Worth mentioning that GEOS itself has been completely reverse engineered and the fully commented 6502 assembler source is up on github: https://github.com/mist64/geos

In theory quite portable to other 'new' 6502/816 systems.

Oh, wow, GEOS is a name I haven't heard in a long time!

I never got the chance to use it on a real C64, but I played with it a lot under CCS64 ( http://ccs64.com/ ). I had a very slow computer at the time, too slow to run many modern games and applications, so I found a lot of interesting things to do under various emulated environments.

Edit: since we're mentioning (possibly) obscure operating systems, WingsOS ( http://wingsos.org/ ) also deserves a note :-).

I did use GEOS on a real C64 back in the day, and what I remember was that every click of the joystick button led to a disk seek to find the graphics for the next screen. If you knew the commands to display a directory, etc, GEOS made things maddeningly slow. I remember similar sentiments being expressed back in the DOS to early Windows days.

GEOS felt kind of pointless to me, like early Windows but worse. It ate so much of the limited resources on the machine that you wouldn't be able to do much while it was still in memory.

The built in word processor had a lot of cool text effects. 13 year old me thought it was much more impressive than my mom's Paperclip. It has been very many years since I used it, but I recall multiple drives improved the experience, and these days I see it is popular to run GEOS with a ram expander.

I used GEOS, and later GEOS 128, to do a lot of my homework in high school. GeoWrite was much better than an other word processor I'd used in the 64 or 128, and it used proportional fonts rather than the printer's built-in monospaced font. Printing took forever, though.