand that there are ways of making better mousetraps.
"Windows still way behind some of the features in the Workplace Shell."
And Linux too. It pains me so much to still have to deal with the fact that every application has a little file system viewer built-in so that you can open and save content. The Template system used in the WPS and the desktop metaphor was so seamlessly familiar to how our brains manage physical data storage it just felt like the right way to do things.
Of the things I wish survived from the days of OS/2 Warp, Taligent, OpenDoc, etc I wish the Workplace Shell survived. The attack on Netscape really was one of the key turning points since it took years before anyone took Mozilla serious and so Microsoft's Internet Explorer became THE standard for businesses and that also meant only Windows thrived.
OpenDoc is the 2nd thing I wished had survived since it would have opened up the ISV market to include tons of specialized component vendors instead of every ISV having to recreate world plus dog in every application.
Running OS/2 back in the day allowed me to not only run OS/2, DOS and Win16 or Win32s applications, it also allowed me to run *nix applications including XWindow applications via XFree86 and EMS(?). For a time, I developed UNIX apps on my OS/2 machine and later ported them(recompiled) to run on full blown UNIX workstations. And the ability to run so many other OS APIs felt seamless. Not like running in a VM sandbox.
[ I devised the title and now have to come up with some content for my comment ]
So clearly there is a business case for supporting OS/22 if the bearded one built a company around Arca Naeiou and is plugging it in the marketplace. The fact that nobody appears to have bothered with an OSINE similar to WINE so that you can cling on to 1-2-3 and do your development in Borland C++ 2.0 tells me that the O/S was never a particular winner. It's dead.
I have long had this idle interest in writing a simple O/S that would work a bit like the 8-bit PCs of the 1980's but with access to 64GiB+ RAM, etc. Where my attempt ended was in the sheer complexity of figuring out how to properly support HDDs on the PCI bus and supporting USB (etc., etc.) in the absence of a usable BIOS. Just getting into protected mode without triple faulting – and turning your bootloader into a sudo reboot now that installs in the partition table – is already a major achievement.
* South African English**. Pronounced "Ach no man!!!!". Basically said in frustration so mean something similar to "WTF are you doing?"
** I learned the phrase from my English teacher. But it was never used on me because I was teacher's pet =====>
So clearly there is a business case for supporting OS/22 if the bearded one built a company around Arca Naeiou and is plugging it in the marketplace. ..... Sceptic Tank
A deeper understanding of the development would have some/one declaring .....So clearly there is a business case for shorting OS/22 if the bearded one built a company around Arca Naeiou and is plugging it in the marketplace. ....... as future fiddling with developments in the field transfer sensitive controversial and contentious legacy OS/2 Warp/IBM hardware tasks to postmodern remote virtual machine software servers .... although that is a question of the business case to deny has any validity because of the repercussions and disturbances in the force such developments secure and deliver and reinforce and protect.
Progress in deed. indeed, but not in any way as one was expecting it ..... for new futures available from the future .... rather than dragging up models built in the past for media resurrections to present you with what you are to believe is happening and is going to shortly arrive and change lives/existences/scenes and landscapes, which is your present default World Order Operating System, is it not?
Anyone remember the original floppy release of Office 95 before M$ switched to proprietary high-density floppies to get it down to, IIRC, 24? I think it was 31... I have an original set, probably unreadable now of course, but I’m pretty sure I imaged them to my archive server, you know, just in case.
I was a Windows early adopter and workplace evangelist back when I bought my copy of Windows 3, then 3.1, and then moved to NT 3.51; I read about O/S 2 and would have liked to play with it but I was too busy being an accountant and honing my SuperCalc, Lotus 1-2-3 and then Excel skills at the time - how things can change!
Great series Liam, I always enjoy your articles even though they’re often out of my humble league - please keep them coming! :)
[Author here]
> I hope it's available on 3.5" floppies.
One of the big deals in ArcaOS 5 is that it can boot and install straight from a USB key.
(A dedicated key, but still.)
eComStation must be written to an actual CD medium and booted from an optical disk drive. A USB one will do, but it's mandatory.
So, while I know you're kidding, the truth is in some ways uglier than you think.
This is a very interesting operating system. Main downsides from my perspective: no chance of the ever supporting 64-bit (because they don't have the OS/2 source code I think), no chance of this ever becoming open source (because of IBM) and no free trial version to play around with it.
BY the way I have really enjoyed these recent articles on ancient operating systems that are still alive - Haiku, RISC OS, FreeDOS, ArcaOS. I hope there will be more of this stuff, maybe AmigaOS, MorphOS, AROS, Plan 9, A2/Oberon, or even OpenVMS?
