1. Process Description and Control
Chapter 3
All multiprogramming OS are built around the concept of processes.
A process is also called a task or a job.

2. Roadmap How are processes represented and controlled by the OS?
Process states and state transitions which characterize the behaviour of processes.
Data structures used to manage processes.
Ways in which the OS uses these data structures to control process execution.
Overview of process management in UNIX SVR4. 2

3. OS Requirements for Multiprogramming Process Management
Process management: fundamental OS task
OS must:
interleave the execution of several processes to maximize CPU usage while providing reasonable response time
allocate resources to processes while
protecting processes
enabling synchronization of processes
avoiding deadlock
support inter-process communications to share data and exchange information, and user creation of processes

4. Concepts
Computer platforms consists of a collection of hardware resources
Computer applications are developed to perform some task
OS provides an interface for applications to use hardware resources
OS provides a representation of resources that can be requested and accessed by application
Recapping chapters 1 & 2 4

5. The OS Manages Execution of Applications
Resources are made available to multiple applications
The processor or CPU is switched among multiple applications
The processor and I/O devices can be used efficiently 5

6. What is a “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, and an associated set of system instructions/data
More than just source program and data 6

7. Process Image A process is comprised of:
Executable program (possibly shared)
A set of data
A number of attributes describing the context of the process: process-specific context
Process state
Processor status
Other resource data
Stacks 7

8. Process Elements
While a process is running, it has a number of elements, including:
Identifier
State
Priority
Program counter
Memory pointers
Context data
I/O status information
Accounting information (?)
... 8

9. Process Control Block (PCB)
Contains the process elements
Created and managed by the operating system
Allows support for multiple processes
Process switching
Interrupt handling for process switching
Process Control Block contains sufficient information so that it is possible to interrupt a running process and later resume execution as if the interruption had not occurred. 9

10. Dispatcher (short-term scheduler): Process Control and Scheduling
Is part of the OS that moves the processor from one process to another
It prevents a single process from monopolizing processor time
It decides which goes next and when according to a scheduling algorithm (to be discussed later)
The CPU will always execute instructions from the dispatcher while switching from process A to process B

11. Quiz 3-1 A process consists of True or False Source program
Data/stacks
Context/attributes
Give 2 attributes of a process
What are stacks for?
A PCB (Process Control Block) is allocated for
Each process
Multiple processes that are running at the same time
A few processes that shared the same program and data
Program portion is executable code, not source code

12. Roadmap How are processes represented and controlled by the OS.
Process states and state transitions which characterize the behaviour of processes.
Data structures used to manage processes.
Ways in which the OS uses these data structures to control process execution.
Discuss process management in UNIX SVR4. 12

13. Exercise What are some of the states for a Student Registrar System?
What are some valid state transitions for a Student Registrar System?
What are the similarities and differences between a Student Registrar System and an OS?
Registrar: applied, accepted/rejected, 1st year, 2nd year, ...., leave of absence, graduated, ...
Registrar vs. OS
Students : proceses 13

14. Two-State Process Model: A Simple Model
Process may be in one of two states
Running
Not-running 14

15. Process Queue and Dispatcher
… processes moved by the dispatcher of the OS to the CPU then back to the queue until the task is competed 15

16. Process Birth and Death
Creation
Termination
New batch job
Normal Completion
Interactive Login
Memory unavailable
Created by OS to provide a service
Protection error
Spawned by existing process
Operator or OS Intervention
See tables 3.1 and 3.2 for more 16

17. Process Creation The OS builds a data structure to manage the process
Traditionally, the OS creates all processes
But it can be useful to let a running process to create another one
fork() in Unix/Linux (in Lab 2 and assignment 1)
This action is called process spawning
Parent Process is the original, creating process
Child Process is the new process 17

18. Process Creation Assign a unique process identifier
Allocate space for the process image
Initialize process control block
many default values (ex: state is New, inherited values from Parent, …)
Set up appropriate linkages
Ex: add new process to linked list used for the scheduling queue
Create or expand other data structures

19. Process Termination
There must be some way that a process can indicate completion.
This indication may be:
A HALT instruction generating an interrupt alert to the OS.
A user action (e.g. log off, quitting an application)
A fault or error
Parent process terminating

20. Process States Let us expand to these states: The Running state
The process that gets executed
The Not Running state
The Ready state
any process that is ready to be executed
The Blocked state
when a process cannot execute until some event occurs (ex: the completion of an I/O)

21. Other Useful States The New state
OS has performed the necessary initialization actions to create the process
has created a process identifier
has created data structures or tables needed to manage the process
but has not yet committed to execute the process (not yet admitted)
because resources are not allocated or limited

22. Other Useful States The Exit state
Termination moves the process to this state
It is no longer eligible for execution
Tables and other info are temporarily preserved for auxiliary program
Ex: accounting program that cumulates resource usage for billing the users
The process (and its tables) gets deleted when the data is no more needed

23. Process Transitions Ready --> Running Running --> Ready
When it is time, the dispatcher (scheduler) selects a new process to run
Running --> Ready
the running process has expired his time slot
the running process gets interrupted because a higher priority process is in the ready state
How about I/O operations?
Is the process still Ready?
What is the new state?

