1. Introduction

2. Why Study OS? Understand model of operation Learn to design an OS
Easier to see how to use the system
Enables you to write efficient code
Learn to design an OS
Even so, OS is pure overhead of real work
Application programs have the real value to person who buys the computer

3. System Overview
A computer system consists of hardware and software that are combined to provide a tool to solve specific problems
Hardware includes CPU, memory, disks, printers, screen, keyboard, mouse ...
Software includes
System software
A general environment to create specific applications
Application software
A tool to solve a specific problem

4. Perspectives of the Computer
cut
malloc()
print
open()
read-disk
start-printer
send
save
fork()
track-mouse
Application
Software
Application
Software
Application
Software
System
Software
System
Software
System
Software
Hardware
Hardware
Hardware
End User
View
(b) Application
Programmer View
(c) OS Programmer
View

5. System Software (“Middleware”)
Independent of applications, but common to all
Examples
C library functions
A window system
A database management system
Resource management functions

6. Purpose of an OS (What is Resource Management?)
Process: An executing program
Resource: Anything that is needed for a process to run
Memory
Space on a disk
The CPU
“An OS creates resource abstractions”
“An OS manages resource sharing”

7. Abstract Resources User Interface Application Abstract Resources (API)
Middleware
OS Resources (OS Interface) OS
Hardware Resources

8. Hardware Resources Processor: execute instructions
Memory: store programs and data
Input/output (I/O)controllers: transfer to and from devices
Disk devices: long-term storage
Other devices: conversion between internal and external data representations

9. Hardware Resources – cont.

10. Hardware Interface – cont.
Everything that a programmer needs to know in order to write programs that perform desired operation on the hardware
Disk drive is an example
Disk interface provides functions to move disk head, transfer data
Monitor
Monitor interface provides functions to move the cursor, display characters/graphics

11. Software Classification
System software
Provides a general programming environment in which programmers can create specific applications
Application software
Intended to solve a specific problem

12. Software Classification - continued
Loader OS
Database
Management
System
Window
Application
Programmer
System Software
Libraries
Compiler
Hardware
Command
Line
Interpreter
Software API

13. What is an Operating System?
The operating system is the part of the system software that manages the use of the hardware used by other system software and all application software
It is the system program that acts between the hardware and the user programs

14. What is an Operating System? - continued
It provides services to user programs
Through system calls / message passing
File system services
Memory services
I/O services
It hides hardware from user programs
When your program shows a message on the monitor, it does not need to know the details
When your program generates a new file, it does not need to know where the free space is on your hard drive

15. Differences between OS and System Software
Major differences between OS and general system software
General system software relies on the abstractions provided by OS
OS abstracts the hardware directly
OS provides the fundamental trusted mechanisms for resource sharing
A general purpose OS is domain-independent

16. Operating System Functions
Resource manager
manage hardware and software resources
Resource abstraction and sharing
A nicer environment
implement a virtual machine for processes to run in
A program in execution is called a process
a nicer environment than the bare hardware

17. Resource Management Functions
Transform physical resources to logical resources
Resource abstraction
Make the hardware resources easier to use
Multiplex one physical resource to several logical resources
Create multiple, logical copies of resources
Schedule physical and logical resources
Decide who gets to use the resources

18. Resource Abstraction
Provides an abstract model of the operation of hardware components
Like data abstraction in Object-Oriented programming
Interface functions
Internal functions and status
Eliminates the tedious details that a programmer would otherwise have to handle

19. A Disk Device Abstraction
Three interface functions
load(block, length, device)
Copies block of memory to device buffer
seek(device, track)
Positions read/write head
out(device, sector)
Writes the data to disk sector

20. A Disk Device Abstraction – cont.
An abstract function for writing a block of memory to disk
write(char *block, int len, int device,
int track, int sector) {
...
load(block, length, device);
seek(device, track);
out(device, sector); }

21. A Disk Device Abstraction – cont.
load(…);
seek(…);
out(…);
void write() {
seek(…)
out(…) }
int fprintf(…) {
...
write(…) …
(a) Direct Control
(b) write()
abstraction
(c) fprintf()
Application
Programmer
OS Programmer

22. Resource Abstraction – cont.
The dark side …
While simplifying the way application programmers can control hardware, abstraction can limit the flexibility by which specific hardware can be manipulated
Certain operations may be impossible to achieve using the abstraction

23. Resource Sharing Concurrent execution Parallel execution
Two or more processes appear to be (or actually are) executing at the same time
Parallel execution
Two or more processing actually executing simultaneously
Processes which are concurrent, or parallel, must share the computer

24. … … … Abstract Machines Abstract Machines Idea Program Result Physical

25. Resource Sharing Two types of sharing Time multiplexed sharing
time-sharing
schedule a serially-reusable resource among several users
Space multiplexed sharing
space-sharing
divide a multiple-use resource up among several users

26. Resource Sharing To control sharing, must be able to isolate resources
OS usually provides mechanism to isolate, then selectively allows sharing
How to isolate resources
How to be sure that sharing is acceptable
Concurrency

27. The OS as a Conductor
The OS coordinates the sharing and use of all the components in the computer

28. Multiprogramming … … Abstract Machine Pi Abstract Machine Pj Abstract
Machine Pk …
OS Resource Sharing
Pi Memory
Pk Memory …
Pj Memory
Time-multiplexed
Physical Processor
Space-multiplexed
Physical Memory

