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Operating Systems and Linux
Partha Sarathi Dasgupta
MIS Group
Indian Institute of Management Calcutta
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What does an Operating System do?
• An Operating System exploits the hardware resources of one
or more processors to provide a set of services to system users.
• An Operating System also manages secondary memory and
input/output devices on behalf of its users.
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Structural Elements of a Computer
• Processor: Controls the operation of the computer and its
Data Processing functions.. A single processor is often termed as
the Central Processing Unit.
• Main memory: Stores data and programs. This memory is
typically volatile. Contents of disk memory are retained even
when the computer is shut down.
• I/O modules: Moves data between the computer and its external
environment, such as disks, data communication equipments,
and terminals.
• System bus: Provides for communication between processors,
main memory, and I/O modules.
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Structural Elements of a Computer: Schematic view
CPU
0
PC
1
MAR
2
MBR
IR
I/O AR
Execution
Instruction
Instruction
Instruction
I/O BR
Data
Data
Data
Data
Buffers
n -2
n -1
Input-Output Module
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Interrupts
• A mechanism by which other modules (I/O, memory) may
interrupt the normal sequencing of the processor
• Interrupts help to improve processor utilization.
• Classes of interrupts:
 Program: generated by some condition that occurs as a result of an
instruction execution, such as arithmetic overflow, division by zero,
attempt to execute an illegal machine instruction, and reference outside
a user’s allowed memory space.
 Timer: Generated by a timer within the processor. This allows the
operating system to perform certain functions regularly.
 I/O: Generated by an I/O controller, to signal normal completion of an
operation or to signal a variety of error conditions.
 H/W failure: Generated by a failure, such as power, etc.
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An Example
A processor has two programs P1 and P2 to execute:
P1 is simply reading data from memory and writing the data into
the disk. P2 is doing only calculations.
Processor may begin P1 first, issue a write command to the
external device, and then proceed with P2.
Two cases:
• Writing to disk initiated by P1 might finish before P2 completes
calculations.
• P2 completes calculations before the disk writing initiated by
P1 finishes.
What are the consequences of these two cases?
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Memory Hierarchy
Registers
Cache
Main Memory
Magnetic disk, CD-ROM, CD-RW
DVD-RW, DVD-RAM
Magnetic tape
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Cache
• a portion of the main memory that can be used as a buffer
• temporarily holds data that are to br read out to disk
• Caches improve the performance in two ways:
 Disk writes are clustered.
 instead of having many small transfers of data, few large
transfers are possible.
 some data meant for write-out (from disk) may be directly
accessed from the cache very fast.
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Operating System
• It is a program that
 controls the execution of application programs
 acts as an interface between applications and computer hardware
 manages the resources associated with a computer
Objectives of an O.S.
 Convenience of use of the computer
 Efficient use of the computing resources
 Should be such that it can permit effective development, testing and
introduction of new system functions without affecting he service.
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Operating System as an Interface
Application Programs
End user
Utilities
Programmer
Operating System
Computer Hardware
OS System
Designer
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Operating System Services
• Program Development – variety of services and utilities such as
Editors and debuggers. They are usually referred to as application program
development tools.
• Program Execution – involves a number of steps, such as loading
Instruction and data in the main memory, initialisation of I/O devices and
files, and other resources to be allocated. OS handles the scheduling.
•Access to I/O devices – Each I/O device has its own set of
Instructions and control signals. O.S. hides these details to provide a
common interface to the user/programmer.
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Operating System Services
• Controlled access to files- nature of I/O device (disk drive, tape
drive) and structure of the data stored in the files in the storage medium. In
the case of multiple users accessing the processor concurrently, proper
protection mechanisms have to be used.
• System access- For shared or public systems, the O.S. controls access to
the system as a whole and to specific system resources. Access system
provides protection of resources, and data from unauthorised users, and
resolves resource conflicts
• Error detection and response – Variety of errors may occur in
computer system. Responses may range from program termination, retrial of
the same operation, or reporting error to the application.
• Accounting – collecting usage statistics, CPU usages, monitor
performance, etc.
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Evolution of Operating Systems
• Serial Processing
 From 1940s to mid-1950s
 Programmer interacted with the hardware
 Programs in machine code were loaded via the input device (card
reader)
 In case of any error, they were indicated with lights
Associated problems:
 Scheduling: Hardcopy sigh-up sheet for reserving time
 User could reserve for 45 mins and finish in 30 mins => wastage of time
 User may not be able to finish in scheduled time
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Evolution of Operating Systems
• Simple Batch Processing
 for maximizing machine utilisation
 batch O.S. used
 a piece of software called the monitor used
 monitor reads in jobs one at a time from the input device
 as a job is read in, control is passed to the current job; control returns
back to the monitor when the job is finished
 results of a job are sent to an output device such as the printer
 Processor is executing the instructions from those of the monitor
 Processor will then execute the instruction of the user program until it
encounters an ending or error condition
 instructions for the monitor are contained in the JCL
monitor
Interr Device Job
upt drivers seque
proce
ncing
ssing
Control
language
interpreter
User
Program
Area
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Modes of a user program
• A user program executes in user mode, in which certain areas
of memory are protected from the user’s use, and
• certain instructions may not be executed.
