O LEVEL- IT TOOLS & BUSINESS SYSTEM
TRICKS TO CRACK FIRST PAPER:
M1-R4: IT TOOLS & BUSINESS SYSTEMS
SOME IMPORTANT TOPICS WHICH FREQUENTLY ASKED EVERY YEAR:--
OPERATING SYSTEM---
An Operating System (OS) is an
interface between a computer user and computer hardware. An operating system is
a software which performs all the basic tasks like file management, memory
management, process management, handling input and output, and controlling
peripheral devices such as disk drives and printers.
Some popular Operating Systems
include Linux, Windows, OS X, VMS, OS/400, AIX, etc.
Definition
An operating system is a program
that acts as an interface between the user and the computer hardware and
controls the execution of all kinds of programs.
Following
are some of important functions of an operating System.
- Memory Management
- Processor Management
- Device Management
- File Management
- Security
- Control over system performance
- Job accounting
- Error detecting aids
- Coordination between other software and users
Memory Management
Memory management refers to
management of Primary Memory or Main Memory. Main memory is a large array of
words or bytes where each word or byte has its own address.
Main memory provides a fast storage
that can be accessed directly by the CPU. For a program to be executed, it must
in the main memory. An Operating System does the following activities for
memory management −
· Keeps
tracks of primary memory, i.e., what part of it are in use by whom, what part
are not in use.
· In
multiprogramming, the OS decides which process will get memory when and how
much.
· Allocates
the memory when a process requests it to do so.
· De-allocates
the memory when a process no longer needs it or has been terminated.
Processor Management
In multiprogramming environment, the
OS decides which process gets the processor when and for how much time. This
function is called process
scheduling. An Operating System does the following activities for
processor management −
· Keeps
tracks of processor and status of process. The program responsible for this
task is known as traffic
controller.
· Allocates
the processor (CPU) to a process.
· De-allocates
processor when a process is no longer required.
Device Management
An Operating System manages device
communication via their respective drivers. It does the following activities
for device management −
· Keeps
tracks of all devices. Program responsible for this task is known as the I/O controller.
· Decides
which process gets the device when and for how much time.
· Allocates
the device in the efficient way.
· De-allocates
devices.
File Management
A file system is normally organized
into directories for easy navigation and usage. These directories may contain
files and other directions.
An Operating System does the
following activities for file management −
· Keeps
track of information, location, uses, status etc. The collective facilities are
often known as file system.
· Decides
who gets the resources.
· Allocates
the resources.
· De-allocates
the resources.
Other Important Activities
Following are some of the important
activities that an Operating System performs −
· Security − By means of password
and similar other techniques, it prevents unauthorized access to programs and
data.
· Control over system performance −
Recording delays between request for a service and response from the system.
· Job accounting − Keeping track of
time and resources used by various jobs and users.
· Error detecting aids − Production
of dumps, traces, error messages, and other debugging and error detecting aids.
· Coordination between other softwares and
users − Coordination and assignment of compilers, interpreters,
assemblers and other software to the various users of the computer systems.
LINUX ----
Linux is one of popular version of
UNIX operating System. It is open source as its source code is freely
available. It is free to use. Linux was designed considering UNIX
compatibility. Its functionality list is quite similar to that of UNIX.
Components of Linux System
Linux Operating System has primarily
three components
· Kernel − Kernel is the core part of Linux. It is
responsible for all major activities of this operating system. It consists of
various modules and it interacts directly with the underlying hardware. Kernel
provides the required abstraction to hide low level hardware details to system
or application programs.
· System
Library − System libraries are special functions or programs using
which application programs or system utilities accesses Kernel's features.
These libraries implement most of the functionalities of the operating system
and do not requires kernel module's code access rights.
· System
Utility − System Utility
programs are responsible to do specialized, individual level tasks.
Kernel Mode vs User Mode
Kernel component code executes in a
special privileged mode called kernel
mode with full access to all resources of the computer. This code
represents a single process, executes in single address space and do not
require any context switch and hence is very efficient and fast. Kernel runs
each processes and provides system services to processes, provides protected
access to hardware to processes.
Support code which is not required
to run in kernel mode is in System Library. User programs and other system
programs works in User Mode which
has no access to system hardware and kernel code. User programs/ utilities use
System libraries to access Kernel functions to get system's low level tasks.
Basic Features
Following are some of the important
features of Linux Operating System.
· Portable −
Portability means software can works on different types of hardware in same
way. Linux kernel and application programs supports their installation on any
kind of hardware platform.
· Open
Source − Linux source code is freely available and it is community
based development project. Multiple teams work in collaboration to enhance the
capability of Linux operating system and it is continuously evolving.
· Multi-User −
Linux is a multiuser system means multiple users can access system resources
like memory/ ram/ application programs at same time.
· Multiprogramming − Linux is a multiprogramming
system means multiple applications can run at same time.
· Hierarchical
File System − Linux provides a standard file structure in which system
files/ user files are arranged.
