[Alan Anderson]

I'm not sure you have a good grasp of the "hardware software model", as you call it. You're asking questions that seem to assume things are not as they are. The IFI Robot Controller system is set up as a main processor with a coprocessor. The software isn't running a "dual thread" scheme with interrupts to switch between tasks; the fast user loop is the "main loop".

The system is set up with an amazingly useful feature: hardware inputs are automagically read and made available to the program as variables, and hardware outputs are automagically generated based on variables set by the program. The minor nuisance of having to call the get_data and send_data communication routines explicitly is worth it.

One thing you're leaving out is the cost of the system. Adding "just one more small PIC" for PWM outputs, and "just one more small PIC" for modem communication, and "just one more small PIC" for a motor drive system...it adds up.