Thread Priority

When a thread is created, it is assigned a priority which indicates its relative importance to other threads within the system. A thread priority is a byte value that can range from 0 (best) to 254 (worst)

in a Shared mode INtime for Windows configuration, the Windows thread runs at priority 254
you can not use priority 255

INtime schedules threads according to their thread priority. If a thread with a better priority than the running thread becomes ready, it will pre-empt the running thread immediately. The running thread will go to the ready state.