If you are not writing this for commercial purposes are you writing it for a project or for fun/edification?. If it is for a low volume project I suggest you look at the Philips XA processor (sorry, Craig)it has 8 "software interrupts" which makes multitasking and priority switching a breeze. Also for the XA there is an open source RTOS written in C by Philip Labrosse(see philips appnotes)

Have fun,

Erik

PS
If I seemed to turn you off in my previous posting, I merely want to make sure you know what a gargantuan task you are undertaking, if you decide to go on best of luck, you will need it.