[Author here]
> no chance of this ever becoming open source (because of IBM)
More to the point, because of MS. MS co-developed OS/2, and so much of that code has MS copyright all over it.
IBM went on to develop Workplace OS/2, which runs on PowerPC and uses the Mach kernel as also used in Mac OS X, DEC Tru64 and MkLinux.
There is more change _that_ could be open-sourced, but I doubt it will ever happen, partly for reasons I outlined recently:
https://www.theregister.com/2022/10/04/you_cannot_buy_software/
As for the others:
> Plan 9
[1] https://www.theregister.com/2013/11/01/25_alternative_pc_operating_systems/?page=2
[2] https://www.theregister.com/2022/11/02/plan_9_fork_9front/
[3] https://www.theregister.com/2022/11/29/redox_os_version_08/
> A2/Oberon
[1] https://www.theregister.com/2015/12/02/pi_versus_oberton/
[2] https://www.theregister.com/2022/03/29/non_c_operating_systems/
[3] https://www.theregister.com/2022/12/16/gcc_13_will_support_modula2/
> OpenVMS
https://www.theregister.com/2022/05/10/openvms_92/
I don't have anything that can run modern AmigaOS, sadly. I'd like to!
I do have an old MorphOS machine. I could take a fresh look at that.
I did OS/2 applications work on contract from about 1990 (with OS/2 v1.1) to 1992 (with OS/2 v1.3 and 2.0). I leveraged those OS/2 skills to get contracts at companies in the back half of the 1990s, although sadly, that often meant decomissioning OS/2 in favour of something else (Linux, Windows NT 3.1 or 3.5, or even... Windows 95).
I still work with companies that have an installed base of OS/2 deployments 25 year later. They are usually in turnkey solutions for control systems. eComStation and now Arca Noae have been lifesavers.
Lots of companies have clients with OS/2 software that can't be replaced or upgraded. Often the original vendor no longer exists. If it does exist, it often doesn't have the source for the legacy system, and even if they do, they don't have the skills to maintain it, let alone upgrade it. You want an upgrade to our 25 year old OS/2 product? Here's our Windows 10 version.
Unfortunately, for some clients, that's not an option. They're perfectly happy to keep running the original code, which works as well today as it did in 1997. That's especially true when talking about PCs that weren't networked. Unfortunately, when that 1997 era Pentium Pro dies, finding a replacement that will even load OS/2 is the problem.
Sure, there are virtual boxes, but a lot of OS/2 device drivers are fussy about running in VMs.
That's where Arca Noae has been a godsend. I've had customers with $25M installations with six OS/2 boxes that had a box die, then another, then another, and then another. The cost of upgrading the software is often quoted at $1M or more, when all they need are four replacement PCs that can run OS/2 v4 (or v3, or even v2.1).
No version of OS/2 will install on a modern PC. It doesn't know about USB, or SATA, or pretty much any other innovation that took place after 1996.
But an Arca Noae installation that gets the customer up and running again is worth its' weight in gold. Does it handle quad cores properly? We don't care. As long as it runs as fast as that 1998 era Pentium, and it can handle the custom device driver for the weird 128 port output controller, it spares the customer spending $1M in needless upgrades.
I can't see myself ever doing any new development in C/Set2, and I doubt many others do, either. But just keeping existing OS/2 installs alive makes Arca Noae a viable business.
[Author here]
All good points.
> Does it handle quad cores properly? We don't care.
That's fair.
But I guess there are 2 choices here:
The zero choice: fail to run on modern multicore CPUs.
Or #1, the easy route: only enable 1 core of 1 CPU, like say DOS does, and put the rest to sleep;
Or #2, use the eServer kernel, enable and schedule them all, and possibly pick up the business of the handful of customers who were CPU-bound, or needed SMP, or who can use it.
This post has been deleted by its author
"Would it work on a Sony VAIP P? Being around so long, OS/2 might have drivers?"
If the Sony VAIO P Wiki page specs list is correct, it looks like it would work, mostly.
You will get only unaccelerated VESA graphics for the Intel graphics, and no connectivity since Arca Noae doesn't have support for wireless. A cheap USB-to-Ethernet adaptec could get you going, however. Audio looks also promising since it is derived from ALSA.
HTH.
We are STILL running OS/2 Warp-4 on an IBM PS/2 Model-80 tower machine with the ORIGINAL IBM 1024x768 pixel CRT display! It's a 1980's-era museum piece but STILL works GREAT!
We use it to run very old but very high-end industrial machines that build and stamps things out super-strong metals. All we have had to do is replace capacitors on the motherboard itself and connect MODERN power supplies to the old connectors. Still works great too! Those towers were built like tanks!
