Lecture 1
Introduction

What is an Operating System?

• Tool to make programmer's job easier

• Resource allocator
  • Must be fair; not partial to any process

  • Must discriminate between different classes of jobs with different service requirements

  • Must do the above efficiently

• Control program

• Tool to facilitate efficient operation of computer system

• Virtual machine that is easier to understand and program

• Layered architecture

• Must be a wizard that maintains the illusion that the user has the sole attention of the computer.

History 1

• 1945 - 1955

• Bare machines -- vacuum tubes and plugboards
  • Example: ENIAC

  • Electronic Numerical Integrator And Computer

  • Designed by J.W. Mauchly and J.P. Eckert at University of Pennsylvania in 1945

  • Commissioned by the Ballistics Research Lab at Aberdeen Proving Ground

  • Made up of 18,000 vacuum tubes and 1,500 relays

• No operating system

• Black box concept -- human operators

• No protection

History 2

Second Generation Systems

• 1956 - 1965

• Transistors and batch systems

• Clear distinction between designers, builders, operators, programmers and maintenance personnel

• I/O channel

• Read ahead; spooling

• Interrupts; exceptions

• Minimal protection

• Libraries; JCL

History 3

We're still not making good use of computing resources. Even though I/O has been separated from the CPU, the program sometimes has to wait for I/O to complete before executing more instructions. Programs and data do not always fill memory. Etc. Solution: Have several programs on the computer and have them trade off on the CPU. One can be running while the other is waiting for I/O.

• 1965 - 1980

• Integrated circuits

• Multiprogramming

• Spooling (simultaneous peripheral operation on-line)

• Time sharing

• On-line storage for
  • System software

  • User programs and data

  • Program libraries

• Virtual memory

• Multiprocessor configurations

• MULTICS
  • Multiplexed Information and Computing Service

  • Design started in 1965; completed in 1972

  • Collaboration between GE, Bell Labs, MIT (Project MAC)

  • Aims:
    • Simultaneous computer access by large community of users

    • Ample computation power and data storage

    • Easy data sharing between users, if desired

    • Protection

  • See Multics home page (access from course home page).

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  History 4

  Fourth Generation and Beyond

  Finally, the hardware gets cheap and people expensive. It's more cost efficient to put a computer on every desk so the user's time is not wasted. Since there is less need for general-purpose time-sharinf systems, a lot of attention is turned toward developing special-purpose computers for scientific and engineering applications.

  • Personal computers and workstations

  • MS-DOS and Unix

  • Massively parallel systems
    • Pipelining

    • Array processing / SIMD

    • General multiprocessing / MIMD

    • Symmetric multiprocessing / SMP
      • Any process or thread can run on any available processor

  • Computer networks -- network operating systems

  • Distributed computing -- distributed operating systems

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  Operating System Concepts

  • Process

  • Kernel

  • Files

  • System calls

  • Shell

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  Processes

  The Process Concept
  • "A program in execution"

  • May be stopped and later restarted by OS

  • Process table contains Process Control Block for each process

  • PCB records information about the process
    • Program code

    • Data

    • Stack

    • Program counter, stack pointer, other registers

  • Core image

  • Process has a parent and may create other processes (children)

  • Communication through messages

  • Process ownership determined by UID and GID

  • Process id identifies the process

  • Special process ids in Unix:
    • Swapper -- 0

    • sbin/init -- 1

    • Page daemon -- 2

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  The Kernel

  Two modes of execution: user mode and kernel mode.
  • User mode
    • Processes can access own instructions and data but not kernel's

    • Cannot execute certain privileged machine instructions

  • Kernel mode
    • Processes can access kernel and user instructions and data

    • No limit to which instructions can be executed

  Services provided by the kernel
  • Always resides in main memory

  • Controls execution of processes by allowing their creation, termination or suspension

  • Schedules processes fairly for execution on CPU

  • Allocates main memory for an executing process

  • File system maintenance

  • Allows processes access to peripheral devices (terminals, tape drives, disk drives, network devices)

  Services provided by kernel transparently
  • Always resides in main memory

  • Recognizes that a given file is a regular file or device but hides the distinction from the user processes

  • Formats data for internal storage; hides format from user; returns unformatted byte stream

  • Allows shell to read terminal input, spawn processes dynamically, synchronize process execution, create pipes, redirect I/O