1. Operating Systems
Part I: Introduction

2. “I think that there is a world market for five computers”
- Thomas J. Watson (1945)

3. Why Study Operating Systems?
We want to have an efficient O/S because it
consumes more resources than any other program.
is the most complex program.
is necessary for any use of the computer.
is used by many users.
Efficiency is measured through
Functionality
Performance: Time and Utilization
Convenience and Cost

4. Goals of This Course Understand what an operating system is
Understand the key components of an operating system
Have a deeper understanding of common operating systems in the market (e.g. Windows, Unix, MS-DOS) and the issues associated with them
To be able to use performance measures

5. What is an O/S? A layer of abstraction between the HW and SW
A resource coordinator
Virtual machine
Reactive system

6. Operating Systems: A Definition
A collection of programs that integrate the hardware resources of the computer and make those resources available to the user in a productive, timely, and efficient manner

7. Operating Systems Ease the Pain
Performs the interface task with the hardware (file operations, memory paging, etc.) which should have been done by the user if the OS did not exist
High-level interface (GUI, command line a.k.a. CUI)
The O/S’s capability for multiuser and multitasking utilize the hardware efficiently
Makes visible the “virtual” component of the system
Allows program interaction

8. Why are Operating Systems Difficult to Create and Maintain?
Size
Too big for one person; current systems have millions of lines of code and involve man years to build
Lifetime
Operating systems remain longer than the programmers who originally wrote them. Code is written and rewritten and original intent is forgotten (Unix designed to be cute, small system - now several volumes thick!)

9. Why are Operating Systems Difficult to Create and Maintain?
Complexity
The system must do difficult things -- deal with ugly I/O devices, multiplexing/juggling act, handle errors
Multitasking
Must do several things at once.
General purpose

10. A Brief History: Early 1950’s, Mainframes Rule!
Early systems
No O/S! Programmer is also operator
Large machines run from a console; programs loaded through switches and card readers
Simple batch systems were the first real OS
Setup time was a problem -> hire an operator
Operator ran related jobs together
O/S was a simple program stored in one part of memory
Loads a single job from card reader into memory
Transfers control from one job to the next

11. Offline Processing Allowed jobs to be read ahead of time onto tape CPU
Card
Reader
CPU
Line
printer
On-line processing
Card
Reader
CPU
Line
printer
Tape
Drive
Tape
Drive
Off-line processing
Tape
Drive
Tape
Drive

12. History: Spooling Allowed jobs to be read ahead onto disk
Spool (Simultaneous Peripheral Operation On-Line)
disk
Card
Reader
Line
printer
CPU

13. Multiprogrammed Systems
Multiprogrammed batch systems provided increased utilization
Keeps several jobs in memory simultaneously
I/O processing of one job overlaps with computation of another
Analogy: Lawyer working on several cases; while waiting to go to trial on one, can work on another
Needs CPU scheduling

14. Timesharing/Multitasking Systems
Timesharing supported interactive use
Each user feels as if he/she has the entire machine
Tries to optimize response time
Based on time-slicing; divide CPU equally among others

15. Desktop Systems First appeared in the 1970s
More popularly known as personal computers (PCs)
Breakthroughs in hardware allowed downsizing from expensive mainframes

16. Multiprocessor Systems
Also known as parallel systems or tightly-coupled systems
Three main advantages
Increased throughput (more CPUs = more work in less time)
Economy of scale (saves money, CPUs share peripherals)
Increased reliability (provides redundancy and fault tolerance)
Symmetric multiprocessing (SMP): All CPUs do the same thing
Asymmetric multiprocessing: each CPU has specific role (usually master-slave)

17. Distributed (Loosely-Coupled) Systems
Facilitates use of geographically distributed computing resources
Supports communications between parts of a job or different jobs
Supports sharing of distributed resources, both hardware and software
Client-server systems vs. Peer-to-peer systems

18. Clustered Systems
Makes several CPUs work together to accomplish computational task
Most likely share storage and linked through a local area network (LAN)
Possible clustering schemes:
Symmetric mode (two or more hosts running applications and monitoring each other)
Asymmetric clustering (one is in hot standby mode while another is running applications; switches to backup if active fails)

19. Real Time Systems
Used for specialized applications: subway systems, flight-control, factories, power plants
Basic idea: O/S guarantees response to physical events will occur within a fixed amount of time
Problem faced : Schedule activities so as to meet all time constraints
Hard real time system
Deadline is critical
Typically used to control a device
Soft real time system
Deadline is important but not critical
Example : Video applications (Use in PC environment)

20. Handheld Systems Used in PDAs and cellular phones Common concerns:
Limited main memory
Processor speed
Small display screens

21. General Structure of an O/S
Resident Programs
Programs which are critical to the operation of the system
KERNEL
Non-resident Programs
Loaded into memory only when needed

22. An Example: MS-DOS Structure
Memory resident components
Command interface shell (eg. ver, dir, date, time) : COMMAND.COM
Set of I/O routines which control each I/O devices (drivers) -- e.g. BIOS : IO.SYS
File Management System : MSDOS.SYS
Non-resident components
FORMAT.COM, XCOPY.EXE, LABEL.EXE, etc.

23. How MS-DOS Programs are Loaded in Main Memory
At System Start-up
Running a Program

24. Operating System Components
Process Management
Process: a program in execution
Keeps track of each process and it’s state
Create, delete, suspend, resume processes; synchronize process communications, handle deadlocks
Possibly support threads (executable parts of a process)

25. Operating System Components
Main Memory Management
Keep track of which parts of memory are in use
Allocates and deallocates memory as needed
Decides which processes must be loaded in main memory when space becomes available

26. Operating System Components
File Management
Keeps tracks of available space on the system
Maintains directory structure and hierarchy
Supports file manipulation commands
Keeps track of file information (inode, name, timestamp)
I/O System Management
Allows for a standard methodology for access of each device
Use of device drivers for modularity

27. Operating System Components
Secondary Storage Management
Free space management
Storage allocation
Disk scheduling
Networking
Transfer protocols
Routing and connection strategies

28. Operating System Components
Protection System
Provide mechanism for controlling access to programs, processes, or users
Essential in multitasking and multiuser systems
Command Interpreter System
GUI vs. Command Line Interface
Redirection, Piping, and Parameter Passing

29. Operating Systems Architectures
Monolithic
Layered
Virtual Machine
Microkernel

30. Monolithic Easy to implement “The Big Mess” – virtually no structure!
Kernel is one large structure
Each procedure is visible to every other procedure
Not used anymore

31. Layered
Not easy to implement because some functionalities are mutually dependent.
Inefficient because it requires a high number of traversals of interfaces

32. Virtual Machine
Non-virtual Machine
Virtual Machine
Each user has a “virtual machine” and he can choose which OS to run on that machine
Elegant, but does not deal with questions of resource management and responsiveness

33. Microkernel Used in Mac/OSF/NT
Takes out as much functionality as possible from kernel -- allows modularity and portability across platforms
Interactions between processes involve the kernel, thereby requiring high efficiency in transfer