3/6/99 ICSS440 - Overview 1
What is an Operating System?
• An operating system (OS) is a collection of software that acts as an intermediary between users and the computer hardware
• One can view an OS as a manager of system resources
• An operating system is a control program
• There is no universally accepted definition of what is part of the OS and what is not
3/6/99 ICSS440 - Overview 2
Components of a Computing System
user 1
user 2
user 3
user n
computer hardware operating system application programs
compiler quake emacs Oracle
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Resources
• Process: An executing program
• Resource: Anything that is needed for a process to run
– Memory – Space on a disk – The CPU
• An OS creates resource abstractions
• An OS manages resource sharing
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Abstract Resources
Hardware Resources OS
OS Resources (OS Interface) Middleware Abstract Resources (API)
Application User Interface
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System Software
• A common term used to describe the software responsible for running a computer system (sometimes the OS is included)
• Independent of applications, but common to all
• Examples
– C library functions – A window system– A database management system – Resource management function
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Goals of an OS
• The primary goal of an operating system
– is to make the computer system convenient to use• A secondary goal of the operating system
– is to use the computer hardware in an efficient manner• OS is pure overhead, the OS performs no useful function by itself
• Application programs have the real value to
person who buys the computer
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Why Study OS?
• Understand model of operation
– Easier to see how to use the system – Enables you to write efficient code• Learn to design an OS
• See application of CS theory
• Study a large and complex software system
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A Religious Note
• There are many different operating systems available today
• Each one provides some service that users want
– size, cost, ease of use– interface, availability of software – efficiency, networking
• Don’t be an OS snob, know the concepts, know the needs, and learn how to pick the right tool for the task at hand
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A Course Note
• This course is not about
– how to use an OS – how to administer an OS• This course is about
– the theory behind OS– the study of large software systems – what makes an OS tick
– concurrent processes
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Early Systems
• Early computers were large, expensive, machines run by a single user from a console
• The user/programmer/operator would
– write a program– load program into memory – start execution
– monitor progress – debug/collect results
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Software Libraries
• Initially programmers would be responsible for writing all of the code required to run their programs
• Soon collections of common functions, software libraries, were created
• Rather than rewriting code, the appropriate routines was copied from the library
• The routines that provided input/output (I/O),
device drivers, were especially important3/6/99 ICSS440 - Overview 12
Language Tools
• Soon tools that allowed programmers to code in something other than machine language were developed
– assemblers – FORTRAN, COBOL
• Made programming easier, but running programs
became much more difficult
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Running a Program
Load FORTRAN
compiler
Run compiler
Load Assembler
Load Linker
Run Linker
Run Assembler
Load Program
Run Program
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Simple Batch Systems
• Job setup was a real problem.
• Computers were extremely expensive and owners needed high utilization
• Two-fold solution
– professional computer operators were hired
• quicker setup
• programmers programmed
– Jobs with similar needs were batched together and run through the computer as a group
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Programming in a Batch System
• A typical run of a program
– programmer completes code and submits job (includes compilation and linking instructions) to the operator – operator groups jobs into batches and runs a batch when
appropriate
– when the batch is done, perhaps days later,the output from each job is returned to the programmer
• The delay between job submission and completion is called turnaround time (often measured in days)
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More Problems
• When a job stopped
– operator would have to notice it stopped – determine why it stopped
– if program terminated abnormally, operator would copy the contents of memory for the programmer to analyze (called a core dump)
– start the next job
• In between these steps, the computer would be idle
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Automatic Job Sequencing
• Automatic job sequencing used a small program, called a resident monitor, that transferred control from one job to another
• The monitor is always resident in memory
• Control cards were used to tell the monitor what programs to execute
• This led to the development of control-card
interpreters and job control languages (JCL)3/6/99 ICSS440 - Overview 18
The First OS
• Resident monitors were the first, rudimentary, operating systems
– monitor is similar to OS kernel that must be resident in memory
– control-card interpreters eventually become command processors or shells
• There were still problems with computer
utilization. Most of these problems revolved
around I/O operations
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Off-line Processing
• One common solution to the I/O problem was to replace slow devices with faster ones
– card images were often transferred to tape (maybe disk) before the program was run
– when the tape was full, it was brought to the operator to be processed
– output was often copied to tape first and printed later
• The card-readers/line printers were operated off-
line, rather than by the main computer3/6/99 ICSS440 - Overview 20
The Big Gain
• The obvious advantage is that the computer was no longer bound to operate at the speed of the slower I/O devices
• Another big advantage, was the ability to use multiple reader-to-tape and tape-to-printer systems for one computer
• A disadvantage was slightly longer turnaround time
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Spooling
• One big problem with tape systems is that it was impossible to simultaneously read and write
• Disk systems changed this
– card/print images were placed in files on the disk which can be accessed randomly by the computer
• This form of processing is called simultaneous peripheral operation on-line (spooling)
• In essence the disk is used as a large buffer, for reading/writing as far ahead as possible
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Overlapped CPU and I/O Operations
• The big advantage in spooling was the ability to overlap CPU and I/O operations
– the spooler may be reading the input of one job while printing the output of another
– during this time, another job (or jobs) may be executed, reading cards and printing output from/to disk
• This also started people thinking about virtual devices, and device independent I/O
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Scheduling
• Spooling provides an important data structure: a job pool
– a job pool consists of a collection of jobs, sitting on a disk, waiting to be run
• Now the computer can select which job to run on some basis other than on a first-come, first-served basis
• Decisions can be made based on program size, estimated run time, etc.
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Multiprogramming
• Most computer programs follow a pattern
• During the I/O steps, even with disks, the CPU sits idle waiting for the operation to finish
CPU I/O CPU I/O CPU I/O ...