· Shell −
Linux provides a special interpreter program which can be used to execute
commands of the operating system. It can be used to do various types of
operations, call application programs. etc.
· Security −
Linux provides user security using authentication features like password
protection/ controlled access to specific files/ encryption of data.
Architecture
The following illustration shows the
architecture of a Linux system −
The
architecture of a Linux System consists of the following layers −
· Hardware layer − Hardware consists of all peripheral devices (RAM/
HDD/ CPU etc).
· Kernel −
It is the core component of Operating System, interacts directly with hardware,
provides low level services to upper layer components.
· Shell − An interface to kernel, hiding
complexity of kernel's functions from users. The shell takes commands from the
user and executes kernel's functions.
· Utilities −
Utility programs that provide the user most of the functionalities of an
operating systems.
FOLLOWING 6 WAY OF
LINUX BOOTING PROCESS--
1. BIOS
§ BIOS
stands for Basic Input/Output System
§ Performs
some system integrity checks
§ Searches,
loads, and executes the boot loader program.
§ It
looks for boot loader in floppy, cd-rom, or hard drive. You can press a key
(typically F12 of F2, but it depends on your system) during the BIOS startup to
change the boot sequence.
§ Once
the boot loader program is detected and loaded into the memory, BIOS gives the
control to it.
§ So,
in simple terms BIOS loads and executes the MBR boot loader.
2. MBR
§ MBR
stands for Master Boot Record.
§ It
is located in the 1st sector of the bootable disk. Typically /dev/hda, or
/dev/sda
§ MBR
is less than 512 bytes in size. This has three components 1) primary boot
loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr
validation check in last 2 bytes.
§ It
contains information about GRUB (or LILO in old systems).
§ So,
in simple terms MBR loads and executes the GRUB boot loader.
3. GRUB
§ GRUB
stands for Grand Unified Bootloader.
§ If
you have multiple kernel images installed on your system, you can choose which
one to be executed.
§ GRUB
displays a splash screen, waits for few seconds, if you don’t enter anything,
it loads the default kernel image as specified in the grub configuration file.
§ GRUB
has the knowledge of the filesystem (the older Linux loader LILO didn’t
understand filesystem).
§ Grub
configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this).
The following is sample grub.conf of CentOS.
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-194.el5PAE)
root (hd0,0)
kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/
initrd /boot/initrd-2.6.18-194.el5PAE.img
§ As
you notice from the above info, it contains kernel and initrd image.
§ So,
in simple terms GRUB just loads and executes Kernel and initrd images.
4. Kernel
§ Mounts
the root file system as specified in the “root=” in grub.conf
§ Kernel
executes the /sbin/init program
§ Since
init was the 1st program to be executed by Linux Kernel, it has the process id
(PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.
§ initrd
stands for Initial RAM Disk.
§ initrd
is used by kernel as temporary root file system until kernel is booted and the
real root file system is mounted. It also contains necessary drivers compiled
inside, which helps it to access the hard drive partitions, and other hardware.
5. Init
§ Looks
at the /etc/inittab file to decide the Linux run level.
§ Following
are the available run levels
§ 0
– halt
§ 1
– Single user mode
§ 2
– Multiuser, without NFS
§ 3
– Full multiuser mode
§ 4
– unused
§ 5
– X11
§ 6
– reboot
§ Init
identifies the default initlevel from /etc/inittab and uses that to load all
appropriate program.
§ Execute
‘grep initdefault /etc/inittab’ on your system to identify the default run
level
§ If
you want to get into trouble, you can set the default run level to 0 or 6.
Since you know what 0 and 6 means, probably you might not do that.
§ Typically
you would set the default run level to either 3 or 5.
6. Runlevel programs
§ When
the Linux system is booting up, you might see various services getting started.
For example, it might say “starting sendmail …. OK”. Those are the runlevel
programs, executed from the run level directory as defined by your run level.
§ Depending
on your default init level setting, the system will execute the programs from
one of the following directories.
§ Run
level 0 – /etc/rc.d/rc0.d/
§ Run
level 1 – /etc/rc.d/rc1.d/
§ Run
level 2 – /etc/rc.d/rc2.d/
§ Run
level 3 – /etc/rc.d/rc3.d/
§ Run
level 4 – /etc/rc.d/rc4.d/
§ Run
level 5 – /etc/rc.d/rc5.d/
§ Run
level 6 – /etc/rc.d/rc6.d/
§ Please
note that there are also symbolic links available for these directory under
/etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.
§ Under
the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and
K.
§ Programs
starts with S are used during startup. S for startup.
§ Programs
starts with K are used during shutdown. K for kill.
§ There
are numbers right next to S and K in the program names. Those are the sequence
number in which the programs should be started or killed.
§ For
example, S12syslog is to start the syslog deamon, which has the sequence number
of 12. S80sendmail is to start the sendmail daemon, which has the sequence
number of 80. So, syslog program will be started before sendmail.
There you have it. That is what happens during the Linux
boot process.
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