The CPU moves to IDLE every time a process finishes its timequantum or executes a syscall (after moving the process). It then executes in the order of these steps: unblock any sleeping processes unblock any processes waiting for all their spawned processes (children) to terminate unblock any completed I/O commence any pending I/O commence/resume the next READY process otherwise the CPU remains idle... And it always executes in this order. So if after running a process, it can then wake something up, it will. The last thing that occurs in IDLE is moving the next process from READY into RUNNING. So it kinda has the least priority. Hence you don't just get processes consistently on the CPU not letting anything wake up.