1. CMPT 300: Operating Systems I Dr. Mohamed Hefeeda
School of Computing Science
Simon Fraser University
CMPT 300: Operating Systems I
Dr. Mohamed Hefeeda

2. Course Objective
Understand the basic principles of designing and implementing operating systems
Why study OS?
Be better developer (system and application levels)
Be better computer user
Use OS concepts in many different CS areas and courses

3. Course Info Textbook Grading Web page
Silberschatz, Galvin, Gagne, Operating System Concepts, (We will refer to it as OSC)
Grading
Assignments: 25% (problem sets and programming projects)
Midterm exam: 25%
Final exam: %
Web page

4. Topics Operating System Structures Processes and Threads
CPU Scheduling and Process Coordination
Memory Management
Storage Management and File Systems
I/O Systems
Security and Protection (time permits)

5. Chapter 1: Introduction

6. Objectives To provide coverage of basic computer system organization
To provide a grand tour of the major operating system components

7. Computer System Structure
Computer system has four components
Hardware – provides basic computing resources
CPU, memory, I/O devices
Operating system
Controls and coordinates use of hardware among various applications and users
Application programs – define the ways in which system resources are used to solve computing problems
Word processors, compilers, web browsers, database systems, video games
Users
People, machines, other computers

8. Computer System Components

9. Operating System Definition
OS is a program that acts as an intermediary between users and computer hardware
OS is a resource allocator
Manages all resources
Decides between conflicting requests for efficient and fair resource use
OS is a control program
Controls execution of programs to prevent errors and improper use of the computer
OS performs no useful function by itself;
It provides environment for programs to do useful work
Can you give an example entity from real life?
Well, the government!

10. Operating System Goals
Make the computer system convenient to use
Execute user programs and make solving user problems easier
Emphasized in PCs, handheld devices
Use computer hardware in efficient manner
Emphasized in large-scale computers with many users and expensive hardware (e.g., airline reservation system)
Note:
The OS kernel: is a program running at all times
Everything else is either a system program (ships with the operating system) or an application program

11. Computer System Organization
One or more CPUs, device controllers connect through common bus to a shared memory

12. Computer System Operation
I/O devices and CPU can execute concurrently
Each device controller
is in charge of a particular device type
has a local buffer
I/O is performed from device to local buffer
CPU moves data between main memory and local buffers
Device controller informs CPU that it has finished its operation by causing an interrupt

13. Interrupt Handling
OS preserves state of CPU by storing registers and the program counter
The corresponding interrupt handler is called to process the interrupt
How would OS locate the correct handler?
In many systems (Windows, Linux), an interrupt vector is stored in the lowest memory locations
Each entry contains the address of a handler routine in the memory
Each interrupt number is mapped to an entry in the interrupt vector

14. Storage Systems: Hierarchy
Speed
Cost
Volatility
Cost
Speed

15. Comparison of Storage Systems

16. Caching
Caching
Information is copied from slower to faster storage
performed at many levels in a computer (HW, OS, SW)
Faster storage (cache) checked first to determine if information is there
If it is, information used directly from cache (fast)
If not, data copied to cache and used there

17. Caching Example: Migration of Integer A from Disk to Register
Movement between levels of storage hierarchy can be explicit or implicit
Multitasking environments must be careful to use most recent value, no matter where it is in the storage hierarchy
Cache consistency

18. Operating System Operations
OS is interrupt (event) driven: sits idle till something happens
Interrupts are generated by hardware devices
Traps (or exceptions) are software-generated interrupts due to
software errors, e.g., divide by zero, illegal memory access
Request for operating system services (system calls)
OS operates in two modes
User mode and kernel mode
Mode bit provided by hardware to indicate current mode

19. Operating System Operations (cont’d)
Privileged Instructions
Subset of instructions that may harm the system
Can only be executed in kernel mode
E.g., I/O control, timer management, interrupt management
Why dual mode?
To enable OS to protect the hardware and itself from users’ codes (which may be buggy and/or malicious), and to protect users from each other
If privileged instructions can only be executed in kernel mode, how can users’ codes use them?
Using System Calls (interrupts generated by software)

20. Transition from User to Kernel Mode
User’s code issues a system call
Mode is changed to kernel mode
OS first checks that everything (e.g., parameter values) is in order and legal
Then, OS executes system call which may contain multiple privileged instructions
mode is set again to user mode

21. More Protection: Timer
Timer to prevent program from holding resources (CPU) for too long, e.g., infinite loop
How it works
Before giving control to a user program, OS sets a timer to a specific value
After period expires, an interrupt is issued and OS regains control
OS then decides whether to grant more time for the program or terminate it

22. Multiprogramming in OS
Multiple jobs are kept in memory so that CPU always has something to execute
needed for efficiency, a single job may not keep CPU and I/O devices busy at all times
One job is selected to run via CPU scheduler
When the job has to wait (for I/O for example), OS switches to another job
Timesharing (multitasking)
CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing

23. Summary
OS is a layer between user and hardware to make life easier for user and use hardware efficiently
Computer organization
CPU(s), memory, and I/O devices connect to a common bus
Devices request CPU attention through interrupts
Storage hierarchy: speed, cost, volatility
Caching: copy frequently-used data to faster storage
Multiprogramming: multiple jobs in memory  efficiency
Timesharing: frequently switch between jobs  interactive
Dual mode operation: user and kernel modes
Protect OS and users from each other
Privileged instructions executed only in kernel mode
Timer to prevent processes from holding resources forever