1. Maxim Shevertalov Jay Kothari William M. Mongan
Operating Systems Lecture 1: Introduction Review of System Architecture and Concurrent Programming
Maxim Shevertalov
Jay Kothari William M. Mongan
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Operating Systems

2. Introduction What is an Operating System? How are they designed?
September 4, 1997
Introduction
What is an Operating System?
How are they designed?
Why study them?
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Operating Systems

3. What is an Operating System?
September 4, 1997
What is an Operating System?
Facilitation + Performance
Think of Air Traffic Control: they are in charge of lots of airplanes, and sometimes they tell airplanes to wait their turn.
Despite what we may sometimes think, their intent is not to prevent us from doing what we want by imposing complicated rules.
Rather, they are there to facilitate aircraft in flight.
On its own, the air traffic controller does not accomplish much; but, together with the aircraft he supports, he aims to maximize performance for all the participating airplanes (not any one!), while protecting them from one another.
It’s all about coordination.
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Operating Systems

4. What is an Operating System?
September 4, 1997
What is an Operating System?
Can we use a computer system without an operating system?
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Operating Systems

5. What is an Operating System?
September 4, 1997
What is an Operating System?
Can we use a computer system without an operating system?
We would have to directly interface with the computer hardware.
Pros: speed?, control?
Cons: What if we need to run on a different machine? Or change the network card? Install a new OS? What if new features like multiprocessors or a cluster is introduced to the program?
The operating system mediates between application (the airplanes) and the shared resources (the runways and airspace) in a way that provides abstraction and generality for the applications.
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Operating Systems

6. What is an Operating System?
Trusted software interposed between the hardware and application/utilities to improve efficiency and usability
Most computing systems have some form of operating systems
Hard to use computer systems without OS
Applications and Utilities
Operating System
Computer Hardware
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Operating Systems

7. Why Study Them? Take a deeper at system architecture
September 4, 1997
Why Study Them?
Take a deeper at system architecture
Modular, layered software design
All of our software runs on top of it! It is the common denominator of everything we write.
How often do you make a syscall? Or invoke some system library?
Learn to build complex systems
Engineering Issues
Business Issues (OS as a government)
Security (viruses, worms, etc.)
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8. Computer System Organization
Computer-system operation
One or more CPUs, device controllers connect through common bus providing access to shared memory
Concurrent execution of CPUs and devices competing for memory cycles
Memory
CPU
Disk Controller
USB Controller
GPU
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Operating Systems

9. Operating System Design: Modularity
September 4, 1997
Operating System Design: Modularity
Process Control: Threads
Synchronization and Scheduling
Protection, Memory Management, Caching
Demand Paging
File Systems
Networking and Distributed Systems
Protection and Security
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10. One Size Fits All? Requirements Dictate Design
September 4, 1997
One Size Fits All? Requirements Dictate Design
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11. Evolution of the OS In the beginning... Computers Cost millions of $$$
Were hard to operate (performed using console switches)
Were hard to program (in particular I/O devices)
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12. Utilizing Expensive Hardware
Expensive computers could not idle; utilization should be very high
Batch processing was devised
Programming done offline and subroutine were created for common tasks
I/O and processing overlap obtained via buffering and interrupts
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13. Factors in OS Evolution
Cost of computers (i.e., computer time)
Cost of people (i.e., user time)
user wait time
computer utilization
time sharing systems
early batch systems
personal systems
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14. September 4, 1997
Major OS Functions
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15. Major OS Functions Control access and provide interfaces
To the OS and devices attached to the system
Provide interfaces for human-machine and machine-machine transactions
Manage resources
Mediate resource usage among different tasks
Implement policies
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Operating Systems

