I know it's a bit annoying, but.. I've did some testing 286 vs OS/2 1.x over here.
(^We weren't always dead serious in that thread, btw. A bit of poking fun at each others..)
The OS/2 compatibility box is quite limiting, but it has good technical reasons here for.
The whole first 640KB (default setting) are essentially being exclusive for the synthetic DOS.
That's why 640KB are being "lost" for OS/2 and OS/2 needs more memory than DOS without any further doing.
In addition, a small HDD cache is being running, too.
If OS/2 is running, it can use its own drivers for networking and filesystem (in theory).
It's not needed to load any DOS drivers for this into first 640KB.
So ideally, those 640KB in the DOS "penalty box" are usually free to DOS applications.
On a contemporary 286 PC running PC-DOS 3.30, even less memory would be available.
On the other hand, the latest 16-Bit OS/2 (v1. 3) was around when MS-DOS 5/6 and memory managers like QEMM/EMM386 were available.
And they could "out source" parts of DOS and applications beyond 640KB, liquidishing the need for 16-Bit OS/2 here.
But then, there's the filesystem. The latest 16-Bit OS/2 supported HPFS, which had advantages over FAT.
And like with Windows NT, OS/2 could run DOS applications on a foreign filesystem.
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