• monitor executes in a system mode, or a kernel mode, in
which privileged instructions may be executed and protected
memory segments accessed.
• Batch O.S. – machine time alternates between monitor and the
user programs.
• Effect: some memory given to the monitor, and some of the
machine time is consumed by the monitor.
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An example of System Utilization
Read one record from a file = 19 μs
Execute 100 instructions =
2 μs
Write one record to a file =
19 μs
---------
TOTAL
40 μs
Percent CPU utilization = 2/40 = 0.005 = 0.5%
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An example of Multiprogramming
P1
Run
P2
Wait
P3
P1, P2, P3
Wait
Run
Wait
Run
Wait
Run
Run
Run
Run
Wait
Run
Wait
Wait
Run
Wait
Run
Run
Run
Wait
Wait
Any Advantage?
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Multiprogramming/Multitasking: Some Figures
Multiprogramming Operating systems are more complex
• Memory management
• Scheduling
Uniprogramming
Multiprogramming
Processor use
20%
40%
Memory use
33%
67%
Disk use
33%
67%
Printer use
33%
67%
Elapsed time
30 min
15 min
Throughput
6 jobs/hr
12 jobs/hr
Mean response time
18 min
10 min
Effect of Multiprogramming on Resource Utilization
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Time-Sharing Systems
• For some jobs, e.g., for transaction processing jobs, it is
desirable that users may interact directly with the computer
• Processor time is shared among multiple users through
time sharing
• multiple users simultaneously access the system through
terminals, with the O.S. interleaving the execution of each
user program in time quanta.
• For n users actively requesting service at one time (for
instance, all editing files), each user will only see on the
average 1/n of the effective computer capacity (approx.)
• Given the slow human reaction time, response time is
similar to that on a dedicated machine
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Multiprogramming and Time-Sharing
• Multiprogramming maximises CPU utilization
• Time-sharing minimizes user response rime
What about the Throughput?
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Major theoretical advances in development of O.S.
• Processes
• Memory management
• Information protection and security
• Scheduling and resource management
• System structure
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Processes
A process is
• a program in state of execution
• An instance of a program running on a computer
• The entity that can be assigned to and executed on a processor
• A unit of activity characterised by a single sequential thread of
execution, a current state, and an associated set of system
resources
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Why Processes need to be scheduled?
• Improper synchronization: for example, a process that initiates an I/O read
must wait until the data are available in a buffer. This requires signal from
some other process.
• Mutual exclusion: multiple users might try to access a shared resource
concurrently, e.g., editing the same file at the same time. Mutual exclusion
mechanism required.
• Non-determinate program operation: Results of a program normally
depend on the input to that program only. When programs share memory, and
their execution is interleaved by the processor, they may interfere with each
other. Order of program execution may affect the execution of a particular
program.
• Deadlock: It is possible for two or more programs to be hung up waiting for
each other.
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Parts of a Process
• Components of a process
 An executable program
 Associated data required by the program (variable, work space, buffer,
etc.)
 Execution context: internal data used by O.S. to supervise a process =
process state.
Process list
Main memory
Program code
data
context
Program code
data
context
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Process Control Block
• Identifier: A unique integer associated with a process
• State : A currently executing process is in running state
• Priority : Priority level relative to other processes
• Program counter : Address of the next instruction of the
program to be executed.
• Memory pointers: pointers to the program code and data
associated with the process, and any shared memory blocks
• Context data: Data in the registers in the processor during
process execution.
• I/O status information: outstanding I/O requests, I/O devices
allocated to the process, a list of files is use by the process, etc.
• Accounting information: amount of processor time, etc.
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Key responsibilities of Resource management
• O.S. manages the resources of a computer system
• Factors for scheduling policy:
 Fairness: processes competing for a resource should be given fair
chance of resource access
 Differential responsiveness: O.S. should make allocation and
scheduling decisions to meet total set of requirements, and in a dynamic
manner. E.g., if a process is waiting for an I/O device, O.S. may schedule
that process as soon as the device is free.
 Efficiency: Maximize throughput, minimize response time, and
accommodate as many concurrent users as possible.
