=> R0 = flags (reserved, should be zero)
R1 = routine handle returned from RT_Register, or 0 to change the priority of the foreground process
R2 = new priority (as for RT_Register)
<= R0 = previous priority level (as a number from 0-255)
This SWI allows the priority setting of a routine to be changed on the fly, for example if process has become more time-critical, or if it is blocking a routine of a higher priority. It may cause a thread switch if a different thread is promoted above the priority of the current thread, or if the current thread is demoted below other unblocked threads.
The priority level of the foreground process may be changed, but this is only intended for short-term use and not when the foreground is in USR mode (otherwise system callbacks will also be promoted above some real-time threads, which is undesirable behaviour).
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