1. Process Control Process Control Block (PCB) Process State Information
Process Control Information
Functions of an Operating-System Kernel
Switch a Process
Change of Process State
Execution of the Operating System

2. Operating System Control Structures
Schedules and dispatches processed for execution by the processor
Allocates resources to processes
(Think about struct rlimt and getrlimit)
Responds to requests by user programs
(Think about getopt)
Tables are constructed for each entity the operating system manages

3. Memory Tables Allocation of main memory to processes
Allocation of secondary memory to processes
Protection attributes for access to shared memory regions
Information needed to manage virtual memory

4. I/O Tables I/O device is available or assigned Status of I/O operation
Location in main memory being used as the source or destination of the I/O transfer

5. File Tables Existence of files Location on secondary memory
Current Status
Attributes
Sometimes this information is maintained by a file-management system
(Think about struct stat and stat( ) )

6. Process Table
Process image consists of program, data, stack, and attributes
Attributes
process control block
( Think about Process ID and getpid( ) )
(Think about Resource Limits and getrlimit( ) )

7. Process State Transition Diagram with Two Suspend States - Seven-State Process Model
New
Admit
Suspend
Admit
Dispatch
Activate
Ready,
suspend
Ready
Running
Exit
Suspend
Time out
Event
Wait
Event
Occurs
Event
Occurs
Activate
Blocked,
suspend
Blocked
Suspend

8. UNIX Process State Transition Diagram (1)
fork
Created
Preempted
return
to user
enough
memory
not enough memory
(swapping system only)
User
Running
preempt
swap out
return
reschedule
process
Ready to Run
in Memory
Ready to Run
Swapped
swap in
system call,
interrupt
Kernel
Running
sleep
wakeup
wakeup
interrupt,
interrupt return
exit
Asleep in
Memory
swap out
Sleep,
Swapped
Zombie

9. UNIX Process State Transition Diagram (2)
User running: Executing in user mode.
Kernel running: Executing in kernel model.
Ready to run, in memory: Ready to run as soon as the kernel schedules it.

10. UNIX Process State Transition Diagram (3)
Asleep in memory: unable to execute until an event occurs; process in main memory.
Ready to run, swapped: process is ready to run, but the the swapper must swap the process into main memory before the kernel can schedule it to execute.
Sleeping, swapped: The process is awaiting an event and has been swapped to secondary storage.

11. UNIX Process State Transition Diagram (4)
Preempted: process is returning from kernel to user mode, but the kernel preempts it and does a process switch to schedule another process.
Created: process is newly created and not yet ready to run.
Zombie: process no longer exists, but it leaves a record for its parent process to collect.

12. Process Control Block Process Identification
The collection of attributes is refereed to as process control block.
Unique numeric identifier
may be an index into the primary process table

13. UNIX Process Control Table
Process Identifiers
ID of this process and ID of parent process.
User Identifiers
real user ID, effective user ID
Pointers
To U area and process memory (text, data, stack)
Process Size, Priority, Signal, Timers,

14. Processor State Information
Contents of processor registers
User-visible registers (data/address register).
Control and status registers (program counter, instruction register.
Stack pointers ( points to the top of the stack).
Program status word (PSW)
contains status information - indicate the mode of execution (user or kernel mode) ??????

15. Question 1 Process Control - Exercise/Home Work
The VAX/VMS operating system makes use of four processor access modes as follows:
Kernel: Executes the kernel of the VMS operating system, which includes memory management, interrupt handing, and I/O operations.
Executive: Executes many of the operating system service calls, including file and disk management routines.
Supervisor: Executes other operating system services, such as responses to user command.
User: Execute user programs, plus utilities such as compilers, editors, linkers, and debuggers.

16. Question 1 Process Control - Exercise/Home Work
a) A number of operating systems have two modes (UNIX), kernel and user. What are the advantages and disadvantages of providing four mode instead of two?
b) Can you make a case for even more than four modes?

17. Typical Functions of an Operating-System Kernel (1)
Memory Management
Allocation of address space to processes
Page and segment management
I/O Management
Allocation of I/O channels and devices to processes
Support Functions
Interrupt handling and accounting

18. Typical Functions of an Operating-System Kernel (2)
Process Management
Process creation and termination
Process scheduling and dispatching
Process switching
Process synchronization and support for inter-process communication

19. Receive Message from Child Processes
* wait( ) System Call
* Status Information of wait( )
* Example: communication between parent process and child processes (jg73.cc and jg74.cc)

20. Process Control Information
Additional information needed by the operating system to control and coordinate the various active processes
scheduling and state information
data structuring
interprocess communication
process privileges
resource ownership and utilization ??????

21. Question 2: Multiple Blocked Queues Process Control - Exercise/Home Work
Ready Queue
Release
Dispatch
Admit
Processor
Time-out
Event 1 Wait
Event 1 Queue
Event 1
Occurs
Event 2 Wait
Event 2 Queue
Event 2
Occurs

22. Question 2 Process Control - Exercise/Home Work
Figure “Multiple Blocked Queues” suggests that a process can only be in one Event queue at a time.
a) Is it possible that you would to allow a process to wait on more than one event at the same time? Provide an example.
b) In that case, how you modify the queuing structure of the figure to support this new feature?

23. When to Switch a Process
Trap
error occurred
may cause process to be moved to Exit state
Supervisor call
such as file open

24. When to Switch a Process
Memory fault
memory address is in virtual memory so it must be brought into main memory
Interrupts ??????
Clock
process has executed for the maximum allowable time slice
I/O

25. Question 3 Process Control - Exercise/Home Work
In a number of early computers, an interrupt cased the register values to be stored in fixed locations associated with the given interrupt signal.
Under what circumstances is this a practical technique?
Explain why it is inconvenient in general.

26. Execution of the Operating System
Nonprocess Kernel
execute kernel outside of any process
operating system code is executed as a separate entity that operates in privileged mode
P P Pn
Kernel

27. Execution of the Operating System
Execution Within User Processes
operating system software within context of a user process
process executes in privileged mode when executing operating system code
P P Pn
OS OS OS
Functions Functions Functions
Process Switching Functions

28. Execution of the Operating System
Process-Based Operating System
major kernel functions are separate processes
a process is invoked by the operating system
P1 P Pn OS1 OS OSn
Process Switching Functions

29. Question 4 Process Control - Exercise/Home Work
UNIX is unsuitable for real-time applications, because a process executing in kernel mode may not be preempted . Elaborate.
P P … Pn
OS OS OS
Functions Functions Functions
Process Switching Functions

30. Question 5 Process Control - Exercise/Home Work
The UNIX kernel will dynamically grow a process’s stack in virtual memory as needed, but it will never try to shrink it.
Explain why it would be possible to shrink the stack, and
Why the UNIX kernel does not shrink it.