02-OS Concepts View

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Operating SystemConcepts

CS204 – Operating Systems

By:

Dilum Bandara


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Outlinep OS concepts

n Processes

n Deadlocks

n Memory management

n Input/Output

n Files

n Security

n Shell

p Kernelp User & kernel mode

p System calls


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Processp Process is a program in execution

p Includes:

n executable program

n program data

n

stackn various register status


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Process cont…p In multitasking each process is

temporarily suspended from time to time

n All information about the process should be

saved somewhere while suspending

n Saved in the process table

n When resuming process information is readback from the process table

p A process can create an another process

n Forms a parent child relationship


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A process tree

n A created 2 child processes, B & Cn B created 3 child processes, D, E & F


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Deadlock


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Deadlocks in OSsp Occur due to resource sharing among

processes

p If one process holds a resource & requestfor another resource deadlock could occur

p Example:

CD-ROMTape driveB

Tape driveCD-ROMA

RequestHoldsProcess


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Memory managementp We need to run:

n multiple programs at the same time

n much larger programs

p Protection becomes an issue while running

multiple programsp Large program need large memory address

space

n However main memory is limited

n Need a virtual address spacen More capacity is gained from hard disk

pMake use of swapping


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Input/Outputp Many I/O devices used in modern

computers

p OS should manage them

n Managed by the I/O subsystem

p I/O softwaren Device dependent – device drivers

n Device independent


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Files

p Data is stored in units called files on disks& other media

p Directories are used to organise/groupfiles

p File system deals with files & directories

p Files are located using the path name

p In certain OSs I/O devices are also treated

as files

n lp in Unix - /dev/lp


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Hierarchy of files


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Mounting file systems

p Before mounting,n files on floppy are inaccessible

p After mounting floppy on b,n files on floppy are part of file hierarchy


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Securityp In multi-user systems privacy is important

n Only authorised users should access files

p Example:n -rwxrw-r-- firstyear user Jul 1 05:55 junk

p OS should withstand other attacksn from unwanted intruders

n from viruses

n due to bugs in kernel & utilities


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The shellp It’s another program that executes on top

of the OS

p Act as a command line interpreter

p It reads a command from standard input

& executep Commands can be classified as:

n Internal commands – cd, pwd

n External commands – ls, cp, sort

p Examplesn Bourne, Bash, C, ksh, Command prompt in DOS


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External commandsp Commands which are implement as

individual programs

p Example:

n $ date

p The shell creates a child process & run thedate program

n While the child is running shell waits

n When it’s finishes shell displays the prompt ($)

again & waits for the next commandp External commands may run in foreground

or background


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Internal commandsp Commands which are implemented within

the shell

p Commands which need to keep track ofcurrent state(s) are implemented within

the shelln cd change the current working directory

n pwd reads the current working directory

n Relative path names depend on current

working directory


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More on shellp Login shell

n The shell that automatically executes when

you login

p Changing the shell

n Changing to a new default shell

pNeed to re-login

n Creating additional shell sessions running ontop of, or concurrently with, the default shell


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Kernelp Kernel is the core of an OS

n The real operating system

p It is the program which:

n provides services to applications

n control resources

Physical

devices

Machine language

Operating System

Compilers Editors management

File

Util

ities

Web browser Banking Systems

Log in


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User & Kernel modep OS is the portion of software that runs in

the kernel mode

n Also called the supervisor mode

n It’s protected from user tampering

p Programs that runs on top of the OS runin the user mode

p Programs running in the user mode makeuse of the services offered by the kernel


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System callsp The interface between OS & user programs

is defined by set of system calls

p These system calls exposes the servicesoffered by the OS to user programs

p These are set of functions/methodsn Varies form one OS to another

n Mostly written in assembly language & machine

dependent

n Libraries are provided to make them accessiblefrom languages like C/C++


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Use of System callsp Opening a file

n n = read(fd, buffer, nbytes)

n n – number of bytes read into buffer

n fd – file descriptor

n buffer – pointer to buffer

n nbytes – buffer size


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Use of System calls cont…

………….

number=read(fd,buffer,nbytes);

………….

System call handlerKernelSpace

UserSpace

User program

Trap to Kernel

Put the code to read in register

Return to caller

Library procedure


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Implementations of system callsp POSIX

n Used in UNIX & Linux

n About 100 system calls

p Win32 API

n Windows Application Program Interface

n Supported since Windows 95

n More than 1000 system calls

n With each OS release new system calls areintroduced

n Most of these calls are carried out in user mode

n Much more complex compared to POSIX


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POSIX & Win32 API

Get the current timeRemove an empty directory

Create a new directory

Write data to file

Read data from file

Create or open an existing

file

Replace the image of aprocess

Create a new process

Description

GetLocalTimetimeRemoveDirectoryrmdir

CreateDirectorymkdir

WriteFilewrite

ReadFileread

CreateFileopen

-execve

CreateProcessfork

Win32 APIPOSIX

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