(i) Software must be available free or at a low cost
(ii) Source code must be included
(iii) Anyone must be allowed to modify the source code.
(iv) Modified versions can be redistributed.

Open source based "Software refers to those categories of software/programs whose licenses do not impose much condition.

The different types of operating system used in the computer
(i) Single User and Single Task Operating System
(ii) Multi User Operating System
(iii) Multi Processing Operating System
(iv) Distributed Operating System
(v) Prominent Operating System

This is a one of the features of Operating System. It has two or more processors for a single running process (job).

The Operating System provides three levels of securities to the user end. They are
(i) File access level
(ii) System level
(iii) Network level.

(i) To hide details of hardware by creating abstraction.
(ii) To allocate resources to processes.
(iii) Provide a interactive and effective interface.

The functions of OS
(i) Process Management
(ii) Device Management
(ill) Memory Management
(iv) File Management
(v) Security Management
(vi) Storage Management

(i) Easy interaction between the human and computer.
(ii) Starting computer operation automatically when power in turned on (Booting).
(iii) Controlling input & output devices.
(iv) Managing the utilization of main memory.
(v) Providing security to user program.

(i) Distributed computing is a field of computer science that studies distributed systems.
(ii) A distributed system is a model in which components located on networked computers communicate and coordinate their actions by passing messages.
(iii) The components interact with each other in order to achieve a common goal.

(i) Multiprocessing is the use of two or more CPUs within a single computer system.
(ii) A system is said to be multiprocessing if it has more than one physical processors. Since it has multiple processors available, multiple processes can be executed at a time.
(iii) In Multiprocessing, processes can be assigned to a different processor for its execution.
(iv) With the help of multiprocessing system more work can be done in a shorter period of time.
(v) Multiprocessing system provides increased reliability means that if one processor fails, the other processor will continue its working. For example, if there are10 processors and 1 fails then the work does not halt, instead the remaining 9 processor will share the work of 10 th processor.
(vi) Multiprocessing system increases the Throughput i.e. if we increase the number of processors, more work can be completed.
(vii) As multiprocessing occurs by parallel processing, it is cost saving since it parallelly shares memory, buses, peripherals, etc.
(viii) Multiprocessor refers to hardware (CPU units) rather than software (running programs).

(a) System Software
(b) Provide a platform to run application programs and activate the basic level hardware.
(c) Act as an interface between user, software and hardware.
(d) 1. Real Time OS
     2. BOSS
     3. Interactive OS
(e) 1. Process Management
     2. Storage Management
     3. Device Management.

Memory Management:
(i) Memory Management is the process of controlling and coordinating computer's main memory and assigning memory block (space) to various running programs to optimize overall computer performance.
(ii) The Memory management involves the allocation of specific memory blocks to individual programs based on user demands. At the application level, memory management ensures the availability of adequate memory for each running program at all times.
(iii) The objective of Memory Management process is to improve both the utilization of the CPU and the speed of the computer's response to its users via main memory. For these reasons, the computers must keep several programs in main memory that associates with many different Memory Management schemes.
(iv) The Operating System is responsible for the following activities in connection with memory management.
(v) Keeping track of which portion of memory are currently being used and who is using them.
(vi) Determining which processes (or parts of processes) and data to move in and out of memory.
(vii) Allocation and de-allocation of memory blocks as needed by the program in main memory. (Garbage Collection).

(i) The user interface determines how command are given to the computer and how data is displayed on the screen.
(ii) User interface allows user to interact with computer effectively.
(iii) User interface is the first impression of a software where the user interacts with computer.
(iv) The user interface is mostly of two types:
1. Command line interface
2. Graphical user interface
(v) Command line interface: It indicates user must know machine language and program language.
(vi) Graphical user interface: In this user gives command by clicking or selecting the icons displayed on the screen.
The following points should be noted while creating a user interface for an Operating system:
(i) The user interface should satisfy the customers based on their needs.
(ii) The user interface should reduce number of errors committed by the user.
(iii) The user interface should save user time.
(iv) The user interface should make simple, common task easy.
(v) The user interface should be flexible and tolerant.