Semaphore_Process

Document Sample
Semaphore_Process Powered By Docstoc
					Q 1. Consider the following situations in a typical Multi Programmed Computer
system:
• The O.S. of the given Computer System employs pre emptive FIRST
COME FIRST SERVED Scheduling.

• It employs semaphores to achieve Mutual Exclusion of the single printer it
possesses.
      Any process wishing to use the printer , must do so in the KERNEL mode.
      All processes in the KERNEL mode, possesses the Highest Priority.
• The Printer is currently printing the Output generated by the process PA. The
process PA has got some more tasks to complete even after printing is done.
• Another process PB has been waiting for sometime for the printer.
• The CPU is currently executing the process PC.
• Two other processes PD & PE are ready to be executed. The Process PE having
arrived earlier.

Q.1.(a) i. Identify the states of each of the processes PA, PB, PC, PD & PE along
with proper justifications.
                                                                          [3]
Ans. 1 (a).

 Sl.                   Current State                Remarks / Justification
 No.   Process

  1       PA              Blocked             Carrying out printing i.e. I/O Activity
  2       PB        Waiting in Blocked        Had asked for I/O Activity waiting for
                           State                       Printer to be Free
  3       PC              Running                  Currently being Executed
  4       PD        In READY Queue                   Ready to be Executed
                         behind PE
  5       PE       In READY Queue in                  Ready to be Executed
                        front of PD



Marking Policy : 1 Mark for Identifying Processes in Ready State.
                 1.5 Marks for Identifying Processes in Blocked State.
                  0.5 Mark for Identifying Process in Running State.

                                                                             Contd. 2
                                        - 2-
Q. 1(a) ii. When the printer finishes printing the job associated with the process
PA, depict the sequence of events that take place in a stepwise fashion. What
will be the new states of each of the processes PA, PB, PC, PD & PE ?

                                                                               [4+2=6]
Ans. 1 (b) Sequence of Events that take place after printing is over

1) The Printer interrupts the CPU.
2) The CPU on completion of the current instruction of the presently running process
   PC switches mode to KERNEL mode service the printer interrupt.
3) The printer interrupt service routine brings the blocked process PA , as well as the
   Waiting process PB , both of which happens to be in KERNEL mode, to READY
   state. PC switches mode back to USER mode & resumes normal operation.
4) Subsequently the Process PA , is scheduled by the Scheduler on the top priority basis
   (it being in the Kernel Mode ) i.e. the Process PA is brought into the running state by
   pre-empting process PC.
5) While in RUNNING state the process PA does the following :

        a) It releases the printer by executing some System Code [SIGNAL] so
            that the process PB can use it.
        b) Next it comes back to the user Mode and normally is pre-empted .

 6) Next the scheduler schedules afresh and when the process PB is scheduled i.e. it
comes the running state, then it may issue the printer request again and assuming all
processes has the same priority , the process PB gets to use the printer , while using the
printer , PB remains blocked and a new process is selected by the scheduler to run next
which should be PE.
Marking Policy : 2 Marks for Points 1), 2) & 3)
                  1 Mark for Point 4) , 1 Mark for the Rest.

 Sl.                          New State                    Remarks / Justification
 No.     Process

  1        PA        In READY Queue at the            Finished I/O, Some more Task may
                             End                                   be Left
  2        PB               Blocked                  Carrying out printing i.e. I/O Activity
  3        PC        In READY Queue at the                   Ready to be Executed
                             End
  4        PD       In READY Queue at front                  Ready to be Executed
                            of PA
  5        PE              Running                         Currently being Executed

Marking Policy : 1 Mark for Identifying Processes in Ready State.
                 0.5 Marks for Identifying Processes in Blocked State.
                  0.5 Mark for Identifying Process in Running State.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:6
posted:11/18/2011
language:English
pages:2