We even have an old set of VAX 9000 series 32-bit supercomputers from 1989 which are used to run financial trading stuff using multiple pairs of 128 kilobits per second ISDN communications links. Their slower clock speeds and the unusually ROBUST NATURE OF VAX VMS operating system are used for SLOW TRADING where the SPECIFIC day and night minute-by-minute timing and the SPECIFIC location of a SINGLE TRADE is far more important than multiple fast-buy-sell-responses.
The 1980's-era VMS operating system is highly-tuned toward interrupt-style programs that need synchronous clock-work timing on multiple users rather than haphazard multi-threading of multiple apps! It would take too long to convert all the software we run to use modern Windows and Linux computers although we ARE in the midst of moving everything over to highly-commented robustly-tested C++ object-oriented libraries that run on real-time custom-tape-out RISC hardware that has one-millisecond hardware interrupts to ensure SYNCHRONOUS operations and communications for timing-exact missions and applications! Unfortunately, we won't be finished until 2030 for that conversion project (i.e. even with 20 highly paid programmers and 10 of MSc.EE microcontroller design engineers on this!) so all our OS/2 code and VMS code MUST STILL WORK until at least 2030!
V
With the MKS Toolkit installed, you had a very good approximation to a Unix system (and you could log into it remotely). I came to this after several years on Sun pizza boxes.
But the fondest memories are when (1) a Windows-based colleague created a Word document that would crash his machine upon opening but OS/2 could open it with no problem; and (2) when we were doing some PC-card DOS-based drivers -- it the dosbox crashed, just open up another one and carry one, but under Windows, the entire box crashed (and sometimes needed re-installation). Also the best debugger that I have ever encountered was Borland's debugger for C.
Despite being a major OS/2 zealot in the 90s, nowadays I use it for fun - I have OS/2 1.3, 2.11 and 3 on a 486, Warp 4 on a pentium II, and ArcaOS on a ridiculously over specified multiprocessor Dell Precision T7400 that I picked up cheap off ebay (the machines also run other old stuff including PCDOS - thanks for pointing me at that, NT 3.51, and Unixy stuff).
ArcaOS is a little swings and roundabouts. Vastly easier to install and configure, a moderate amount of acceptably modern software that enables day to day usage. You probably wouldn't want to actually use it as a daily driver.
Not all hardware co-operates well with the multiprocessor kernel (I had issues with an adaptec 2940 and a scanner), but that's expecting a lot considering the driver was first developed when even OS/2 2.11 SMP didn't exist. I tried getting a DAT72 drive working and that resulted in a lot of driver unhappiness, but again this is technology released considerably after the software was written, and I was using free software from Hobbes that had the disclaimer it might not work with other hardware.
SNAP graphics generally works extremely well as does Panorama, modern network chipset support wasn't an issue either.
Similarly, after spending a fair bit of time specifically selecting a graphics card (AMD X800) that supports multi screen operation I discovered that as programs see it as one big screen[1] a quick game of Galactic Civilizations 2 was unusable as it puts an unresizable window across two monitors!
All in all, if you do want to run OS/2 ArcaOS is a very convenient way of getting it up and running. Just don't expect modern Windows or Unix levels of functionality.
[1] As far as I'm aware OS/2 simply doesn't have multi monitor APIs at all. I have a vague memory of it being possible to query multiple outputs.
I don't even see the novelty of running this. Tinkered with OS/2 WARP 3 or 4 back in the day, not memorable. Tried getting a copy of Windows 7 pro 64 bit to run, it is useless as there are no compatible drivers for modern hardware. At only 13 years old it is already dead.
"Look always forward. In last years nests, there are no birds this year." -Movie: Man of la Mancha, 1972
Probably the wrong question.
A necessary condition: no one left with any practical use or need for it but clearly not a sufficient condition.
In the case of OS/2 there are enough really expensive and now abandoned but largely irreplaceable scientific instruments that were managed by OS/2 warp to keep the OS on life support. I had an ancient magazine (APC) CDRom with OS/2 warp 4(?) which some poor soul in the early noughties used to reinstall OS/2 on a replacement PC for some microscopy instrument. You can still get industrial control ISA motherboards (cost$$) but back then there were plenty of retired PCs available.
The right question is probably "What does it take to keep an old OS alive?"
A sufficient condition: one or more people who care, for whatever reason, to keep it running.
As an example if I really wanted to run Multics on real hardware I would have to build a GE645 workalike but there are the people who cared to write an emulator and others who have build the open sourced Multics system for this emulator.
If you have read Organick's "The Multics System" and perused the Multics PL/1 source code you can appreciate these people are seriously ah... serious - definitely not for the fainthearted.
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