A computer can be broken down into four logical areas, as shown in this diagram:
A diagrammatic representation of the four parts of a computer.
Computers must have data to work on.
Data must be moved from outside the computer and put into it.
The methods of getting data into a computer have traditionally been discussed by talking about using
manual methods of data input
or automatic methods of data input.
Peripherals (h)
Peripherals: External devices that are connected to the computer
Input Devices: Enable a turn things from the real human physical world into data for the digital computer.
(Keyboard, Mouse, Microphone, Sensor, Wii remote, bar code scanner, Wii Fit board etc)
Output Devices: Take data from the digital world and communicate it with the real human physical world.
(Monitor or VDU, Speakers, Printer etc).
Those that produce hard copy (i.e. a physical printout)
Those that produce soft copy (only exists electronically (e.g. a Monitor / VDU screen)
Storage Devices: Backing Storage is used to save data and retrieve it later.
It is non-volatile so remains saved after power is turned of.
(Hard Drive, DVD, CD Rom, USB memory stick, floppy disk, Magnetic tape etc
2 Types of Output Device
NB. A few devices can be both Input & Output (e.g a touch screen)
Each year pupils move from various primary schools to secondary school.
Parents are sent a form to fill in, requesting such details as name, address, date of birth and contact numbers. This information is then typed in by the secretary.
This is an example of a manual data input system.
The data is collected and then it is entered in via a keyboard by the secretary.
It isn't entered into the computer by 'feeding' the data into the computer automatically using MICR, or OCR, or any other 'automatic' method (see later in the chapter).
The data hasn't been prepared by coding it into a form that the computer can read automatically e.g. a barcode.
Input Devices
Most computers have QWERTY keyboards. These devices can be used to enter in data manually. They are very efficient in the right hands and not so efficient if a user has limited training. In fact, they can be very slow and mistakes are easy to make, although software tools can automatically fix many of them. A lot of work has been done to ensure that keyboards are designed ergonomically. This means that they are designed in a way that takes into account the 'design limitations' of a human being. There is a health and safety problem known as Repetitive Strain Injury (RSI) for people who use keyboards all the time, such as secretaries. The joints in their fingers can become 'worn'.When transferring paper-based data (questionnaire) to computer manually, the design of the user interface (HCI) is crucial.
Form-based interfaces should be used to help the user enter data quickly and efficiently.
The form on the computer and the paper form should be laid out in a very similar manner.
There should be clear sections.
Also, tabbing should be used and validation and verification techniques employed.
Touch-sensitive keyboards & Concept keyboards
Take information from the real world and puts it into the computer as data
(eg Mouse, Keyboard, Scanner,
Sensor, Microphone, Joystick,
Wii controller, Digital Camera,
Bar Code Scanner etc...)
Dirt can get into normal QWERTY keyboards and cause them to malfunction but the protective plastic cover on touch-sensitive keyboards stops this happening. They can be used to customise keyboards because some touch-sensitive keyboards (called 'concept keyboards') allow you to program what you want to happen when an area is pressed.
e.g. To design a concept keyboard for a two year old child, who may have limited co-ordination and cannot yet read. You could provide a keyboard with four big brightly coloured areas. When the child wants to make any selection at all, they just need to touch the right coloured area.
Another manual data input method is the graphics tablet. These are touch-sensitive pads that allow you to 'draw' on them with a stylus. The pressure from the stylus on the pad is sent to the computer, which reproduces what was done on the pad in a drawing program or CAD program. These types of data input devices are far more natural for designers to use than trying to use a keyboard and mouse to draw with.
This is another manual data input method. A touch screen enables a user to touch their VDU screen to make selections. A plastic cover that has fine wires running through it can be placed over a VDU's screen. A user makes a selection by touching the screen with their finger. The exact position can be calculated from the signals sent back by the wires. Touch screens allow very fast selections from choices. They could be used in places where people need to find out information but may have zero computer skills, for example, an information system in a library or a museum. They would be of limited use if you had to type in a letter, for example.
A mouse is a pointing and selecting device used with graphical user interfaces (GUI). There are different kinds of mice around, each with their own advantages and disadvantages although they all broadly do they same thing: point and select. A search on the Internet should enable you to quickly identify different sorts of mice.
These include:
Standard 2/3 button mice.
Mice with a scrolling wheel to allow you to better navigate applications and web pages.
Optical mice that aren't prone to collecting fluff and dirt and so don't need cleaning.
Mice that use radio waves to connect to the computer instead of wires, so that there is less clutter on the desk.
Ergonomically designed mice, are mice that are very small for the small hands of children.
Mice that can work with a serial port, a PS/2 port or a USB port.
Checkout tills such as McDonalds use symbols to make ordering faster and easier.
