Real Operating Systems - warwick.ac.ukwhite Real Operating Systems Introduction DOS Windows Unix &...

Real Operating SystemsLecture #12

Department of Computer Science and TechnologyUniversity of Bedfordshire

Written by David Goodwin,based on the lecture series of Dayou Li

and the book Understanding Operating Systems 4thed.by I.M.Flynn and A.McIver McHoes (2006).

Operating Systems, 2012

bg=whiteReal Operating

Systems

Introduction

DOS

Windows

Unix & Linux

MemoryManagement

DOS

Windows

Linux

ProcessManagement

DOS

Windows

Linux

Outline

IntroductionDOSWindowsUnix & Linux

Memory ManagementDOSWindowsLinux

Process ManagementDOSWindowsLinux


Systems

Introduction

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MemoryManagement

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ProcessManagement

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INTRODUCTION TO DIFFERENTO/Ss

I Three typical operating systemsI Disk operating system (DOS)I WindowsI Unix/Linux


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ProcessManagement

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DOS

I HistoryI In 1980, IBM looked for an operating system for its

soon-to-be-released 6-bit personal computersI Digital Research offered CP/M-86I Softech offered P-SystemI MS

I MS also looked for OS for its 16-bit computersI Seattle Computer Products offered 86-DOSI MS bought it and renamed it MS-DOS

I IBM chose MS-DOS in 1981 and called it PC-DOSI MS-DOS evolved from v 1.0 to v 6.22 from 1981 to

1994


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DOS

I FeaturesI Single user, stand-alone

desktopI Command-lineI Commands are based on

wordsI Examples

I COPY – copy a fileI DEL – delete a fileI PRINT – print files on a

printerI DIR – list files in this

directoryI MD – make a new

directoryI CHKDSK – check the diskI COMP – compare two

files


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Layers of DOS

I Three layers:I Top layer – command processor

I Sends prompts to userI Accepts commandsI Executes commands (including interpret commands to

machine language)I Issues responses


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ProcessManagement

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Layers of DOS

I I Middle layer – DOS kernelI A program containing routines that are needed for

interfacing diskI Stored in MSDOS.SYS fileI Read to memory during initialisation time


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Layers of DOS

I I Bottom layer – BIOS (Basic Input/Output System)I Interfaces I/O devices such as printer, monitor and

keyboardI Controls data flow to and from these devicesI Receive statues information about these devices


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WINDOWS

I HistoryI Initiative

I To allow users to not have to remember and usesystem commands but via a user-friendly interface –GUI

I Not a replacement of DOS

I Early versions (1985 to 1992)I Windows 1.0 to 3.1 are only “interfaces” between GUI

and DOSI Single-user rather than networked

I True O/S since 1992I Windows 95 is first true O/S

I Network O/SI Windows NT version 3.1 in 1993 led by David Cutler


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WINDOWS - Design goals

I Extensibility – be easily enhanced to meet changes overtime to support new hardware and software technologies

I Privileged process and non-privileged processesI Kernel mode refers to the privileged mode of a

processorI All instructions are allowedI System memory is accessible

I Use mode refers to the non-privileged mode of aprocessor

I Only certain instructions are allowedI System memory is not accessible

I O/S executes in kernel modeI Application programs (protected subsystems) run in

user mode

I Modular structure

I Drivers for new file systems, devices and networks

I Objects – abstract data types


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I Portability – ability for O/S to operate on differentmachines that use different processors or configurations

I Code is modularI Standard high-level programming language c/c++ is

used for implementationI Hardware abstraction layer (HAL) providing isolation

from hardware dependenciesI HAL abstracts hardware such as caches with a layer of

low-level software so that higher-level code needs nochange when moving from one platform to another


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I Reliability – predictability in responding to errorconditions, including hardware failures

I Modular designI NTFS to recover all types of errorsI US government-certifiable securityI Virtual memory strategy to prevent one user from

reading or modifying memory that is occupied byanother user

I Compatibility – ability of an O/S to execute programswritten for other O/Ss

I Execution environments for applications that differ fromWin32 API

I Source-level compatibility to POSIX (PortableOperating System Interface for Computer Environment)

I Supporting already-existing file systems


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I Performance – fast response timesI Crucial processes such as system calls and page faults

are tested and optimisedI LPC (local procedure call) to guarantee fast

communications among the protected subsystemsI Carefully designing the environment subsystems to

ensure the speed of frequently used systems servicesI Critical elements of Windows’ networking software are

built in the privileged protion


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UNIX AND LINUX

I Advantages shared by Unix and LinuxI PortableI Powerful utilitiesI Device independent


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UNIX AND LINUX

I Evolution of UNIX


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UNIX AND LINUX

I Evolution of LINUX


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UNIX AND LINUX - Design goals

I Supporting software development

I Keeping its algorithms as simple as possible


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DOS MEMORY MANAGEMENT

I ROM and RAMI ROM contains a section of BIOS for

starting up a computerI RAM is mail memory where programs

are loadedI Interrupt vectorsI BIOS interfaceI DOS kernelI Buffer cacheI Installable drivesI Resident part of COMMAND.COMI TSRI User memoryI Transient part of COMMAND.COMI Reserved for BIOS


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I TPA allocation “policy”I Reason for allocating memory blocks in TPA for more

than one programsI Improving efficiency when executing or accessing the

next program/file after executing one program

I “Policy”I Dynamic allocationI Modification – modifying (normally giving more)

memory blocks to a running program when it requiresmore for, e.g., I/O purposes

I Release of main memory– after part of a program isexecuted

I For .EXE files – allocating the max memory needed ifTPA has enough free memory, otherwise, giving themin memory

