The keyboard is used to input data into a computer by pressing the keys on it, the data maybe input into a verity of different applications from word processors to program generators. A keyboard has 102 keys on it as standard; these keys allow the inputting of numerical data, textual data and commands (By using the ‘F’ or function keys) into specially designed programs. The character keys are what are used to input textual data, this maybe into a word processor or Internet order form. The arrow keys are used to move selected items around, this can be useful in a word processor so that the user can put the cursor in the correct place before they start typing. The number pad is used for inputting numbers. In the case of a cash till system this would be used to key in card numbers or percentages for discounts. The function keys are used to make the execution of commands simpler. By assigning a certain key a property so that when it is pressed it brings up a menu it is possible to operate the whole system from these keys alone. The ESC or escape key is used to abort an instruction, this means if a user of a system accesses the wrong menu by hitting this key they will be able to leave this menu returning to the previous one. Some keyboards have extras on them like www, back, forward, stop, refresh and volume keys. The manufacturer adds these additional keys to make everyday tasks simpler like surfing the web or adjusting the master volume.
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The display of a computer system is essential for the user to be able to see the output from the system. The monitor gives a visual representation for the user of the task they are performing in real time. If a program requires inputs visual messages can be displayed on the screen that the user can read and respond to. Without a display it is almost impossible for a human to use a computer for any good use. Displays come on many different forms these all have their own benefits and drawbacks. The most common type of display is that of a desktop monitor. A desktop monitor will normally come as standard with all systems, the quality of the monitor is what makes the monitors different. The screen size can be as small as 8 inches up to and exceeding 32 inches, typically systems will have a monitor of around 17 inches. Refresh rate is also another major quality point; the simple rule is that the higher the refresh rate the better the image quality and less chance of health risks occurring like eye strain or migraines. A new variation of the standard CTR monitor is the LCD display, this is just the same as the CRT but it does not have much depth to it. In other words it’s a lot lighter and takes up less space. Projectors are another way to display the output from a computer system, this is especially useful when presenting data to many people at once, such as at a meeting or conference. Projectors are very expensive so are not a standard display device.
A CRT monitor a projector
The mouse is used to move a floating cursor around on the screen of the computer system. This cursor can have various different uses including:
Bringing up submenus
A mouse will have 2 buttons or more on them, these all have different uses. The first mouse button will be used to selecting objects on screen (single click on icon) or executing a command such as start a program (double clicking on the icon).
The second button on the mouse is used for bring up drop down menus that gives the user a list of commands that are associated with the item. Some mice have a third button that can be used in some programs for added ease of use. The scroll wheel is used to move pages displayed on the screen up and down without having to let go of the mouse or click on anything.
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Disks drives are used to store data on that the user requires to access at a later time, there are two types of storage, fixed and removable.
This type of storage is always removable. Floppy disks are used in special drives that only have reading and writing hardware in them, the actual disk that the information is stored on is that of a removable disk. Floppy disks range in storage capacity starting as low as (now redundant) 400kb on old 5 ¼ inch floppies, up to multiple gigabytes that the newer JAZ disks offer. All computer systems currently come with a removable disk drive, the standard is a 3½ inch floppy disk drive of some sort. The 3 ½ inch floppy disk has a storage capacity of 1.44Mb, this is a fairly small amount of storage in today’s standards and is slowly being phased out by the newer ZIP disk format that is now becoming standard in most ‘off the shelf’ machines.
Hard disks are non-removable storage this means that the disk is permanently stored internally inside the machine. The capacity of a hard disk dive is much greater then that of a floppy disk. The capacity of a hard disk can range from as low as 50mb up to around 120 gigabytes. The speed at which a hard disk drive runs at is far greater then that of a floppy disk. A hard disk will be able to write huge amounts of data in a short period of time compared to that of what a slow floppy disk can save in the same time span. The hard disk drive will be where the operating system and all the programs run under the OS will be stored. Hard disk drives can be classed as permanent storage as they do not lose the information they house when the power supply is switched off.
A CD-ROM is a drive that uses compact disks to save massive amounts of data like operating system or program installation files. A CD-ROM is a read only device, but this does not cause any problems as the reason for the drive is mainly for installing new software to the machine. Current CD-ROM’s are running at around 52X, this means that the spindle rotation speed is 52 times faster then that of the first CD-ROM’s on the market. This high data transferee rate allows the execution of programs directly from the CD with out having to install any files to the systems hard disk; also it allows video play back of higher quality to be run from the drive.
