Representing a process - the PCB

A process is represented by the PCB (Process Control Block) or PD (Process Descriptor), which contains the following :

• Process Identifier (parent identifier)
• Context
  Program Counter (PC)
  Processor Status Word (PSW)
  Stack Pointer (SP)
  Registers (perhaps implemented as a stack)
  Memory addressing registers
• Process State
• Priority
• Accounting Information : start time, CPU used
• Resources Allocated
  memory allocated (base and limit regs, page and segment tables)
  root directory
  open files
  other i/o devices
• Exception address
• Pointer for queues - why is this needed ?

  
Figure:

Assume - ReadyQ is priority ordered

BEGIN
  disable interrupts
  initialise data structures
  create initial processes

  REPEAT (FOREVER)
    Dispatch process
     

  ENTER :
   save registers of current process
   determine the source of interrupt
   service interrupt or call
 END (repeat)
END

Things to consider :

• The Dispatcher selects the highest priority process, enables the interrupts and runs it. The priority at which a process is run is defined by the PSW (Processor Status Word)
• The saving of registers and determining interrupt source is determined by the First Level Interrupt Handler (FLIH)
• Note : Servicing an interrupt or call may state the change of the current or another process, and cause a reschedule to take place. Hence, it is possible that another process may be run after an external event has occurred instead of the one intended prior to the event. Hence, the dynamic element of scheduling
• The amount of stuff that is stored in the PCB depends on the need. Hence, it is possible to control the degree of information that needs to be recorded on a context switch

The Dispatcher does the following (more later) :

    REPEAT (FOREVER)

	Run thread (process)
 	Save state (into PCB 
	   or TCB (thread control block))
	Choose new thread (process) to run
	Load state into PCB (TCB)

    END