That would likely be launching 'init' (or a shell if it fails).
Look at linux/init/main.c for the code.
This of course is after any kernel threads have been started
(like 'kflushd' and 'kswapd').
A _MINIMUM_ Linux system is the kernel and a shell, though it
isn't a very useful system.
-- Andrew E. Mileski --
mailto:aem@ott.hookup.net
http://www.redhat.com/~aem/