1. Chapter 3 Process Description and Control
Operating Systems: Internals and Design Principles, 6/E William Stallings
Chapter 3 Process Description and Control
Patricia Roy Manatee Community College, Venice, FL ©2008, Prentice Hall

2. Major Functions of an Operating System
Interleave the execution of multiple processes, to maximize processor utilization while providing reasonable response time
Allocate resources to processes
Support interprocess communication and user creation of processes

3. Process A program in execution
An instance of a program running on a computer
The entity that can be assigned to and executed on a processor
A unit of activity characterized by
the execution of a sequence of instructions
a current state
an associated set of system resources

4. How to describe a process? What elements are included in it?
Process Elements (I)
Identifier
State
Priority
Program counter
Memory pointers: to code and data
How to describe a process? What elements are included in it?

5. Process Elements (II) Context data: value of CPU registers
I/O status information
Outstanding I/O requests
Assigned I/O devices and used files
Accounting information
Amount of processor time & clock time used
Time limits
Account numbers

6. Process Control Block Contains the process elements
Created and managed by the operating system

7. Process Control Block
Allows support for multiple processes

8. Trace of the Process
Sequence of instruction that execute for a process
Dispatcher switches the processor from one process to another

9. Example Execution

10. Trace of Process

11. Combined Trace of Process

12. Two-State Process Model
Process may be in one of two states
Running
Not-running

13. Queuing Diagram

14. Process Creation (I)

15. Example: Application Modularity

16. Example: Concurrent Server
Request dispatched to a worker process
Dispatcher process
Server
Worker
process

17. Process Creation (II) What does the OS do to create a process?
Build data structures that are used to manage the process;
Allocate address space in main memory to the process.

18. Process Termination

19. Process Termination

20. Queuing Diagram

21. Queuing Processes Not-running ready to execute Not-running block
Dispatcher must scan list to find process not-running, ready, and in queue the longest

22. A Five-State Model
Running
Ready
Blocked
New
Exit

23. Five-State Process Model
New: a process that has just been created but has not yet been admitted to the pool of executable processes by the OS. Typically, a new process had not been loaded into main memory, although its process control block has been created.

24. Process States

25. Using Two Queues

26. Multiple Blocked Queues

27. Suspended Processes
Processor is faster than I/O so all executable processes could be waiting for I/O, while there are some new processes waiting to be admitted
Swap these processes to disk to free up more memory to admit new processes
Blocked state becomes suspend state when swapped to disk
Two new states
Blocked/Suspend
Ready/Suspend

28. Two Suspend States

29. Reason for Process Suspension

30. Processes and Resources

31. Which state transition is impossible?
In-Class Exercise
Running 2 1 6 3 5
Blocked
Ready 4
1: process blocks for input
2: scheduler picks another process
3: scheduler picks this process
4: input becomes available
5: process blocks for input
6: input becomes available
Which state transition is impossible?

33. Operating System Control Structures
Information about the current status of each process and resource
Tables are constructed for each entity the operating system manages
So, what control tables does an OS have

34. OS Control Tables

35. Memory Tables (1)
Used to keep track of both main (real) memory and secondary (virtual) memory
Some of main memory reserved for OS
The remainder is available for processes
Processes are maintained on secondary memory

36. Memory Tables (2) 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

37. I/O Tables
Used by OS to manage I/O devices and channels of the computer system
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

38. File Tables Existence of files Location on secondary memory
Current Status
Attributes
Sometimes this information is maintained by a file management system, OS has little or no knowledge of files
In other OS, detailed file management is by OS itself
Status: text file, binary file, directory etc.

39. Process Tables Manage processes What is a process composed of?
Process image is the collection of program, data, stack, and attributes

40. Process Stack
Figure 4-5. (a) Parameter passing in a procedure call: the stack before the call to read. (b) The stack while the called procedure is active.

41. Process Location Where are the processes located?
Secondary memory, usually disk
To manage and execute a process, at least a small portion of its image must be maintained in main memory
OS must know the location of each page of each process image, achieved by process tables

42. Process Images

43. PCB: Process Control Information

44. PCB: Process Control Information
Data Structuring
A process may be linked to other process in a queue, ring, or some other structure. For example, all processes in a waiting state for a particular priority level may be linked in a queue. A process may exhibit a parent-child (creator-created) relationship with another process. The process control block may contain pointers to other processes to support these structures.

45. Modes of Execution User mode System mode, control mode, or kernel mode
Less-privileged mode
User programs typically execute in this mode
System mode, control mode, or kernel mode
More-privileged mode
Kernel of the operating system

46. Process Creation Assign a unique process identifier
Allocate space for the process
Initialize process control block
Set up appropriate linkages
Create or expand other data structures

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

48. When to Switch Process Trap Supervisor call (system call)
error or exception occurred
may cause process to be moved to Exit state
Supervisor call (system call)
I/O operation such as file open
Lead to a transfer to an OS routine

49. How to Switch Process?

50. Process Switch (1)
Save context of processor including program counter and other registers
Update the process control block of the process that is currently in the Running state
Move process control block to appropriate queue – ready; blocked; ready/suspend

51. Process Switch (2) Select another process for execution
Update the process control block of the process selected
Update memory-management data structures
Depending on how address translation is managed, memory management data structures need to be updated
Restore context of the selected process

52. Execution of the Operating System
Non-process Kernel
Execute kernel outside of any process
Operating system code is executed as a separate entity that operates in privileged mode
Execution Within User Processes
Operating system software within context of a user process

53. Execution of the Operating System
Process-based operating system
Implement the OS as a collection of system processes

54. Execution of the Operating System

55. OS Executes in User Space

56. Process Switch
vs.
Mode Switch

57. Process Switch (1)
Save context of processor including program counter and other registers
Update the process control block of the process that is currently in the Running state
Move process control block to appropriate queue – ready; blocked; ready/suspend

58. Process Switch (2) Select another process for execution
Update the process control block of the process selected
Update memory-management data structures
Restore context of the selected process

59. Mode Switch
Save context of processor including program counter and other registers
Update the process control block of the running process
Mode changes to kernel mode, finishes the OS routine
Mode changes back, restore the context, and continue the running process in user mode
A mode switch may occur without changing the state of the process that is currently in the Running state. Therefore, the context saving and subsequent restoration involve little overhead.

60. UNIX Process States

61. UNIX Process State Transition Diagram

62. fork()
Process creation in Unix is made by means of the kernel system call, fork()
When a process issues a fork request, the OS:
Allocates a slot in the process table for the new process;
Assigns a unique process ID to the child process;
Makes a copy of the process image of the parent, with the exception of any shared memory;
Increments counters for any files owned by the parent, to reflect that an additional process now also owns those files;
Assigns the child process to the Ready to Run state;
Returns the ID number of the child to the parent process, and a 0 value to the child process.

64. Reminder 13 students still haven’t formed project teams
Team needs to be formed by this Friday!

65. Appendix

66. Elements of a Process Control Block
How to assign a process an identifier?
may be an index into the process table
or process identifier  process table index
useful for cross-reference by other tables, inter-process communication

67. Elements of a Process Control Block

68. Review Questions What does it mean to preempt a process?
What is swapping and what is its purpose?
For what types of entities does the OS maintain tables of information for management purposes?
List three general categories of information in a process control block.