16. Major OS Functions Control access and provide interfaces
To the OS and devices attached to the system
Provide interfaces for human-machine and machine-machine transactions
Manage resources
Mediate resource usage among different tasks
Implement policies
Provide abstractions
Hide the peculiarities of the hardware.
Example: device independent I/O
Consume resources
OS runs on the system that is being managed… so it is going to consume resources!
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17. What is the Access Problem?
User wants to access OS, why?
User wants to access devices connected to the system
It all starts with recognizing the user’s intent to access!
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18. Access Problem: Handling I/O
Programmed I/O
The CPU transfers the data from (or to) the device buffers.
After issuing an I/O operation the CPU continuously checks (polls) for its completion
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19. Access Problem: Handling I/O
Interrupt-driven I/O (slow speed, character device)
The CPU issues an I/O operation and goes on; Device notifies (interrupts) the CPU as data arrives; CPU processes the data
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20. How Does the OS Service Interrupts?
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21. Access Problem: Data Processing
Once data is captured from device (e.g., keyboard) what is next?
Imagine you want to write a word processor
Capture some keystrokes – for arrow and other control keys
Capture words (cooked or raw form?)
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22. Accessing Devices vs. OS Services
OS interposes itself between the hardware and applications/utilities/users
Controlling access to OS ---> Controls device access
User OS
Device
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23. How Do You Control Access to the OS?
Provides a two level architecture:
Trusted mode
Untrusted mode
The OS (at least the core part – called Kernel) runs in the trusted mode
User applications/utilities run in the untrusted mode
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24. System Call via Trap
OS provides system calls for accessing OS services from applications
System calls are special procedure calls
Control transfer
Switch protection domain
System calls also:
Validate the call
Allow authorized actions to take place
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25. How are Syscalls Processed?
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26. UNIX Syscall Examples Process control File manipulation
fork(), exec(), wait(), abort()
File manipulation
chmod(), link(), stat(), creat()
Device manipulation
open(), close(), ioctl(), select()
Information maintenance
time(), acct(), gettimeofday()
Communications
socket(), accept(), send(), recv()
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27. How Does an OS Manage Resources?
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28. Process: Avatar of the Application
A process represented process table entry
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29. Simple Process Model Below is a two-state process model
Running (on the CPU)
Not running (waiting to get the CPU or at I/O)
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30. Resource Management: Processor Time
Processor time is the primary resource managed by the OS
How it is managed depends on the type of OS:
Batch vs Time-sharing
Uniprogramming vs Multiprogramming
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31. Example: Uniprogramming (only one program running on the system)
What is the CPU Utilization?
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32. Example: Multiprogramming (multiple programs simultaneously running)
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33. Comparing System Utilization
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34. Process Scheduling
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35. Process Scheduling: Concerns
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36. Process Scheduling Concerns
Fairness
Give equal and fair access to resources
Differential responsiveness
Discriminate among different classes of jobs
Efficiency
Maximize throughput, minimize response time, and accommodate as many users as possible
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37. Memory Management Memory management is key to multiprogramming
Memory is a space sharing resource
Memory management is necessary to protect co-residing programs from each other
Depending on the occupancy pattern, a program would not know the location until load time
Memory management should provide:
Protection
Relocation
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38. How Does an OS Manage Memory?
Idea:
Divide the memory into partitions and allocate them to different processes
Partitions can be fixed size or variable size
A process
Cannot access memory not allocated to it
Can request more memory from OS
Can release already held memory to OS
May only use a small portion of the allocated memory
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39. How Does an OS Manage Memory?
We can load more processes than that fitting the memory
Where do the rest go?
Answer: Virtual Memory
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40. Virtual Memory in Action
Some memory chunks (pages) are in memory
Rest in disk
OS is responsible for retrieving the pages from and flushing the pages to disk
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41. To Use Storage, We Need a Filesystem
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42. Storage Management: Component View
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43. Topics Covered Fundamentals (Operating Systems Structures)
Process Control and Threads
Synchronization and scheduling
Protection, Address translation, Caching
Demand Paging
File Systems
Networking and Distributed Systems
Protection and Security
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