Recognition Devices

Character and mark recognition devices are scanners that are able to recognize special characters and marks. Essentially used for certain applications.

Types

MICR (Magnetic Ink Character Recognition):

• Used by banks to read numbers written on cheque.
• Special purpose machine reads character made of ink containing magnetized particles.

OCR (Optical Character recognition):

• Special preprinted characters that can be read by light source and changed into machine readable form.
• Used in department stores to read retail price tags by reflecting light.

OMR (Optical Mark Recognition):

• An OMR device senses the presence or absence of a mark such as pencil mark.
• Used to calculate or store multiple choice tests.
  
  
  

Bar Code Reader

• An electronic device used for reading printed bar codes.
• Consists of light source, lens, and light sensor for translating optical impulses into electrical ones.
• Also consists of decoder circuitry to analyze the barcode image data provided by sensor.
• This bar code identifies the product to the supermarket computer which has latest description and price.
• Computer automatically tells the electronic cash register the price.
• These are easy to use and widely used in electronic shops and malls.

Output Devices

An output device is any device used to send data from a computer to another device or user. Most computer data output that is meant for humans is in the form of audio or video. Thus, most output devices used by humans are in these categories. An output device is any peripheral that receives data from a computer, usually for display, projection, or physical reproduction.Examples include monitors, projectors, speakers, headphones and printers.

Output of processed data can be divided into two classes:

Hard Copy

Output that is in the form of print document, which can be read directly, long last and permanently stored. E.g. A word document containing text and images printed by the printer on a paper.

Soft Copy

Output that is in the form of metallic or audio form, which is cannot be read directly by the user. E.g. A collection of music files and folders on a CD.

Accordingly the output devices are classified as hard copy devices and soft copy devices.

• Hard Copy Devices : printers, fax machine, photographic output devices etc.
• Soft Copy Devices : monitors, data projectors, speakers etc.

Types

• Monitors
• Printers
• Data projectors
• Speakers
• EBook reader
• HDTV

Mouse, Joystick, Light pen

Mouse

Mouse is most popular pointing device. It is a very famous cursor-control device having a small palm size box with a round ball at its base which senses the movement of mouse and sends corresponding signals to CPU when the mouse buttons are pressed.
Generally it has two buttons called left and right button and a wheel is present between the buttons. Mouse can be used to control the position of cursor on screen, but it cannot be used to enter text into the computer.

Advantages

• Easy to use
• Not very expensive
• Moves the cursor faster than the arrow keys of keyboard.

Joystick

Joystick is also a pointing device which is used to move cursor position on a monitor screen. It is a stick having a spherical ball at its both lower and upper ends. The lower spherical ball moves in a socket. The joystick can be moved in all four directions.
The function of joystick is similar to that of a mouse. It is mainly used in Computer Aided Designing(CAD) and playing computer games.

Light Pen

Light pen is a pointing device which is similar to a pen. It is used to select a displayed menu item or draw pictures on the monitor screen. It consists of a photocell and an optical system placed in a small tube. When the tip of a light pen is moved over the monitor screen and pen button is pressed, its photocell sensing element detects the screen location and sends the corresponding signal to the CPU.

Input Devices

Those means or devices by which we feed data (any type) to computer. These devices have capability to convert that data in digital form i.e. in 0 & 1. Also known as peripherals and computer hardware.
Following are few of the important input devices which are used in a computer:

• Keyboard
• Mouse
• Joy Stick
• Light pen
• Track Ball
• Scanner
• Graphic Tablet
• Microphone
• Magnetic Ink Card Reader(MICR)
• Optical Character Reader(OCR)
• Bar Code Reader
• Optical Mark Reader(OMR)

Keyboard

Keyboard is the most common and very popular input device which helps in inputting data to the computer. The layout of the keyboard is like that of traditional typewriter, although there are some additional keys provided for performing additional functions.
Keyboards are of two sizes 84 keys or 101/102 keys, but now keyboards with 104 keys or 108 keys are also available for Windows and Internet.
The keys on the keyboard are as follows:

Sr.No	Keys	Description
1	Typing Keys	These keys include the letter keys (A-Z) and digit keys (0-9) which generally give same layout as that of typewriters.
2	Numeric Keypad	It is used to enter numeric data or cursor movement. Generally, it consists of a set of 17 keys that are laid out in the same configuration used by most adding machines and calculators.
3	Function Keys	The twelve function keys are present on the keyboard which are arranged in a row at the top of the keyboard. Each function key has unique meaning and is used for some specific purpose.
4	Control keys	These keys provide cursor and screen control. It includes four directional arrow keys. Control keys also include Home, End, Insert, Delete, Page Up, Page Down, Control(Ctrl), Alternate(Alt), Escape(Esc).
5	Special Purpose Keys	Keyboard also contains some special purpose keys such as Enter, Shift, Caps Lock, Num Lock, Space bar, Tab, and Print Screen.

