Well worth looking at
CP/M is a surprisingly decent operating system --- it's a miracle of minimalism: the absolute smallest number of features required to do a decent abstraction over the hardware, and not a single one more.
On CP/M 2, the OS proper, the BDOS, was 3kB, and provided some I/O streams, a filesystem, and... nothing else (see http://www.gaby.de/cpm/manuals/archive/cpm22htm/ch5.htm). The command shell, the CCP, was another 2kB and could be overwritten if applications needed the RAM. These talked to the underlying hardware via the BIOS, typically another 2kB. On a 64kB system this meant apps got about 59kB of available RAM.
And these modules all talked to each other via standard, platform-independent interfaces. The CCP and BDOS supplied in binary form by Digital Research; the BIOS you could write yourself (and they provided all the tooling, docs and a sample BIOS for you). The BIOS interface was ludicrously tiny (see http://www.gaby.de/cpm/manuals/archive/cpm22htm/ch6.htm#Section_6.6) and porting was straightforward, particularly as the hardware requirements were so minimal --- you didn't even need interrupts!
It's worth playing with. There's still a wealth of ultra-minimal software out there, including actual C compilers (provided you like K&R). You can actually get proper work done on it, including internet access.
The later versions supported bank switching, network file systems, multiple concurrent users (yes, with time slicing!) and stuff like that, but to my mind they don't have the beautiful simplicity of CP/M 2.