24. Process Transitions Running --> Blocked Blocked --> Ready
When a process requests something for which it must wait
a service that the OS is not ready to perform
an access to a resource not yet available
initiates I/O and must wait for the result
waiting for a process to provide input (IPC)
Blocked --> Ready
When the event for which it was waiting occurs

25. A Five-state Process Model
Ready to exit: A parent may terminate a child process

26. Quiz 3 What are the states when a process is not in the Running state?
Describe the state transitions:
Ready  Running
Running  Ready
Ready  Blocked
Running  Blocked
Blocked  Running
Ready  Running
Running  Ready
Ready  Blocked (not a valid transition)
Running  Blocked
Blocked  Running (not a valid transition)

27. Suspended Processes – The Need for Swapping
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 and use processor on more processes
Ready/Blocked state becomes suspend state when swapped to disk
Two new states
Blocked Suspend
Ready Suspend
Mention that the aim is to fully utilize the processor. 27

28. New state transitions (mid-term scheduling)
Blocked --> Blocked Suspend
When all processes are blocked, the OS will make room to bring a ready process in memory
Blocked Suspend --> Ready Suspend
When the event for which it as been waiting occurs (state info is available to OS)
Ready Suspend --> Ready
when “less” ready processes in main memory
Ready--> Ready Suspend (unlikely)
When there are no blocked processes and must free memory for adequate performance

29. A Seven-state Process Model

30. Reason for Process Suspension
Comment
Swapping
The OS needs to release sufficient main memory to bring in a process that is ready to execute.
Other OS Reason
OS suspects process of causing a problem.
Interactive User Request
e.g. debugging or in connection with the use of a resource.
Timing
A process may be executed periodically (e.g., an accounting or system monitoring process) and may be suspended while waiting for the next time.
Parent Process Request
A parent process may wish to suspend execution of a descendent to examine or modify the suspended process, or to coordinate the activity of various descendants.
Table 3.3 Reasons for Process Suspension 30

31. Quiz 3-2 Identify the 7 possible process states
Describe the following state transitions
Blocked  Blocked Suspend
Blocked Suspend  Ready Suspend
Ready Suspend  Running
Ready Suspend  Running (not a valid transition) 31

32. Roadmap How are processes represented and controlled by the OS.
Process states and transitions which characterize the behaviour of processes.
Data structures used to manage processes.
Ways in which the OS uses these data structures to control process execution.
Discuss process management in UNIX SVR4.
Progress indicator 32

33. Operating System Control Structures
For the OS is to manage processes and resources, it must have information about the current status of each process and resource.
Tables are constructed for each entity the operating system manages
Memory tables (to be discussed later)
I/O tables (to be discussed later)
File tables (to be discussed later)
Process tables: For OS to manage processes, OS needs to know details:
Current state
Process ID
Location in memory
etc

34. OS Control Tables

35. Process Image User program User data Stack(s)
for procedure calls and parameter passing
Process Control Block (execution context)
Data needed (process attributes) by the OS to control the process. This includes:
Process identification information
Processor state information
Process control information

36. Location of the Process Image
Each process image is in virtual memory
may not occupy a contiguous range of addresses (depends on the memory management scheme used)
both a private and shared memory address spaces are used
The location if each process image is pointed to by an entry in the Primary Process Table
For the OS to manage the process, at least part of its image must be loaded into main memory

37. Process Identification (in the PCB)
Each process is assigned a unique numeric identifier.
Many of the other tables controlled by the OS may use process identifiers to cross-reference process tables
A few other numeric identifiers are also used, e.g.,
User identifier
the user who is responsible for the job
Identifier of the process that created this process

38. Processor State Information (in PCB)
Contents of processor registers
User-visible registers
Control and status registers
Stack pointers
Program status word (PSW)
contains status information
Example: the EFLAGS register on Pentium machines

39. Pentium II EFLAGS Register
Also see Table 3.6 39

40. Process Control Information (in PCB)
The most important data structure in an OS. It defines the state of the OS:
interprocess communication
may hold flags and signals for IPC
process privileges
Ex: access to certain memory locations...
memory management
pointers to segment/page tables assigned to this process
resource ownership and utilization
resource in use: open files, I/O devices...
history of usage (of CPU time, I/O...)

41. Process Control Information (in PCB)
Scheduling and state information
Process state (i.e., running, ready, blocked...)
Priority of the process
Event for which the process is waiting (if blocked)
Data structure information
may hold pointers to other PCBs for process queues, parent-child relationships and other structures

42. Queues as linked lists of PCBs

43. Roadmap How are processes represented and controlled by the OS.
Process states which characterize the behaviour of processes.
Data structures used to manage processes.
Ways in which the OS uses these data structures to control process execution.
Discuss process management in UNIX SVR4.
Progress indicator 43

44. Modes of Execution
To provide protection to PCBs (and other OS data) most processors support at least 2 execution modes:
Privileged mode (a.k.a. system mode, kernel mode, supervisor mode, control mode )
manipulating control registers, primitive I/O instructions, memory management...
User mode
For this the CPU provides a (or a few) mode bit which may only be set by an interrupt or trap or OS call

45. Switching Processes Key for multiprogramming
Several design issues are raised regarding process switching
What events trigger a process switch?
We must distinguish between mode switching and process switching.
What must the OS do to the various data structures under its control to achieve a process switch?

