Chapter 5 -Storage.

STORAGE

The main memory contains 2 different types of memory: RAM & ROM ..

• Random access memory [RAM] 
  RAM memory is like a blackboard on which you can scribble down notes, read them, and then rub them out when you're finished with them. In the computer, RAM is the working memory. It's a memory device in the form of a collection of integrated circuits. A copy of all the programs that are executed have to be first loaded in RAM. RAM chips can be both read from and written to. by the computer, but their contents are lost when the power is switched off. Such memory is called volatile memory. This is why it is so important to save your work because if the computer gets turned off, RAM will lose all of your work..
  
  So, if none of your work is saved, and it's all on the ram, then when you switch your computer off it's all going to fly with the wind.
  For example RAM is used to store:
• The program which is running and the data it is using.
• Data being transferred to and from peripherals.
• The contents of the screen.

• Read only memory [ROM]
  ROM is good at remembering things, but sucks when it comes to changing the information. So, if you have information on ROM, it's going to be always there, but you can never change it. That's why it's called read only memory. ROM can only be read by the computer .

                             ROM                              RAM

1.  Can only be read by the           2. Can be both read from and                                                                                  
              computer                      -        written to by the computer
2. The contents of the chips are  -  2. Contents are lost when the                
         not lost when the power                 power is switched off
    is switched off  [non-volatile]                       [volatile]


The difference between main memory and backing storage:
 Main memory characteristics: 

• Main memory is fast because it is accessed electronically and no mechanical components are used. 
• Main memory is expensive and its storage size on a computer system is usually limited when compared with backing storage. 

Backing Storage characteristics: 

• Access is much slower than that of the main memory. 
• They are non-volatile. 
• Usually used for backup of files. 

Backing Storage:
Backing storage is used to store data while a computer is switched off. When programs or data are used they are copied into main memory (RAM) for faster access. 

A storage medium is like a USB flash drive, floppy disk, magnetic tape ecc. While a storage drive is the piece of equipment which rotates the storage medium and accesses data on it. 

Magnetic Media:

• Magnetic Tape:
  Narrow plastic ribbon coated with an easily magnetisable material on which data can be recorded. The magnetic tape was very popular in the 1950s and 1960s .Information is recorded in binary form.
  
• Magnetic Discs:
  A typical magnetic disk which has 2 sides. Each surface holds data in circular tracks and each track is divided into a number of sectors . 
  When you format a disk, you will be actually dividing the disk into a number of tracks and sectors and hence creating the addressing system.
  

Chapter 3- Logic Circuits

Logic Gates :

Logic circuits are very small electronic decision-making hardware devices that are fed one or more inputs and give back one output. Logic gates are just like binary. They are only fed ones and zeros  and they only give ones and zeros. 

There are 3 types of logic gates:

• The NOT gate
• The OR gate 
• The AND gate

Truth Tables :

A truth table is used to define the output of a logic gate or a logic circuit for all possible combinations of inputs. 
The truth tables are actually used to calculate the output for given inputs at each point along the logic circuit. 

NOT GATE -> The NOT gate accepts only 1 input and gives its inverse as output. If a 1 is input ,then 0 is given as an output, but if 0 is an input, then 1 is the output. So basically, in the truth table of the NOT gate, the inputs and outputs are always opposite. The NOT gate is also known as an inverter.

AND gate -> The AND gate accepts 2 or more inputs, and gives only one output. In the AND gate, the output is only 1 if all the inputs are 1 as well. 

OR gate -> The OR gate accepts 2 or more inputs and gives only one output, which will be 1 when either one or all of the inputs are 1. So, the OR gate always gives a 1 as an output, unless they're all 0. 

The NOT gate can have only one input, but the AND and OR gate can have two, three and even four inputs. 

Chapter 4 -Input and Output devices :)

These are examples of Input Devices: 

Mouse:

Application: Menu Selection

A Mouse is a most widely used pointing device on personal computers. A mechanical Mouse has a rubber ball on its underside to detect movement of the mouse. An optical mouse uses devices that emit and sense light to detect the mouse's movement. A chordless mouse (either mechanical or optical) transmits data using wireless technology such as radio waves or infrarred waves.

 

Trackball:

A trackball is a stationary pointing device with a ball mechanism on its top. A trackball is good for limited desk space because the user does not have to move the entire device.

Touchpad:

Is a small pressure-and motion- sensitive area usually used on a portable computer that you can use to move the mouse pointer. When you move your finger across the pad, the mouse pointer on the screen moves in the same direction. A touchpad is an alternative to a mouse.

 

 

 

Joystick:

Application: Games and simulations

A joystick is a pointing device with a vertical lever mounted on a base. The lever usually includes buttons called triggers, which activate certain events when pressed. Joysticks are mainly used for computer games and ultrasound scanners in hospitals.

An advantage is that a joystick allows fast interactions required in most games.

Bar Code Reader.Application: Point of Sale (POS)

A bar code reader uses laser beams to read bar codes. A bar code reader reads a bar code by using light patterns that pass through the bar code lines of different widths and spaces. A bar code reader represents a set of unique numbers that define a specific product (not the price). These numbers specify the country of origin, the manufacturer, the item's code etc. So the numbers represent different things. 

An advantage of the bar code reader is that the process of data is fast and quite accurate.

While a disadvantage is that the bar code reader may misread a bar code if there is any dirt or even a mark on the code. 

Magnetic Ink Reader.

Application: Bank

A magnetic ink character (MICR) reader can read text printed with magnetic ink. MICR is used most exclusively by the banking industry for processing cheques. The characters represent the check number, the bank number, and the account number of customers. 

An advantage is that - MICR is difficult to forge                               - Documents can still be read when folded or                    written on  

A disadvantage is that- The system can accept only a few different character sets .  

Scanner
Application: Converting hardcopy to machine readable form. 

A scanner is a light-sensing input device that converts printed text and graphics into a digital form that can be further processed by the computer.

There are 2 types of popular scanners, "THE FLATBED SCANNER" & "THE HANDHELD SCANNER". 

The flatbed scanner works like a copy machine. The difference between a copy machine and the flatbed scanner is that the flatbed scanner creates a file of the document rather than a paper-copy. 

 

                                                     

The handheld scanner can be manually passed over the image to be scanned.It is a portable device you can use to copy an image paper to your computer, just like a normal scanner. Hand-held scanners are ideal for capturing small images, such as signatures and logos. A hand-held scanner is smaller, less expensive and also more portable than a flatbed scanner. So most people would rather use a handheld scanner rather than a a flatbed scanner. 

Scanners are mainly used for image processing, which consists of capturing, storing, analysing, displaying, printing, and manipulating images. Many scanners also include optical character recognition (OCR) software, which can convert an image with text included into a text file which one can edit. 

 Advantages of a scanner include: Using the appropriate software, you can edit your photos and insert them.  :)

Disadvantages of a scanner include: A scanner is best for two-dimensional objects only. :(