[jmts]

My understanding of PC game development at the time was that most games would re-implement their own drivers for system hardware, hence why you would often need to select what kind of graphics card, sound card, and their settings during the setup. As such, a game running from a boot disk is closer to just skipping DOS and having no OS rather than implementing a custom OS, although from another perspective you might just say that the game is the OS.

[contingencies]

Sort-of. IIRC DOS games that wanted to use more than 512KB(?) of memory had to use one of a series of particular high memory access drivers. HIGHMEM.SYS ("XMS"?), EMM386.SYS ("EMS"?), etc. You had to load these and other drivers (in particular sound and mouse drivers, antivirus programs, etc.) when booting up in your CONFIG.SYS and/or AUTOEXEC.BAT. These were referred to as 'TSR' (terminate and stay resident) programs. There were all sorts of tricks to get things working... LOADHIGH, etc. In the end, most games required one particular approach to high memory, and you had to have a CONFIG.SYS/AUTOEXEC.BAT that left enough base memory free as well. It was a huge hassle. In later days most programs used DOS4GW ... https://en.wikipedia.org/wiki/DOS/4G ... which made things a lot easier.

[ajxs]

I was a 90s kid, and was totally head over heels for Origin Systems' games (Ultima, Wing Commander, etc). Par for course, I had a selection of DOS boot disks to set up the right operating environment for each game. Ultima VII featured a memory manager[0] based on 'Unreal Mode', which made getting it running even more precarious. The quote at the bottom of that link from the developer is really telling.

0: https://www.wiki.ultimacodex.com/wiki/Voodoo_Memory_Manager

[sumtechguy]

Then making a boot disk using 4DOS to manage the whole lot of it. As each game had its own idea of what being 'setup' looked like with different other TSRs (mouse, cd, etc).