Criteria for comparing OS

• 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– 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

– 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– 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• 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– File manipulation– Status information– File modification– Programming language support

Home Work

• Read some example OS:– MS DOS– Microsoft Windows 95, 98 and 2000– Microsoft Windows NT– Unix– Linux