29. Multiprogramming(2)
Technique for sharing the CPU among runnable processes
Process may be blocked on I/O
Process may be blocked waiting for other resource, including the CPU
While one process is blocked, another might be able to run
Multiprogramming OS accomplishes CPU sharing “automatically” – scheduling
Reduces time to run all processes

30. How Multiprogramming Works
Process 1
Process 2
Process 3
Time-multiplexed CPU
Process 4
Space-multiplexed Memory

31. Speeding Up the Car Wash
Vacuum
Inside
Wash
Dry
(a) The Sequential Car Wash
Vacuum
Inside
Wash
Dry
(b) The Parallel Car Wash

32. Time-multiplexing the Processor
- Resulted in concurrent execution or concurrency

33. Time-multiplexing the Processor – cont.
Using the processor
I/O operation / paused ti
Pi’s Total Execution Time, ti
(a) Pi’s Use of Machine Resources P1 P2 Pi PN …
(b) All Processes’ Use of Machine Resources
Improves performance

34. Space-multiplexing Memory

35. Time-multiplexing I/O Devices

36. Space-multiplexing the Disk

37. Issues in Resource Sharing
Resource isolation and sharing
Protection
Prevent unauthorized access of resources by one process when they are currently allocated to another
Sharing
Resource allocation
Scheduling

38. Do We Need an OS? Not always But they are very useful
When resource abstraction or sharing is not needed
Some programs run “stand-alone”
Early computers did not have a sophisticated OS
OS was evolved along the hardware technology
But they are very useful
Reusable functions
Easier to use than the bare hardware

39. Operating Systems Strategies
Several different strategies have been used
Earliest computers were dedicated to a single program and there was no multiprogramming and no OS
Batch systems
Timesharing systems
There are a few other recent strategies
Personal computers and workstations
Embedded systems
Small, communicating computers
Network technology

40. Batch Processing Systems
Reduce setup time by batching similar jobs
Automatic job sequencing – automatically transfers control from one job to another. First rudimentary operating system.
Resident monitor
initial control in monitor
control transfers to job
when job completes control transfers back to monitor

41. Batch Processing Systems - continued
Job 19
Input Spooler
Output Spooler
Job 3
Input Spool
Output Spool

42. Memory Layout for a Simple Batch System

43. Batch Processing Uses multiprogramming
Job (file of OS commands) prepared offline
Batch of jobs given to OS at one time
OS processes jobs one-after-the-other
No human-computer interaction
OS optimizes resource utilization
Batch processing (as an option) still used today

44. Spooling
Overlap I/O of one job with computation of another job. While executing one job, the OS
Reads next job from card reader into a storage area on the disk (job queue).
Outputs printout of previous job from disk to printer.
Job pool – data structure that allows the OS to select which job to run next in order to increase CPU utilization

45. Multi-programmed Batch Systems
Several jobs are kept in main memory at the same time, and the
CPU is multiplexed among them.

46. OS Features for Multi-programming
I/O routine supplied by the system
Memory management – the system must allocate the memory to several jobs
CPU scheduling – the system must choose among several jobs ready to run
Allocation of devices

47. Time-sharing Systems
The goal is to enable users to interact with the computer system
Batch processing systems do not allow user interactions
On-line communication between the user and the system is provided
When the operating system finishes the execution of one command, it seeks the next “control statement” not from a card reader, but rather from the user’s keyboard.
On-line system must be available for users to access data and code.

48. Time-sharing Systems - continued
Physical
Machine
Abstract
Machines …
Command
Result

49. Timesharing Systems – cont’d
Uses multiprogramming
Support interactive computing model (Illusion of multiple consoles)
Different scheduling & memory allocation strategies than batch
Tends to propagate processes
Considerable attention to resource isolation (security & protection)
Tend to optimize response time

50. Personal-computer Systems
Personal computers – computer system dedicated to a single user.
I/O devices – keyboards, mice, display screens, small printers.
User convenience and responsiveness.
Can adopt technology developed for larger operating system
often individuals have sole use of computer and do not need advanced CPU utilization of protection features.

51. Personal-computer Systems - continued
Win32 API
Windows CE
(Pocket PC)
Windows 95/98/Me
Windows NT/2000/XP
Win32 API Subset
Win32 API SubSet

52. Embedded Systems
Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.
Well-defined fixed-time constraints.
Hard real-time system.
Secondary storage limited or absent, data stored in short-term memory, or read-only memory (ROM)
Conflicts with time-sharing systems, not supported by general-purpose operating systems.
Soft real-time system
Limited utility in industrial control or robotics
Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.

53. Parallel systems
Multiprocessor systems with more than one CPU in close communication.
Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.
Advantages of parallel system:
Increased throughput
Economical
Increased reliability
graceful degradation
fail-soft systems

54. Distributed Systems
Distribute the computation among several physical processors
Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines
Advantages of distributed systems
Resources Sharing
Computation speed up – load sharing
Reliability
Communications

55. Distributed systems - cont.
Network operating system
provides file sharing
provides communication scheme
runs independently from other computers on the network
Distributed operating system
less autonomy between computers
gives the impression there is a single operating system controlling the network.

56. Migration of Operating-System Concepts and Features

57. Evolution of Modern OS Timesharing Network OS PC & Wkstation Batch
Real-Time
Memory Mgmt
Protection
Scheduling
Files
Devices
System software
Human-Computer
Interface
Client-Server Model
Protocols
Small Computer
Network storage,
Resource management