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Multiprogramming
• Multiprogramming interleaves program execution to increase CPU, and I/O, utilization
CPU I/O CPU I/O CPU I/O
CPU I/O CPU I/O CPU I/O
CPU I/O CPU I/O CPU I/O
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A Multiprogramming System
Job Pool (ready and waiting to
run)
Monitor
job 1
job 2
job 3
job 4
CPU memory
space-multiplexing
time-multiplexing
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Issues
• Multiprogramming systems present several interesting issues
– job scheduling - How do you decide which job to bring into memory from the job pool?
– memory management - How do you put multiple program in memory? How do you prevent one program from interfering with another?
– CPU scheduling - How do you decide which in memory job to run?
– resource management
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Time Sharing Systems
• Multiprogrammed batched systems provide an environment where resources can be utilized efficiently
• The big problems though are
– long turn around times – non-interactive system– post-mortem debugging is difficult
• Time sharing, or multitasking, is a logical extension of multiprogramming
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Multitasking
• The basic idea is to have the CPU switch between jobs so quickly that the user does not notice
• This allows users to interact with a program while it is running, while still providing good utilization of system resources
• Timesharing systems are even more complex than multiprogramming systems
• More interested in improving response time and decreasing variance in delay
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OS and Hardware
• OS and hardware often develop hand in hand
– first OS was to improve utilization– new hardware was developed to improve OS
• The dinosaur mainframe computers are a thing of the past
• New computing technology, and new uses of this
technology, has resulted in changes in OS
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Personal-Computer Systems
• As hardware costs decreased, it became possible to dedicate a single computer to a single user
• Early operating systems for these types of machines consisted of nothing more than a simple resident monitor (CP/M)
• State of the art systems include support for multitasking and typically provide graphical interfaces
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Parallel Systems
• Machines consisting of many, sometimes thousands, of processors in a single box
• Most OS use symmetric multiprocessing
– each processor runs an identical copy of the OS, andthese copies communicate as needed
• Another model is asymmetric multiprocessing
– each processor is assign a specific task– a master processor controls the system
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Networks
• Networks have changed the face of computing
• LAN (Local Area Network) evolution
– 3Mbps (1975)– 10 Mbps (1980) – 100 Mbps (1990)
• Networks provide new ways to do computing
– Shared files– Shared memory – ???
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Distributed Systems
• Wave of the future
Distributed OS
App App
App App
App App
Multiple Computers connected by a Network
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Process Control & Real-Time Systems
• Computer is dedicated to a single purpose
• Classic embedded system
• Must respond to external stimuli in fixed time
• Continuous media popularizing real-time techniques
• An area of growing interest
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Modern Operating Systems
Modern OS
Batch
Timesharing PC & Workstation Network OS
Real-Time Memory Mgmt
Protection Scheduling
Files Devices Memory Mgmt
Protection Scheduling
System software
Human-Computer Interface
Client-Server Model Protocols
Scheduling Windowing
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Mainstay Systems
• The major systems that are out there include
– Mainframe OS (VMS, CMS, MVS, …) – UNIX (Solaris, IRIX, HPuX, Linux, …) – Windows (Win9x, NT, Win2000)• We will look at primarily UNIX and NT in this course
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Unix
• Develop by AT&T Bell Labs in the late 1960's
– Written by a small group of engineers• Ken Thompson & Dennis Ritchie – User interface built on Honeywell Multics – Research in Operating System
– Idea was to write a "portable" operating system – C Language was invented to write UNIX – Not intended to be commercial product – Developers of UNIX were primary users
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Unix Popularity
• UNIX was readily available to universities with source code
• University of California Berkeley modified UNIX
– Virtual Memory– Faster File System
– TCP/IP Networking (telnet, FTP, SMTP) – Given away freely
• UNIX was used primarily in Universities and Research Labs
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Sun and UNIX
• SUN Microsystems
– Formed by ex Stanford and Berkeley Grad students – Evolved BSD UNIX into a reliable product – Added a network file system (NFS) capability
• Created the workstation market
• Technology Expansion in the 1980's fueled the growth of UNIX
– Writing from scratch was very expensive, UNIX was complete, portable, and inexpensive
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X Windows
• Massachusetts Institute of Technology (MIT) developed the X-Window System
– Graphical User Interface – Client-Server Design – Network Oriented
• Although the implementation of the X-Window System is very different, the user interface is very similar to Windows.
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UNIX Standards
• The portability of UNIX was a benefit and a curse
– many companies created ports of UNIX, which resultedin many different flavors of UNIX
• Standards were developed to make sure that UNIX would be stable across different platforms
– IEEE POSIX
– Open Software Foundation – UNIX International – X/Open
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Windows
• Windows NT has a long history as well
– 1981: PC-DOS 1.0 ships with the IBM PC – 1983: Apple releases the Lisa– 1983: MS-DOS 2.0, announces Windows
– 1984-1985: Many multitasking enhancements for DOS – 1985: Windows 1.0 released
– 1987: IBM and Microsoft announce OS/2 1.0 – 1987: Windows 2.0 ships
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Windows
– 1988: IBM and Microsoft release OS/2 1.1
• has a graphical interface
• still has problems
• IBM & Microsoft split – 1990: Windows 3.0 ships
– 1991: Windows 3.1 ships, Chicago is mentioned – 1992: Windows 3.1 for Workgroups ships – 1993: NT is launched
– 1995: Windows95 ships
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Windows
– 1996: NT 4.0 ships – 1997: NT 5.0 ships – 1998: Windows 98 is released
• last version based on kernel running on top of DOS – ????: Windows 2000
