I'm thinking of experimenting with an RTOS such as freeRTOS or MicroC/OS-II.
I am wondering how the low level interrupt services will affect the RTOS. For example if I had a fast serial port running the time taken for the reception fo a character could be faster than the RTOS tick. Therefore a dedicated ISR is needed to store the data rather than using a basic RTOS task. The problem is not just related to serial ports but any internal or external interrupt that needs servicing beyond the capabilities of the RTOS.
How would this ISR for the serial port affect the RTOS tick? Can I assume that the serial ISR (if i define it as exclusive) will be serviced with only a small resulting delay to the RTOS? What would happen if the serial ISR occured during a context switch?