Criteria for comparing OS
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Character-based or GUI-based
Single or multi tasking
Single or multi threading
Weak or strong security
8,16, 32 or 64 bit processed at a time
Portable or not
Proprietary or not
Types of OS
• Desktop OS
– Least powerful and least secure
– Example, Windows 9x, DOS etc.
• OS for servers
– Computers are connected to each other in a
network through server computers.
– Very powerful and secure.
– Supports multi tasking, multi threading.
– Suitable for mission critical applications like
user account database.
– Example, Windows NT, Unix/Linux etc.
• OS for mainframe
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These were normally custom made and not portable.
Example, IBM mainframe computers used open VMS.
Powerful and secure.
Proprietary systems.
• Handheld OS
– Windows CE, uses touch screen and suitable for
palmtops.
• Embedded OS
– Built inside a particular device with the microprocessor.
– Example, Microwave etc.
– These are dedicated OS and can not be used for general
purpose applications.
– Stored in memory chip like ROM instead of the hard
disk.
File management
• Filenames
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Filename has 2 parts: name + extension.
Filename must be unique.
Extension is used to identify the type of file.
Example, test.doc
• Folder
– Also called directory.
– Contains related documents together.
• Drives
– Also called partition.
– Logical division of the hard disk.
– They are assigned unique letters for identification.
• Paths
– One has to follow a path to locate a file.
– Example, if a file called a.doc is stored in a
folder called abc which is in drive d, then the
path of that file will be d:\abc\a.doc
– If the above file was inside the def folder which
was inside the abc folder than the path will be
d:\abc\def\a.doc
• File managers
– OS uses a separate program that the user can
use to explore drives, files and folders.
– In Windows, it is called the Explorer.
– The concept of explorer came from Web
Browsing where anything can be located by the
click of a button.
Utilities
• In addition to file manager, OS contains
other utility programs. Some common
utility programs are:
– Disk formatting
– Defragmentation. (removal of fragments)
– Disk scanning. (bad sector detection and
locating lost clusters)
Types of processing
• Batch processing
– Collect many transactions or jobs together and
execute them as a batch.
– Example, in a marriage dinner, one batch is
served after another.
• Online processing
– Interaction between computer and user.
– Example, user and ATM booth
• Spooling
– A number of jobs are kept in memory at the
same time and all are ready and waiting to be
processed.
– Example, printer spooler.
• Time sharing OS
– Allows many users to share a computer
simultaneously.
– All users are given small amount of CPU time
and the OS switches the CPU from user to user.
Each user gets the impression that he/she is
using a single independent computer.
• Real-time
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Processing done according to actual time.
Sensors bring data to computer.
Actuators control the system
Example, industrial control system.
In real-time systems, processing must be done
within certain time limits, otherwise the system
will fail.
• Virtual memory
– Portion of the hard disk is used as main
memory.
– Gives the impression that there is more main
memory.
– Needed for programs that need large amount of
memory at a time which can not be satisfied by
RAM.
– Example, high quality games.
Types of OS services
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Program execution environment.
I/O operations.
File system management.
Error detection and reporting.
Resource allocation.
Accounting
How the services are provided
• System call
– Functions.
• System program
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File manipulation
Status information
File modification
Programming language support
Home Work
• Read some example OS:
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MS DOS
Microsoft Windows 95, 98 and 2000
Microsoft Windows NT
Unix
Linux