1. Process Description and Control
Chapter 3

2. Process Requirements of an Operating System
Interleave execution of several processes to maximize processor utilization while providing reasonable response time
Allocate resources to processes
Support interprocess communication and user creation of processes

3. Process Also called a task or job It is a program in execution
it is an active entity

4. Two-State Process Model
Dispatch
Enter
Not
Running
Running
Exit
Pause
State transition diagram
Queue
Processor
Enter
Dispatch
Exit
Pause
Queuing diagram

5. Reasons for Process Creation
Submission of a batch job
User logs on
Created by OS to provide a service such as printing
Spawned by an existing process

6. Reasons for Process Termination
Normal completion
e.g., user logs off, requests termination
Error and fault conditions
e.g., time limit exceeded, memory unavailable
OS intervention
e.g., when deadlock occurs
Parent request

7. Processes Ready Blocked
prepared to execute
Blocked
waiting for I/O
Dispatcher cannot just select the process that has been in the queue the longest because it may be blocked

8. A Five-State Model Running Ready Blocked New Exit
process currently being executed
Ready
prepared to execute
Blocked
waiting for some event to occur
New
created but not admitted to job pool
Exit
released from job pool

9. Five-State Process Model
Dispatch
Release
Admit
New
Ready
Running
Exit
Time-out
Event
Wait
Event
Occurs
Blocked

10. Single Blocked Queue Ready Queue Release Dispatch Admit Processor
Time-out
Event Wait
Event
Occurs
Blocked Queue

11. Multiple Blocked Queues
Ready Queue
Release
Dispatch
Admit
Processor
Time-out
Event 1 Wait
Event 1 Queue
Event 1
Occurs
. . .
Event n Wait
Event n Queue
Event n
Occurs

12. Suspended Processes
Processor is faster than I/O so all processes could be waiting for I/O
Swap these processes to disk to free up more memory
Blocked state becomes suspend state when swapped to disk
Two new states
Blocked, suspend
Ready, suspend

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

14. Operating System Control Structures
Tables are constructed for each entity the operating system manages
memory tables
I/O tables
file tables
process tables

15. Memory Tables Allocation of main memory to processes
Allocation of secondary memory to processes
Protection attributes for main or virtual memory
Information needed to manage virtual memory

16. 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

17. File Tables Existence of files Location on secondary memory
Current Status (e.g., open)
Attributes (e.g., permissions)
Sometimes this information is maintained by a file-management system

18. Process Table Process image is a collection: user program user data
program or set of programs to be executed
user data
locations for local and global variables, defined constants
run-time stack
LIFO stack to keep track of procedure calls, parameters passed
process control block (PCB)
attributes used by the OS for process control

19. Contents of the Process Control Block
Process identification
user id, process id, id of parent
Process state information
user visible registers
control and status registers
stack pointers

20. Contents of PCB (cont’d)
Process control information
scheduling and state information
data structuring
interprocess communication
process privileges
memory management
resource ownership and utilization

21. Typical Functions of an Operating System Kernel
Process management
Memory management
I/O management
Support functions

22. Process Management Process creation and termination
Process scheduling and dispatching
Process switching
Process synchronization and support for inter-process communication
Management of process control blocks

23. Memory Management Allocation of address space to processes Swapping
Page and segment management

24. I/O Management and Support Functions
Buffer management
Allocation of I/O channels and devices to processes
Interrupt handling
Accounting
Monitoring

25. Steps in Process Creation
Assign a unique process identifier
Allocate space for the process
Initialize process control block
Set up appropriate linkages
e.g., add new process to linked list used for scheduling queue
Other
maintain an accounting file

26. When to Context Switch Interrupts Trap Supervisor call
react to an asychronous external event
e.g., clock, I/O, memory fault
Trap
associated with execution of current instruction
e.g, handle error or exceptional condition
Supervisor call
explicit request, e.g., open file

27. Steps in a Context Switch
Save context of “old” process
Update “old” PCB with the new state and any accounting information
Move “old” PCB to appropriate queue
Select a “new’’ process for execution
Update PBC of “new” process
Update memory management tables
Restore context of “new” process

28. Execution of the OS Nonprocess kernel Execution within user processes
execute kernel outside of any process
OS code is executed as a separate entity that operates in privileged mode
Execution within user processes
OS software within context of a user process
process executes in privileged mode when executing OS code
Process-based operating system
major kernel functions are separate processes
a process is invoked by the OS

29. UNIX Process State Transition Diagram
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