ArcaOS 5.1 gives vintage OS/2 a UEFI facelift for the 21st century

In the OS/2 world, ArcaOS 5.1 is a long-awaited release which enables this 32-bit OS from the late 20th century to run on modern PC hardware.

The first version of Arca Noae's distribution of OS/2, ArcaOS 5, was released in 2017, and 5.1 is the ninth release since then. The previous update, version 5.0.8 came out in May, and was the first since December 2021.

5.1 is a significant bump because for the first time it adds native support for PCs with UEFI firmware rather than legacy BIOS, which includes drives partitioned with GPT, the more modern replacement for MBR which allows drives of over 2TB in size.

We chatted with Lewis Rosenthal from Arca Noae early last year, discussing the modernization of this venerable OS.

ArcaOS is based on the kernel from the last IBM version of OS/2, IBM Warp Server for e-Business version 4.52, which was released in 1999, plus all of its later bug fixes and updates. For context, this is the same time frame as Service Pack 6 for Windows NT 4 – when the base system requirements for Microsoft's highest-end professional-level OS were an Intel 80486 at 33MHz or faster, 16MB of RAM, and 110MB of disk space.

ArcaOS 5.1 is a strange combination of old and new

ArcaOS is the second reboot of OS/2, the 32-bit OS which came out of IBM in the 1990s after its partnership with Microsoft broke up. OS/2 1 was developed jointly by the two companies, but OS/2 2 and onward were developed by IBM on its own. OS/2 2.0 came out in 1992, a month before Microsoft released Windows 3.1, and at the time, it was an amazing product – a native 32-bit OS, which could use all 4MB of RAM in a high-end PC, had its own desktop-oriented GUI, and could run DOS or Windows 3.0 in a window, alongside its own native command-line or graphical applications, which included big-name apps such as WordPerfect, CorelDraw, and Lotus 1-2-3.

It was followed by OS/2 Warp 3 in 1994, which had a revamped desktop with a CDE-like launcher, and in 1996 by Warp 4, the final desktop version from IBM, with a loosely Windows 95-like desktop. In 2001, Serenity Systems released an updated and modernized version called eComStation, based on code licensed from IBM. The final version of this, 2.1, came out in 2011.

ArcaOS is a successor to eComStation. It's still OS/2 Warp 4.52 underneath, but with a new installation program, comprehensively updated drivers and tools, a more modern web browser and email client, and more. It supports much more modern hardware, some via generic drivers for the most recent sound and graphics chips. It has power management, multiprocessor support, USB support, and so on. Alongside OS/2's own HPFS filesystem, it also supports IBM JFS, which allows volumes of over 64GB.

Obviously it can run OS/2 apps, but not only those. There are existing subsystems to support DOS apps, 16-bit Windows ones, and Java programs. It has a package manager, YUM, derived from the one formerly used in Fedora Linux, and a POSIX-like environment called kLIBC. Alongside integrated Qt4 support, this means that some Linux tools have been ported across. There's also a WINE-like tool called Odin that allows some Win32 software to run.

Saying all that, though, it's a 32-bit OS. That means that it suffers from some of the same limitations as Windows XP did in the late 32-bit era, of which the most important is that it can address a maximum of 4GB of RAM. For machines with over 4GB, there's a driver which can use Intel's PAE to turn additional memory into a RAMdisk.

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The Reg FOSS desk was an OS/2 2 user when it was new, and back in the era of Windows 3 it stomped all over Microsoft's offerings as well as the then-infant Linux. It was a true 32-bit OS that could multitask several large apps side by side. In its day, it was much more stable than classic MacOS, and unlike the new Windows NT 3, it could run on much lower-end hardware and yet had a weird but capable desktop GUI.

We've been playing with ArcaOS for a while and we plan to come back and do a much more in-depth review soon. It's been very interesting, with some ups and downs.

On the plus side, it's substantially easier to install than eComStation was. It supports USB peripherals, modern displays and onboard sound chips, plug-and-play peripherals, and so on with aplomb. We recently surprised ourselves by how much fun we had with XP on a 2008 Thinkpad, which was far more responsive than any version of Windows has been for 20 years now. Well, ArcaOS is lighter-weight than even Windows XP. We've tried it on a Thinkpad X200 and an X61, both powered by Core 2 Duo processors, and it made them very fast and responsive. We've upgraded a couple of them with old SSD drives, and the result is that it boots in seconds on roughly 15-year-old hardware. It puts the skinniest Linux to shame. There aren't many native OS/2 apps these days, but it's happily run all DOS software we've tried.

But there are downsides too. It is extremely finicky about disk partitioning, so you can more or less forget about dual-booting it with anything else except DOS. Don't try partitioning a disk with Linux tools such as GParted – do it entirely in ArcaOS. Either dedicate a machine to it, or put FreeDOS in a small C: partition and nothing else. We've also had no success with Wi-Fi. On most machines, it can see the wireless controller, and maybe list available networks, but we've not yet got it to connect to a single one. With wired networking, it's perfectly happy. It comes bundled with Firefox 45, which works very well, but the snag is that no later releases work because they contain code that requires compilers that don't support OS/2.

It's great fun, albeit challenging, and makes old, low-end kit fast and responsive again. Compared to any 21st century OS, it's tiny and amazingly snappy. There is a lot of DOS freeware and FOSS software nowadays without the murky semi-legal world of "abandonware" – for instance, the FreeDOS repo has a comprehensive selection. If you can live with just a selection of DOS applications, then on ArcaOS they're running very close to the metal, and you can multitask away to your heart's content.

The Personal Edition is priced at $139 (£110). For owners of older versions and licensed copies of eComStation or IBM OS/2, update packages are available. ®