SRAM vs DRAM

SRAM vs DRAM

There are two types of Random Access Memory or RAM, each has its own advantages and disadvantages compared to the other. SRAM (Static RAM) and DRAM (Dynamic RAM) holds data but in a different ways. DRAM requires the data to be refreshed periodically in order to retain the data. SRAM does not need to be refreshed as the transistors inside would continue to hold the data as long as the power supply is not cut off. This behavior leads to a few advantages, not the least of which is the much faster speed that data can be written and read.

1. SRAM is static while DRAM is dynamic
2. SRAM is faster compared to DRAM
3. SRAM consumes less power than DRAM
4. SRAM uses more transistors per bit of memory compared to DRAM
5. SRAM is more expensive than DRAM
6. Cheaper DRAM is used in main memory while SRAM is commonly used in cache memory

Keys in DBMS

KEY: - A key is a single attribute or combination of two or more attributes of an entity set that is used to identify one or more instances of the set. Eg. The attribute employee soc_sec_no. uniquely identifies while the employee skill would be unique i.e. Identify more than one instance.

PRIMARY KEY: - If there is a case two instances of an entity set could have the same value for all its attributes. Hence to distinguish such instances, we introduce the unique attribute and will identify an instance of the entity set. Such unique entity identifier is referred to as a PRIMARY KEY.

SUPER KEY: - If we add additional attributes to a primary key, the resulting combination would still uniquely identify an instance of the entity set. Such augmented keys are called SUPER KEYS. So here we called Primary Key is minimum SUPER KEY.

CANDIDATE KEY: - Candidate keys are defined as the set of fields from which primary key can be selected. It is an attribute or set of attribute that can act as a primary key for a table to uniquely identify each  record in that table.

COMPOSITE KEY:- Key that consists of two or more attributes that uniquely identify an entity is called Composite key. But any attribute that makes up the Composite key is not a simple key in its own.

ALTERNATE KEY: - In the case of candidate key we must decide which of the candidate key e must decide which of the candidate key will used as the primary key. The remaining candidate keys would be considered as ALTERNATE KEY.

SECONDARY KEY: - It is an attribute or combinations of attributes that classifies the entity set on a particular characteristic. It may not be a candidate key. Eg. The entity set EMPLOYEE having the attribute department, which identifies by its value all instances of EMPLOYEE who belongs to a given department. More than one employee may belong to a department, so the department attribute is not a candidate key for the entity set EMPLOYEE.

FOREIGN KEY :- A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table. The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table. 
Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables.

Database Terms

ENTITIES: - Entities are the basic units used in modelling stones of concrete or abstract objects. All such items about which relevant information is stored in the database are called ENTITIES. ENTITY is the identical objects which are transformed from manual to computer system. E.g.  Building, room, chair, transaction, course, machine, employee.

ATTRIBUTES: - The qualities of the entity which we store as information are called the attributes. OR The properties that characterize an entity set are called as attributes. It is also referred as data item, data element, data field, items, elementary item or object property.

DOMAIN: - Each attribute of an entity set has a particular value. The set of possible values that a given attribute can have is called its DOMAIN. Eg. The set of value that the attribute EMPLOYEE. Salary may have take is a positive number ranging between 0.00 and 9,999,999.00.

ENTITY SET               ATTRIBUTES                                 VALUE

EMPLOYEE                          EMPLOYEE.soc_sec_no                               1254

EMPOYEE.Name                                          Amitesh

                                                EMPLOYEE. Address                                   Patna

                                                EMPLOYEE.Desgnation                               System admin

                                                EMPLOYEE. Skill                                         DBA

                                                EMPLOYEE. Salary                                      45056.00

Process Scheduling

Process Scheduling

The act of determining which process in the ready state should be moved to the running state is known as Process Scheduling.
The prime aim of the process scheduling system is to keep the CPU busy all the time and to deliver minimum response time for all programs. For achieving this, the scheduler must apply appropriate rules for swapping processes IN and OUT of CPU.

Process Scheduler

The process scheduler is the component of the operating system that is responsible for deciding whether the currently running process should continue running and, if not, which process should run next. There are four events that may occur where the scheduler needs to step in and make this decision:

1. The current process goes from the running to the waiting state because it issues an I/O request or some operating system request that cannot be satisfied immediately.
2. The current process terminates.
3. A timer interrupt causes the scheduler to run and decide that a process has run for its allotted interval of time and it is time to move it from the running to the ready state.
4. An I/O operation is complete for a process that requested it and the process now moves from the waiting to the ready state. The scheduler may then decide to preempt the currently-running process and move this newly-ready process into the running state.

Schedulers fell into one of the two general categories :

• Non pre-emptive scheduling. A scheduler is a preemptive scheduler if it has the ability to get invoked by an interrupt and move a process out of a running state to let another process run.When the currently executing process gives up the CPU voluntarily.
• Pre-emptive scheduling. If a scheduler cannot take the CPU away from a process then it is a cooperative, or non-preemptive scheduler. When the operating system decides to favour another process, pre-empting the currently executing process.