What are some tiny open source operating systems? I'm looking for something several orders of magnitude smaller than Puppy Linux, Feather Linux, DSL, etc. I want to run a command-line text editor and compiler; anything else is extraneous. I'm looking for a system I can take apart and acquire a fairly good understanding of the whole thing in a reasonable period of time. Bonus points for something that is portable and well-documented.
There are also completely different research/teaching operating systems around, like TOPSY.
Topsy is a small footprint native multi-threading, multi-tasking operating system based on a micro-kernel architecture with a message passing communication infrastructure.
ANSI C, ~20k lines. It was apparently ported to ia32, MIPS R3000/R4000, StrongARM SA110, Intel IXP1200, Intel IXP2400 and Motorola 68K. You'll find various thesis reports on their website.