Download What is an Operating System?

University of Pennsylvania
CSE 380
Introduction to Operating Systems
Insup Lee
Fall 00
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What is an Operating System?
• A piece of software that provides a convenient,
efficient environment for the execution of user
programs.
Users
Applications software
Systems software
Operating systems
Hardware
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Resource Abstraction and Sharing
• Hides the details of how the hardware operates
• Provides an abstract model of the operation of
hardware components
– generalize hardware behavior
– limit the flexibility in which hardware can be
manipulated
• Computation model: processes, threads
• Resources: memory, disk, files, cpu, etc.
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Why do we need an operating system?
• User viewpoint -- provide user interface, command
interpreter, directory structure, utility programs
(compilers, editors, filters)
• Program environment viewpoint -- enhance the bare machine
higher-level I/O, structure files, notion of independent
processes, improved store (size, protection)
• Efficiency viewpoint -- replace a human operator scheduling
jobs, storing I/O (files), invoking necessary programs such
as compiler
• Economic viewpoint -- allow concurrent uses and good
scheduling of resources
So, the goals are to make the system convenient to use (via
system calls) and to manage resources efficiently.
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Evolution of an OS
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Hardware upgrades
New types of hardware
New services
Fixes
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Brief History of Operating Systems
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1940's -- First Computers
1950's -- Batch Processing
1960's -- Multiprogramming (timesharing)
1970's -- Minicomputers & Microprocessors
Late 1970's, 1980's -- Networking, Distributed
Systems, Parallel Systems
• 1990's and Beyond -- PC’s, WWW, Mobile Systems
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1940's -- First Computers
• Computer dedicated to one user/programmer at a time.
Program loaded manually by programmer, using console
switches. Debugging using console lights.
• Advantages:
– Interactive (user gets immediate response)
• Disadvantages:
– Expensive machine idle most of time, because people are
slow.
– Programming & debugging are tedious.
– Each program must include code to operate peripherals -error prone, device dependencies.
• Libraries of subroutines to drive peripherals are example of
typical OS service.
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1950's -- Batch Processing
• User/programmer submits a deck of cards that describes a
job to be executed.
• Jobs submitted by various users are sequenced
automatically by a resident monitor.
• Tape drives available for batching of input and spooling of
output.
• Advantages:
– Computer system is kept busier.
• Disadvantages:
– No longer interactive; longer turnaround time.
– CPU is still idle for I/O-bound jobs.
• OS issues -- command processor (JCL), protection of
resident monitor from user programs, loading of user
programs after monitor.
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1960's -- Multiprogramming (timesharing)
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The advent of the I/O processor made simultaneous I/O and CPU
processing possible.
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CPU is multiplexed (shared) among a number of jobs -- while one
job waiting for I/O, another can use CPU.
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Advantages:
– Interactiveness is restored.
– CPU is kept busy.
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Disadvantages:
– Hardware and O.S. required become significantly more complex.
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Timesharing - switch CPU among jobs for pre-defined time interval
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Most O.S. issues arise from trying to support multiprogramming -CPU scheduling, deadlock, protection, memory management, virtual
memory, etc.
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CTSS (Compatible Time Sharing System), Multics
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1970's - Minicomputers & Microprocessors
• Trend toward many small to mid-range personal computers,
rather than a single mainframe.
• Early minicomputers and microprocessors were small, so
there was some regression to earlier OS ideas.
– e.g. DOS on PC is still essentially a batch system similar
to those used in 1960, with some modern OS ideas
thrown in (e.g., hierarchical file system).
• This trend is changing rapidly because of powerful new
microprocessors.
• Also, the user interface became more important.
• UNIX, DOS
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Late 1970's, 1980's - Networking
• Powerful workstations (e.g., PDP, VAX, Sunstations, etc.)
• Local area networks (e.g., Ethernet, Token ring) and longdistance network (Arpanet)
• Networks organized with clients and servers
• Decentralization of computing requires more communication
(e.g., resource sharing)
• O.S. issues -- network communication protocols, data
encryption, security, reliability, consistency of distributed
data
• Real-Time Systems – timing constraints, deadlines, QoS
(90’s term)
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1990's and Beyond
• Parallel Computing (tera-flops)
• Powerful PCs, Multimedia computers
• High-speed, long-distance communication links to send large
amounts of data, including graphical, audio and video
• World Wide Web
• Electronic notebooks using wireless communication technologies
• O.S. issues -- Large heterogeneous systems, mobile computing,
etc.
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Basic OS Concepts
• Processes: programs in execution
• Resources (e.g., files, memory, display)
– A process must request it from the OS
– The process is blocked until it is allocated
• System calls
• Shell -- command interpreter
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Services provided by OS
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Program creation
Program execution
Access to I/O devices
Controlled access to files
Error detection and response
– hardware errors (e.g., memory error, device
failures)
– software errors (e.g., overflow, out of memory
boundary)
• Accounting
– collect usage statistics, monitor performance
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Services Provided by OS (2)
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CPU scheduling: allocate computing time among several processes
Memory management: allocate main memory among several processes
Swapping: move processes and their data between main memory and
secondary storage
I/O device drivers: specialized code required to operate I/O devices
File system: organize mass storage (disk) into files and directories
Utilities: date/time, accounting, file copy, etc.
Command interpreter: allow users to enter commands interactively
System calls: allow user programs access to O.S. services
Protection: keep processes from interfering with each other and
system
Communication & Resource sharing: allow users/processes to
communicate (over networks) and share resources (e.g., laser
printers, disks, etc.)
Security: protect machines from intruders, worms and viruses
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Factors in OS Design
• Engineering factors:
– Performance/Efficiency -- CPU utilization, resource
utilization (e.g. disks), response time
– Correctness/Reliability -- error-free (no software
crashes)
– Maintainability -- modular construction, clearly defined
interfaces, well-documented
– Small in size-- main memory and secondary storage
valuable, larger means more error-prone, larger means
takes longer to write
• Non-engineering factors:
– Commercial factors
– Standards and open systems
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Operating System Structure
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Monolithic Systems
Layered Systems
Virtual Machines
Client-Server Model
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