The Concept of a Process

COMPPUTER SCIENCE AN OVERVIEW 11th Edition

One of the most fundamental concepts of modern operating systems is the distinction between a program and the activity of executing a program. The former is a static set of directions, whereas the latter is a dynamic activity whose properties  change  as  time  progresses.  (This  distinction  is  analogous  to  a  piece  of sheet music, sitting inert in a book on the shelf, versus a musician performing that piece by taking actions that the sheet music describes.) The activity of exe-

cuting  a  program  under  the  control  of  the  operating  system  is  known  as  a process.

Associated with a process is the current status of the activity, called the process  state.

This  state  includes  the  current  position  in  the  program  being executed  (the  value  of  the  program  counter)  as  well  as  the  values  in  the  other CPU  registers  and  the  associated  memory  cells.  Roughly  speaking,  the  process state is a snapshot of the machine at a particular time. At different times during the execution of a program (at different times in a process) different snapshots (different process states) will be observed.

Unlike a musician, who normally tries to play only one musical piece at a time, typical time-sharing/multitasking computers are running many processes, all competing for the computer’s resources. It is the task of the operating system to  manage  these  processes  so  that  each  process  has  the  resources  (peripheral devices,  space  in  main  memory,  access  to  files,  and  access  to  a  CPU)  that  it needs,  that  independent  processes  do  not  interfere  with  one  another,  and  that processes that need to exchange information are able to do so.