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Virtual Memory
• A facility that allows programs to address memory from a logical
viewpoint
• Disregards the amount of physical memory actually available
• Meets the requirements of multiple user processes to reside
simultaneously in main memory without any interference
• (Demand) Paging system and Page faults
• Address of a word of a program = page number + offset within page
• Pages vs Blocks mapping
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Problem of contiguous block assignment
File A
10
File B
20
File C
30
Is the contiguous block allocation OK?
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Why use Linux/Unix?
• System is written in a high-level language => easy to read,
understand, change and port to other machines.
• Simpler user interface or user services
• Provides simple routines to write complex programs/develop utilities
• Hierarchical file system for easy maintenance
• Consistent format for files (byte streams) making application
development easier
• Simple, consistent interface to peripheral devices
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UNIX Architecture
• Kernel
Application Programs
• Shell
UNIX commands and
libraries
• Utilities
System call
interface
bash
H/W
Kernel
who
wc
vi
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Traditional Unix Kernel
User programs
Libraries
User level
Kernel level
System call interface
File subsystem
IPC
Process control
Buffer cache
scheduler
subsystem
Character
Block
Memory
management
Device drivers
Hardware control
Kernel level
Hardware level
Hardware
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Some Basic Characteristics of Linux
• Multi-user System
• Multiprogramming/Multitasking system
• Uses Time Sharing
• Access rights for Files and Processes
• Uses File and Process hierarchies
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Linux File System
Characteristics:
• Hierarchical structure
• Consistent treatment of file data
• Ability to create, modify and delete files
• Dynamic growth of files
• Protection of file data
• Treating peripheral devices (terminals, tapes, etc.) as files
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Linux File System Layout (RedHat)
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Directory Structure of Linux File System
Directory Content
/bin
Common programs, shared by the system, the system administrator and
the users.
/boot
The startup files and the kernel, vmlinuz. In recent distributions also grub
data. Grub is the GRand Unified Boot loader and is an attempt to get rid
of the many different boot-loaders we know today.
/dev
Contains references to all the CPU peripheral hardware, which are
represented as files with special properties.
/etc
Most important system configuration files are in /etc, this directory
contains data similar to those in the Control Panel in Windows
/home
Home directories of the common users.
/initrd
(on some distributions) Information for booting. Do not remove!
/lib
Library files, includes files for all kinds of programs needed by the system
and the users.
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Directory Structure of Linux File System
Directory
Content
/lost+found
Every partition has a lost+found in its upper directory. Files that were saved during
failures are here.
/misc
For miscellaneous purposes.
/mnt
Standard mount point for external file systems, e.g. a CD-ROM or a digital camera.
/net
Standard mount point for entire remote file systems
/opt
Typically contains extra and third party software.
/proc
A virtual file system containing information about system resources. More information
about the meaning of the files in proc is obtained by entering the command man proc in
a terminal window.
/root
The administrative user's home directory. Mind the difference between /, the root
directory and /root, the home directory of the root user.
/sbin
Programs for use by the system and the system administrator.
/tmp
Temporary space for use by the system.
/usr
Programs, libraries, documentation etc. for all user-related programs.
/var
Storage for all variable files and temporary files created by users, such as log files, the
mail queue, the print spooler area, space for temporary storage of files downloaded from
the Internet, or to keep an image of a CD before burning it.
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Mounting of file systems
etc
usr
usr
tom
harry
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Types of files in a Linux System
"On a UNIX system, everything is a file; if something is not a file, it is a process."
Regular files : they contain normal data, for example text files, executable files
or programs, input for or output from a program and so on.
Directories: files that are lists of other files.
Special files: Devices treated as files, used for input and output. They exist in
/dev.
Links: a system to make a file or directory visible in multiple parts of the
system's file tree.
(Domain) sockets: a special file type, similar to TCP/IP sockets, providing
inter-process networking protected by the file system's access control.
Named pipes: act more or less like sockets and form a way for processes to
communicate with each other, without using network socket semantics.
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Permissions of a file
•Read
•Write
•execute
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Listing files in a Linux System
The -l option to ls displays the file type, using the first character of each input line:
psdg/Documents> ls –l
total 8
-rw-rw-r-- 1 psdg faculty 31744 Feb 21 17:56 samplefile1.doc
lrw-rw-r-- 1 psdg faculty 41472 Feb 21 17:56 samplefile2
drwxrwxr-x 2 psdg faculty 4096 Feb 25 11:50 os-course/
crw-rw-r-- 1 psdg faculty 41472 Feb 21 17:56 /dev/tty00
Symbol
Meaning
-
Regular file
d
Directory
l
Link
c
Special file
s
Socket
p
Named pipe
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User Accounts
• login nmes
• passwords
• user id
• group id
• Home directory
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