I For .COM files – allocating all memory it may needdespite it will use or not


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I TPA allocation algorithmsI First-fit was used in early version and Best-fit was used

in later versionI A block can be as small as 16 bytes (also known as

paragraph) and as large as the max available memory(TPA)

I The first five bytes have special usage:I Byte 0 – indicator of the last block (90h if yes, 77h

otherwise)I Byte 1– indicator of status of the block (00h for busy)I Byte 2 – pointer to PSPI Bytes 3 and 4 – indicator of the number of paragraph

contained in the block

I List of busy/free blocksI List is a data structure containing a head and a tailI An algorithm searches for a free block in the list for a

file


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WINDOWS MEMORY MANAGEMENT

I Memory manage challenge and solutionI Challenge is to run programs written for Windows, DOS

and POSIX without clashing each other in memoryI Solution is to separate system memory and application

memoryI Example

I 4GB memory with 2GB each allocated for applicationstorage and system storage


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I Virtual memory managerI To allow applications to share memoryI Allocate memory in two stages:

I Reserving memoryI Committing memory

I Read/write protection for virtual memory so read/writeperformed by one process won’t be interrupted by otherprocesses

I Lock virtual memory pages in physical memory toensure that a critical page won’t be removed frommemory while a process is using it

I Retrieve informationI Protect virtual pagesI Rewrite virtual pages to disk


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I ImplementationI Address space management

I System storage section of the virtual memory can onlybe accessed by kernel-mode processes

I Addresses of the lower part of this section aretranslated by hardware to have a fast access speed

I PagingI Fetch policy determines time when copying a page

from memory to diskI Placement policy is a set of rules determining where

the vurtual pages are loaded in memoryI Replacement policy determines which virtual page

must be removed from memory to make room for newpages


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LINUX MEMORY MANAGEMENT

I Swapping and demand pagingI Swapping a job out of memory

I Round robin policy is used – jobs/processes aremanaged by round robin and if a job’s time slice is upor when it generate an I/O interrupt, the entire job willbe swapped out to secondary storage to make room foranother job that is waiting in the READY queue

I Demand pagingI ImageI Program codeI DataI stack


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LINUX MEMORY MANAGEMENT

I UNIX kernelI Responding system calls issued by processesI Set up memory boundaryI Permanently resides in memoryI Uses the least recently used (LRU) page replacement

algorithm


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DOS PROCESS MANAGEMENT

I Designed for single-task and single user environmentI Parent child processes – parent process calls child

process and then goes to sleep and remains asleep whilethe child process is running

I One process runs at a timeI The child process can interrupt the parent processI 256 interrupts


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I Reason for interruptsI No need for having any sophisticated process

management as MS-DOS is designed for single user in asingle task environment

I A task/process sometimes does need to be interrupted ,for example, when it waits for a peripheral device, toimprove efficiency

I Process life cycle: ready – running – waiting – exitI Synchronisation among tasks/processes is needI Synchronisation is achieved in MS-DOS via interrupts


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I Different types of interruptsI Internal hardware interrupts

I Generated by certain event during a program’sexecution, for example, divided by zero

I Every such event is assigned with a specific interruptnumber which is electronically wired into the processorand therefore cannot be modified

I External hardware interruptsI Caused by peripheral device controllersI Also assigned with specified numbers and cannot be

modified

I Software interruptsI Generated by system and application programsI Some are used to activate specialised application

programs


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I Interrupt synchronisationI Stack for

I PSW (Program Statuts Word)I code segment registerI instruction pointer register

I Disables the interrupt system until the current interrupthas been solved

I Placing a 8-bit number on the systems bus


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WINDOWS PROCESS MANAGEMENT

I MultithreadingI Elements of a process

I An executable programI Private memory areaI System resources allocated by an O/SI At least one thread of execution

I Elements of a threadI A unique identifierI The contents of a volatile set of register indicating the

processor’s stateI Two stacks used during the thread exectuionI A private storage area used by subsystems and

dynamic-link libraries


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WINDOWS PROCESS MANAGEMENT

I Multithreading synchronisationI Problem

I Several treads can modify the same global variableindependently of each other

I Competition/racing for single shared resource

I Synchronisation in Win32I Mutexes – only one thread can own the resource at a

timeI Semaphores – multiple threads can own it at a timeI Critical section – a critical section can only be owned

by a process and cannot be shared between processesI Event object – it is sent to all threads to alert them of

an action occurring


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LINUX PROCESS MANAGEMENT

I PriorityI Priority is largely determined by accumulated CPU timeI Computer-to-total-time ratio:

CPU time a process has used

Total time CPU time required by the process

I A process that has used a lot CPU time gets the lowestpriority

I Computer-to-total-time ratio is updated every secondI Round-robin is used to decide which process will run

first among those that have the same priority


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I TablesI Process with sharable

codeI Resident Section

of memory has twotables

I Process Tableshows all processes

I Text Table showsthe relationshipbetween theprocesses andcode, i.e. whichprocesses share thesame code and thememory address ofthe code


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I I I Transient Section has Data area, Code area and UserTable for each process

I Data and code are stored separately because code issharable

I User Table is a map between Data area and Code areaso it controls the access privilege of data/files

I Example – P3 and P4I Processes with nonsharable code

I Data and code are stored in the same areaI Example – P5


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I Fork, wait and execI fork

I Creates a copy of a processI The original one is called parentI The copy is called child


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I I waitI Allows programmers to synchronise process execution


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I I execI Is a family of commands – execl, execv, execls, execlp

and execvpI Are used to start a process from another process

Real Operating Systems - warwick.ac.ukwhite Real Operating Systems Introduction DOS Windows Unix &...

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