46. When to Switch a Process ?
A process switch may occur whenever the OS has gained control of CPU, i.e., when:
Supervisor Call
explicit request by the program (ex: file open). The process will probably be blocked
Trap
An error resulted from the last instruction. It may cause the process to be moved to the Exit state
Interrupt
the cause is external to the execution of the current instruction. Control is transferred to IH

47. When to Switch Processes?
Summary
Mechanism
Cause
Use
Interrupt
External to the execution of the current instruction
Reaction to an asynchronous
external event
Trap
Associated with the execution of the current instruction
Handling of an error or an
exception condition
Supervisor call
Explicit request
Call to an operating system
function
Table 3.8 Mechanisms for Interrupting the Execution of a Process 47

48. Mode Switching
It may happen that an interrupt does not produce a process switch
The control can just return to the interrupted program
Then only the processor state information needs to be saved on stack
This is called mode switching (user to kernel mode when going into IH)
What is the benefit?
Less overhead: no need to update the PCB like for process switching

49. Steps in Process (Context) Switching
Save context of processor including program counter and other registers
Update the PCB of the running process with its new state and other associate info
Move PCB to appropriate queue - ready, blocked
Select another process for execution
Update PCB of the selected process
Update memory-management data structures
Restore CPU context from that of the selected process
Playing detective: find the key words.

50. Quiz 3-4 Identify the causes for process switching
Processing switching: Reorder the tasks:
Update PCB of the selected process
Save context of processor including program counter and other registers
Move PCB to appropriate queue - ready, blocked
Select another process for execution
Update the PCB of the running process with its new state and other associate info
Restore CPU context from that of the selected process
Update memory-management data structures
Work with a peer.

51. Execution of the Operating System – Is the OS a Process?
Up to now, by process we were referring to “user process”
If the OS is just like any other collection of programs, is the OS a process?
If so, how it is controlled?
The answer depends on the OS design.

52. Execution of the Operating System
This figure is explained in the following slides 52

53. Non-process Kernel
The concept of process applies only to user programs
OS code is executed as a separate entity in privileged mode
OS code never gets executed within a process

54. Execution within User Processes
Most OS code gets executed within the context of a user process
On Interrupts, Traps, System calls: the CPU switch to kernel mode to execute OS routine within the context of user process (mode switch)
Control passes to process switching functions (outside processes) only when needed

55. Execution within User Processes
OS code and data are in the shared address space and are shared by all user processes
Separate kernel stack for calls/returns when the process is in kernel mode
Within a user process, both user and OS programs may execute (more than 1)

56. Process-based Operating System
The OS is a collection of system processes
Major kernel functions are separate processes
Small amount of process switching functions is executed outside of any process
Design that easily makes use of multiprocessors

57. Roadmap How are processes represented and controlled by the OS.
Process states which characterize the behaviour of processes.
Data structures used to manage processes.
Ways in which the OS uses these data structures to control process execution.
Discuss process management in UNIX SVR4. 57

58. Unix SVR4 System V Release 4
Uses the model of fig3.15b where most of the OS executes in the user process
System Processes - Kernel mode only
User Processes
User mode to execute user programs and utilities
Kernel mode to execute instructions that belong to the kernel.
UNIX System V makes use of a simple but powerful process facility that is highly visible to the user.
System processes run in kernel mode and execute operating system code to perform administrative and housekeeping functions, such as allocation of memory and process swapping.
User processes operate in:
user mode to execute user programs and utilities
in kernel mode to execute instructions that belong to the kernel.
A user process enters kernel mode by issuing a system call, when an exception (fault) is generated, or when an interrupt occurs. 58

59. UNIX Process State Transition Diagram59

60. UNIX Process States 60

61. A Unix Process
A process in UNIX is a set of data structures that provide the OS with all of the information necessary to manage and dispatch processes.
See Table 3.10 which organizes the elements into three parts:
user-level context,
register context, and
system-level context. 61

62. Process Creation
Process creation is by means of the kernel system call - fork( ).
This causes the OS, in Kernel Mode, to:
Allocate a slot in the process table for the new process.
Assign a unique process ID to the child process.
Copy of process image of the parent, with the exception of data and stacks. 62

63. Process Creation (cont’d)
Increment the counters for any files owned by the parent, to reflect that an additional process now also owns those files.
Assign 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. 63

64. After Creation
After creating the process the Kernel can do one of the following, as part of the dispatcher routine:
Stay in the parent process.
Transfer control to the child process
Transfer control to another process. 64

65. Review The key component for process representation and control?
PCB
Identify the elements that characterize the behaviour of processes.
States and state transitions
Data structures used to manage processes.
Control structure and various tables
Ways in which the OS uses these data structures to control process execution.
Process switching and different process models
Discuss process management in UNIX SVR4.
Assignment 1 65