A concept keyboard is a flat board that contains a grid of buttons. Each button can be programmed to do whatever you want.
An overlay sheet with pictures or symbols is placed on the grid so that the user can tell what pressing on different areas will do.
Concept keyboards are used when fast input is needed and are ideally suited to selecting from a limited range of choices such as fast food restaurants.
Speakers,
Headphones,
Printer (Laser, Bubble Jet, dot matrix)
Robot arm
Automtic garage doors, PIF Light etc ...)
Non volatile storage (i.e. Data is stored after power is switched off)
Divided into 3 main Types:
- Magnetic Storage (e.g Hard disk drive Magnetic Tape)
a series of rapidly rotating disks (platters) stored on top of each other
the data is stored on the surface of the disks in small amounts of magnetism.
Each Platter has a read write head for faster access
The data is stored on a series of concentric rings called Tracks
The tracks are further divided into smaller sections called Blocks
Each block is divided into smaller wedges called Sectors
For More Information:
A detailed look at a hard disk drive HOW STUFF WORKS
- Optical Storage:
Similar to Magnetic storage, but the data is stored as light and read by lasers The CD surface is a mirror covered with billions of tiny bumps that are arranged in a long, tightly wound spiral.
The CD player reads the bumps with a precise laser and interprets the information as bits of data.
The spiral of bumps on a CD starts in the center. CD tracks are so small that they have to be measured in microns (millionths of a meter).
The CD track is approximately 0.5 microns wide, with 1.6 microns separating one track from the next.
The elongated bumps are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers (billionths of a meter) high.
The optical storage device that most of us are familiar with is the compact disc (CD).
A CD can store huge amounts of digital information (783 MB) on a very small surface that is incredibly inexpensive to manufacture. The design that makes this possible is a simple one:
Most of the mass of a CD is an injection-molded piece of clear polycarbonate plastic that is about 1.2 millimeters thick.
During manufacturing, this plastic is impressed with the microscopic bumps that make up the long, spiral track. A thin, reflective aluminum layer is then coated on the top of the disc, covering the bumps.
The tricky part of CD technology is reading all the tiny bumps correctly, in the right order and at the right speed. The CD player has to be exceptionally precise when it focuses the laser on the track of bumps.
When you play a CD, the laser beam passes through the CD's polycarbonate layer, reflects off the aluminium layer and hits an optoelectronic device that detects changes in light.
The bumps reflect light differently than the flat parts of the aluminum layer, which are called lands.
The optoelectronic sensor detects these changes in reflectivity, and the electronics in the CD-player drive interpret the changes as data bits.
- Solid State Storage:
Flash memory is a type of solid-state technology,
(which means that there are no moving parts.)
Inside the chip is a grid of columns and rows, with a two-transistor cell at each intersecting point on the grid. The two transistors are separated by a thin oxide layer.
One of the transistors is known as the floating gate,
and the other one is the control gate.
The floating gate's only link to the row, or wordline, is through the control gate.
As long as this link is in place, the cell has a value of "1."
To change the cell value to a "0" requires a curious process called Fowler-Nordheim tunneling. Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually between 10 and 13 volts, is applied to the floating gate. The charge comes from the column, or bitline, enters the floating gate and drains to a ground.
This charge causes the floating-gate transistor to act like an electron gun. The excited, negatively charged electrons are pushed through and trapped on the other side of the oxide layer, which acquires a negative charge. The electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than fifty percent of the charge, it has a value of "1." If the charge passing through drops below the fifty-percent threshold, the value changes to "0."
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Flash memory uses Fowler-Nordheim tunneling
to alter the placement of electrons.
The electrons in the cells of a Flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply this electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip, which can then be rewritten. Flash memory works much faster than traditional electrically erasable programmable read-only memory (EEPROM) chips because instead of erasing one byte at a time, it erases a block or the entire chip.
Buffers & Interrupts
A Buffer: is a small area of memory that temporarily stores data,
while it is waiting to be processed
or sent somewhere else
Interrupts: are messages sent from other parts of the system to the processor
These messages 'interrupt' the processing in the Fetch-Execute-Cycle
Fetch - Execute Cycle
Click on here for an Animated Model of Register use in the Fetch-Execute Cycle
.
Interrupt Service Routine (ISR) (g)
Current fetch-execute cycle is completed.
The contents of the PC must be stored away safely so it can be restored after servicing the interrupt.
The contents of other registers used by the user program are stored safely for later restoration.
The source of the interrupt is identified.
Interrupts of a lower priority are disabled.
The PC is loaded with the start address of the relevant interrupt service routine.
The interrupt service routine is executed.
The saved values belonging to the user program for registers other than the PC are restored to the processor’s registers.
Interrupts are re-enabled.
The PC is restored to point to the next instruction to be fetched and executed in user program.