The Sampah's Blog

Operating System Support

Posted on: June 13, 2010

Fungsi Sistem Operasi

Sistem operasi merupakan program yang mengontrol eksekusi program aplikasi dan bertindak sebagai antarmuka antara pengguna komputer dengan hardware

Tujuan Sistem Operasi

Sistem Operasi Mempunyai 2 Tujuan :

–          Kenyamanan : Suatu sistem operasi membuat komputer lebih mudah digunakan

–          Efisiensi : Suatu sistem operasi memungkinkan sumber daya sistem komputer dapat digunakan dengan cara yang efisien

Layers and Views of a Computer System


Layanan Sistem Operasi

—  Pembuatan Program

—  Eksekusi Program

—  Access to I/O devices

—  Akses Terkontrol ke file

—  Akses Sistem

—  Pendeteksi Kesalahan

—  Akuntansi

O/S as a Resource Manager


Jenis Sistem Operasi

—  Interactive

—  Batch

—  Single program (Uni-programming)

—  Multi-programming (Multi-tasking)

Early Systems

—  Late 1940s to mid 1950s

—  No Operating System

—  Programs interact directly with hardware

—  Two main problems:

—  Scheduling

—  Setup time

Simple Batch Systems

Prosesor masa awal sangat mahal, dan oleh karena itu sangat penting untuk memaksimalkan pemanfaatan prosesor. Waktu yang disia-siakan dalam kaitan dengan waktu penjadwalan dan waktu setup sangat sulit untuk diterima. Untuk meningkatkan pemanfaatan dikembangkan sistem operasi bacth sederhana.

Fitur-fitur perangkat keras

–          Perlindungan Memori Pada saat program pengguna dieksekusi, maka program itu tidak harus mengubah area memory yg berisi monitor. Jika terjaadi perubahan area memori maka prosesor akan mendeteksi kesalahan dan kontrol transfer ke monitor

–          Pengatur waktu Pengatur waktu digunakan untuk mencegah sebuah tugas memonopoli sistem

–          Instruksi-instruksi istimewa Instruksi yang diistimewakan adalah instruksi I/O

–          Interupsi Fitur yang memberikan SO lebih flexibel dalam melepaskan kontrol ke program pengguna dan mendapatkan kembali kontrol tersebut

Multi-programmed Batch Systems

—  I/O devices very slow

—  When one program is waiting for I/O, another can use the CPU

Single Program


Multi-Programming with Two Programs


Multi-Programming with Three Programs


Time Sharing Systems

—  Allow users to interact directly with the computer

—  i.e. Interactive

—  Multi-programming allows a number of users to interact with the computer

Scheduling

—  Key to multi-programming

—  Long term

—  Medium term

—  Short term

—  I/O

Long Term Scheduling

—  Determines which programs are submitted for processing

—  i.e. controls the degree of multi-programming

—  Once submitted, a job becomes a process for the short term scheduler

—  (or it becomes a swapped out job for the medium term scheduler)

Medium Term Scheduling

—  Part of the swapping function (later…)

—  Usually based on the need to manage multi-programming

—  If no virtual memory, memory management is also an issue

Process States


Process Control Block

—  Identifier

—  State

—  Priority

—  Program counter

—  Memory pointers

—  Context data

—  I/O status

—  Accounting information

Key Elements of O/S


Memory Management

—  Uni-program

—  Memory split into two

—  One for Operating System (monitor)

—  One for currently executing program

—  Multi-program

—  “User” part is sub-divided and shared among active processes

Swapping

—  Problem:  I/O is so slow compared with CPU that even in multi-programming system, CPU can be idle most of the time

—  Solutions:

—  Increase main memory

—  Expensive

—  Leads to larger programs

—  Swapping

What is Swapping?

—  Long term queue of processes stored on disk

—  Processes “swapped” in as space becomes available

—  As a process completes it is moved out of main memory

—  If none of the processes in memory are ready (i.e. all I/O blocked)

—  Swap out a blocked process to intermediate queue

—  Swap in a ready process or a new process

—  But swapping is an I/O process…

Partitioning

—  Splitting memory into sections to allocate to processes (including Operating System)

—  Fixed-sized partitions

—  May not be equal size

—  Process is fitted into smallest hole that will take it (best fit)

—  Some wasted memory

—  Leads to variable sized partitions

Fixed Partitioning

Variable Sized Partitions

—  Allocate exactly the required memory to a process

—  This leads to a hole at the end of memory, too small to use

—  Only one small hole – less waste

—  When all processes are blocked, swap out a process and bring in another

—  New process may be smaller than swapped out process

—  Another hole

—  Eventually have lots of holes (fragmentation)

—  Solutions:

—  Coalesce – Join adjacent holes into one large hole

—  Compaction – From time to time go through memory and move all hole into one free block (c.f. disk de-fragmentation)

Effect of Dynamic Partitioning


Relocation

—  No guarantee that process will load into the same place in memory

—  Instructions contain addresses

—  Locations of data

—  Addresses for instructions (branching)

—  Logical address – relative to beginning of program

—  Physical address – actual location in memory (this time)

—  Automatic conversion using base address

Paging

—  Split memory into equal sized, small chunks -page frames

—  Split programs (processes) into equal sized small chunks – pages

—  Allocate the required number page frames to a process

—  Operating System maintains list of free frames

—  A process does not require contiguous page frames

—  Use page table to keep track

Logical and Physical Addresses – Paging


Virtual Memory

—  Demand paging

—  Do not require all pages of a process in memory

—  Bring in pages as required

—  Page fault

—  Required page is not in memory

—  Operating System must swap in required page

—  May need to swap out a page to make space

—  Select page to throw out based on recent history

Thrashing

—  Too many processes in too little memory

—  Operating System spends all its time swapping

—  Little or no real work is done

—  Disk light is on all the time

—  Solutions

—  Good page replacement algorithms

—  Reduce number of processes running

—  Fit more memory

Bonus

—  We do not need all of a process in memory for it to run

—  We can swap in pages as required

—  So – we can now run processes that are bigger than total memory available!

—  Main memory is called real memory

—  User/programmer sees much bigger memory – virtual memory

Page Table Structure


Segmentation

—  Paging is not (usually) visible to the programmer

—  Segmentation is visible to the programmer

—  Usually different segments allocated to program and data

—  May be a number of program and data segments

Advantages of Segmentation

—  Simplifies handling of growing data structures

—  Allows programs to be altered and recompiled independently, without re-linking and re-loading

—  Lends itself to sharing among processes

—  Lends itself to protection

—  Some systems combine segmentation with paging

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