the OS maintains a number of queues

 

COMPUTER ORGANIZATION AND ARCHITECTURE DESIGNING FOR PERFORMANCE NINTH EDITION

 

 

To do its job, the OS maintains a number of queues. Each queue is simply a
waiting list of processes waiting for some resource. The long-term queue is a list of
jobs waiting to use the system. As conditions permit, the high-level scheduler will
allocate memory and create a process for one of the waiting items. The short-term
queue consists of all processes in the ready state. Any one of these processes could
use the processor next. It is up to the short-term scheduler to pick one. Generally,
this is done with a round-robin algorithm, giving each process some time in turn.
Priority levels may also be used. Finally, there is an I/O queue for each I/O device.
More than one process may request the use of the same I/O device. All processes
waiting to use each device are lined up in that device’s queue.