ShellOS was a project of two electrical engineering students with too much time on their hands.
Those guys are now coders in the int13 team.

Their goal was to transform the HP48 into a true handheld computer with a multitasking OS, friendly user interface and powerful applications.

This project is now discontinued but you may want to see what was possible with a 4bits/4MHz CPU and 32KByte of assembly code.