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Operating SystemsOperating Systems

Introduction

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OS is a Control ProgramOS is a Control Program

• OS Functions are:• 1) OS is a resource allocator. Resources are:

– CPU time– I/O devices– Files– Memory, internal (SIMM, DIMM)– Mass Storage (HD, MTU…)

• 2) To make Computer System user friendly– GUI, Sound, Use of human (like) languages

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CS ComponentsCS Components

Application Programs

Operating System

Computer Hardware

User 1 User 2 User 4User 3 User 5

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Early Computers (Mainframes)Early Computers (Mainframes)

• First Computers had no OS– programmer had to control hardware (011101110…)

• “Single User” machine– User had to book time to use CS

– CS was often not available, or was idle

• Computers were set up to run a (ONE) job– load, run and unload compiler, (removable disk of

5MByte)

– load assembler, …(JOB, FOR,…), another disk

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Batch SystemBatch System

• Job Cards were used to specify steps in the job process – load compiler, one instruction, one card

– run compiler,

– unload compiler,

– load assembler,

– Changes in the program were done by punching new cards

• Human operators had to batch jobs together – jobs requiring the same compiler run in the batch

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Resident Monitor – First OSResident Monitor – First OS

• Monitor Program was doing job scheduling• User is no longer in control of the CS• Resident Monitor was always in memory• RM was first OS, with components:

– Loader

– Job Sequencer

– Control Card Interpreter (Command Interpreter)

• The computer efficiency was increased

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Monitor ProgramMonitor Program

• While the user application is running, it must not be allowed to alter the monitor– CPU hardware is designed to protect the monitor in

memory – an error message is generated if protected memory area

is to be accessed

• Monitor must take control of CS if a application enters an infinite loop– Timer is a hardware to support that

• when timer goes off, monitor program gain control back

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Monitor Monitor

• Monitor prevents possible hardware damage, caused by application software, by using– Privileged instructions:

• CPU allows only the monitor to run instructions which use the I/O devices, and which will report back with an error in case of any failure

• How does CS hardware put Monitor Program back in control of CS?– Traps – Interrupts

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Next Step - Spooled Batch SystemsNext Step - Spooled Batch Systems

• I/O activities are too slow comparing to CPU activities, order of [ms], [sec] vs. [ns]:– Card reader is slow, and following that, CPU is mostly

idle as it waits for data to come in

– Writing output / printing is also slow, and CPU also mostly idle

• Computer CPU time was always expensive, $$$• SPOOL-ing (Simultaneous Peripheral Operation

On Line) was THE solution

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Spooled Batch SystemsSpooled Batch Systems

• Cheaper machines do the Input and record details on a tape or disk– Tape or Disk is then used by the main computer

• Output is written on the disk, or tape, which another cheaper machine reads to print out

• CPU Idle time is down from few min to few sec, ms.

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Multi-programmed Batch SystemsMulti-programmed Batch Systems19651965

• Spooling still means that the CPU is mostly idle while accessing data from– A disk ( few 10s ms) or – A tape (few min)

• Next step: Multi-programmed Batch Systems– Introduce more than one user application into memory.

While I/O is occurring, CPU run another job– Operating System, or Control Program, now have to do

Job Management• Priority, FCFS, RR,…

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Multi-programmingMulti-programming

• Memory allocation for multi-programmed system

Operating System

Job 1, (User)

Job 2

Job 3

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Concurrent JobsConcurrent Jobs

• Operating System is now doing resource allocation for concurrent jobs in memory– By the time an input device is available, few jobs are most likely

waiting ( in a i/o queue for that device)

– OS must ensure that the data goes to the correct job i.e. user application

– OS must protect devices from competing jobs

– Fairness of resource allocation is to be in place

– Efficiency of the system is always an issue

– Deadlock prevention, and/or recovery should be enforced

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New OS functions to Support New OS functions to Support MultitaskingMultitasking

• I/O routines to handle concurrent requests• More sophisticated memory management

– multiple jobs must be placed in memory and – be protected from each other

• CPU scheduling,– to choose which ready-to-run job gets the CPU first

• I/O devices, and Mass storage devices, allocation algorithms are developed to handle concurrent requests– Disk Management: FCFS, SSTF, SCAN, CSCAN, LOOK,

CLOOK– Printer queue: FCFS

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Batch Jobs, still in placeBatch Jobs, still in place

• Modern Operating Systems like Windows (DOS), UNIX,…, can perform batch jobs, as well.

• UNIX batch shell script:

» mkdir new_dir» cp *.txt new_dir» startx» logout

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Multi-user Interactive SystemsMulti-user Interactive Systems

• Batch Systems: There was no interaction between programmer and application during execution– User have to provide all possible inputs in advance,– Question was, how to handle errors, in the process?

• Multi-user Interactive Systems, or Time sharing was introduced in 1970s:– User application and OS can:

• take inputs, interactively, and • Produce outputs, in a reasonable time• CS serve many users (Honeywell, IBM… 128, 256 users)

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Multi-user Interactive SystemsMulti-user Interactive Systems

• CPU schedules a small time-slice to each user application – For n users,

• give each user (job, task) approximately 1/n of the CPU time, over a short interval (Round Robin Algorithm)

• Such OS requires File System with easy interface for an interactive user.

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Finally, PC Systems, late 1970,…Finally, PC Systems, late 1970,…

• Hardware is becoming cheap,• A PC is a single user machine,• As being a single user CS, used to have just a few,

hardware and resources, protection mechanisms • PC is quite sophisticated now, • Well known PC Operating Systems:

– CPM, DOS, WINDOWS,…

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Other CS and OSOther CS and OS

• Parallel Systems, Multiprocessing– More than one CPU sharing the same clock, bus, and

sometimes the same memory– Parallel processing, load sharing

• Two Types– Symmetric systems:

• each CPU runs a copy of the OS. The separate operating systems can communicate with each other when necessary in order to balance workloads and coordinate resources

– Asymmetric systems: • only one OS. Different OS processes can be running on

different CPUs

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Networked SystemsNetworked Systems

• User is able to access files/peripherals across a network and can log in to remote machines

• CPUs time is a network resource as well• Network types:

– LAN, MAN (broadcasting mostly, but switching as well)

– WAN (switching mostly)

• Internet Computing

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Distributed SystemsDistributed Systems

• User sees the system as it is just his local system, – in reality, files and running programs are on different

machines.

• On some systems, running applications are able to migrate from system to system– Internet Computing

• On other systems, a running application has to stay on the computer that started it.

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Real-time OSReal-time OS

• Systems defined according response time.

• Real Time OS guarantees that a request to the OS will be done within a certain time-limit (short time usually).

• Mainly used for embedded systems that need to control processes, or equipment– Automatic pilot,

– medical equipment,

– some military systems,

– power station,…

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Real-time OSReal-time OS

• Hard limit real-time:– Absolutely guarantees that any request will be done

within a time limit • system usually limits types of activities the user application

can perform to ensure a request can be met

• Soft limit real-time:– to do the request on time

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ReferencesReferences

• Silberscharz A. and Galvin P., Operating System Concepts, John Wiley & Sons, 1999

• Tanenbaum A., Modern Operating Systems, 2nd edition, Prentice Hall, New Jersey, 2001