Unit 11

• What is process control management?


  Activities involved in managing the execution of multiple processes in an operating system
• Process states


  As a program executes, it changes states

  • New
  • Ready
  • Ready - Running (running process can be blocked, ready, or terminated)
• The NEW state


  This is when a process has just been created

  Reasons for creating a process:

  • New batch job
  • Interactive logon

  Upon creation, the NEW process will become READY
• The READY state


  It is when a process is waiting to be assigned to a processor

  Ready - Running:

  • It is when instructions are being executed, where a process is using the CPU
• Running process becomes blocked when:

  ...

  • The process cannot be executed because it is waiting for an I/O operation to be completed
  • Waiting for an external event to occur

  From the blocked state, a process will move to the ready state when the event that the process is waiting for is completed
• Running process moves to ready state when:

  ...

  • A process has reached its maximum allowable time for uninterrupted execution
  • A process needs a resource that is not immediately available
  • A process needs an I/O operation before it continues
• Running process becomes terminated when:

  ...

  • The process has been completed/aborted

  Reasons for process termination:

  • Normal completion
  • Invalid instruction
  • Memory unavailable
• Each process in an OS is represented as a process control block, what is a process control block?

  A process control block keeps track of each process, it contains information associated with a specific process

  
  

  PCB diagram:

  • Process state - ready, running, blocked, terminated
  • Program counter - indicates the location for the next instruction
  • CPU scheduling information - process priority
  • Accounting information - statistics on CPU time, job, and process numbers
  • I/O status - list of I/O devices which are allocated to processes
• Process scheduling

  ...

  • As processes enter the system, they are put on the JOB QUEUE
  • The process waits until it is selected for execution (or dispatched) and is given CPU resources

  Once the CPU is allocated and the process is running:

  • The process could issue an I/O request and be placed on a device queue
  • The device could create a new sub-process
  • The process could be forcibly removed
• What is a thread?

  An individual executable part of a process

  There are 2 types:

  • User processes
  • System processes

  Creation of a new process (child) from the older one (parent) is called spawning or forking