My RTOS looks as follows:

workloop:
execute one loop of task 1
execute one loop of task 2
...
goto workloop.

The ISRs do the minimum, mark a bit and then set the timer 2 interrupt flag (timer 2 otherwise unused)

The timer 2 ISR, set to lowest priority, handles "priority processing not done in the short ISR"

This gives 3 priority groups
Highest ISR
Middle T2
Lowest Main

Erik

Erik