Barrapunto

Si está basado en x86, puedes probar minix

WHAT IS MINIX 2.0 MINIX is a free UNIX clone that is available with all the source code. Due to its small size, microkernel-based design, and ample documentation, it is well suited to people who want to run a UNIX-like system on their personal computer and learn about how such systems work inside. It is quite feasible for a person unfamiliar with operating system internals to understand nearly the entire system with a few months of use and study.

MINIX has been written from scratch, and therefore does not contain any AT&T code--not in the kernel, the compiler, the utilities, or the libraries. For this reason the complete source can be made available (by FTP or via the WWW).

MINIX has evolved over the years, so several versions exist. Two of these are still current. The rest are obsolete. The current versions are:

MINIX 2.0 (Intel CPUs from 8088 to Pentium) MINIX 1.5 (Intel, Macintosh, Amiga, Atari, SPARC)

El link es http://www.cs.vu.nl/~ast/minix.html [cs.vu.nl]

Es un UNIX en toda regla, pero (obviamente) muy limitado

La verdad es que no había caído en Minix... y posiblemente sea la opción más adecuada, aunque me gustaría probar alguna otra más. Por cierto, ¿lo has instalado? ¿que tal es el compilador? ¿has probado algún otro unix pequeñito?
En fin, Gracias por la respuesta :)