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A CDROM works by projecting a very thin beam of light called a laser at the surface of the disk. The disk that the drive is reading is covered in tiny pits, a little like that of a vinyl record, these pits are used to represent the data on the disk. Where there is a pit on the disk the drive will read this as a binary 1 and where there is not a pit, the drive will read this as a 0.
It is now possible to get CD writers, these new drives are just the same as the ROM version but also have the ability to write data to special CD’s that allow writing to them. This is useful when transferring larger files between computers that have no other means of data swapping.
Speakers when connected to a suitable sound card allow the user of the system to hear sounds. The sound maybe warning bleeps emitted by the system to say something is wrong i.e. A message warning the user they are about to over write a file, this message is normally accompanied by a beep to alert the user. Speakers can also be used to hear music CD’s, digital audio files or the sounds that are in a game etc.
A modem is a means of connecting a system to other systems or to the World Wide Web. Once a network connection has been established it is then possible to transferee data across this connection. The speed at which data can be sent is all dependant on what modem hardware is installed and the line it is connected to. Standard modems can transferee data at speeds of 56kb a second, this is quite slow for transferring larger files of 2mb plus. 56K modems use analogue phone lines to transferee data, analogue lines are prone to noise on them so can cause errors when transferring data. Newer modem technology is now available this is called ADSL. An ADSL modem uses a digital line to transferee the data instead of an analogue modem. The advantage of digital modems over analogue is that there is only an ON and an OFF state, this means that it is highly unlikely that the data will become corrupted in transit.
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Tape drives are used primarily for back of important system information. Often an image of the entire hard disk will be saved on this device so that, in the case of total system failure, the image can be transferred back to the disk allowing the system to return to an earlier point in time where it was working correctly. Tape drives can store massive amounts of data (the tape drive in the picture below can store 66GB of data on a single tape), this is why they are suitable for back up of whole hard disk drives. However the speed at which a tape drive stores data to its media is not very fast (6MB/sec), it may take an hour or so to transferee the contents of an entire hard disk to tape.
A pixel is the smallest single dot of coloured light that is individually controllable by the monitor. The word pixel is a combination of the words ‘picture’ and ‘element’. This refers to the individual dots that, when grouped, comprise a video display’s picture. If you look very closly at the monitor screen you can see the individual pixels.
A monitor is graded on its ability for clarity, the way that the clarity is improved is by fitting higher numbers of smaller pixels into a square inch. The way this is measured is by the monitors dot pitch, generally .25mm or .28mm. The dot pitch indicates a higher quality tube (CRT or Cathode Ray Tube) with sharper dot (pixel) resolution.
The attributes that are associated with VDU’s can be said to be things such as the resolutions (height x width), colour depth, type of screen and positioning.
The resolution of the display will generally be stated as either 640×480, 800×600, 1024×768 or 1600×1200. These different resolutions allow the VDU to display a higher quality output due to the amount of dots per inch or DPI. As the resolution is increased so does the available work space (see below)
As you can see from the picture on the left the icons are a lot smaller and the task bar is very long due to the fact that a resolution of 1280×1024 is being used. The screen dump on the right shows what can be seen on the screen when a resolution of 640×480 is used. The red dotted line shows the smaller screens equivalent size.
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Color depth is also a screen attribute, a CRT monitor can display up 32bit color. This means 32,000 colors can be on the screen at anyone time. This makes pictures look a lot clearer and more colorful. (See below: notice the blue aura around the hand of the lady, the 16-colour picture has large blocks of color that are used to fill areas that would normally have a smooth transition of color to give a clearer image)
The type of screen can be classed as Color, green, or black and white. Color monitors have now pretty much-phased out Green screens and black and white displays have gone almost entirely.
ASCII (pronounced Ass-key) is a standard character representation code that is recognized world-wide. ASCII stands for American standard code for information interchange. It represents characters by using seven single bits to make up a byte, i.e. this would define its characters being only 7 bits in length, not 8.
Each character is assigned a number between 0 and 127 this gives 128 different characters. As almost all PC’s and Macintoshes use ASCII data can easily be transferred between them with out and converting between character sets.
All displays are able to display this code on them, an example of this would be when using word processors. The characters are converted from their binary ASCII value into a character on the screen.
IBM adopted EBCDIC from punched card codes in the early 1960s and still uses it on mainframes. It exists in at least six versions and has features such as non-contiguous letter sequences, control code values from 0 to 63, and the absence of several ASCII punctuation marks. However, when transferring data to or from IBM systems you will often encounter data that is in EBCDIC.
This Character code will be displayed when data is being transferred between PC’s, the characters will be printed to the screen as the data arrives at the receiving terminal.
To output an image on a display the computer must have a video card that has memory on it, the memory is where whatever is to be displayed is stored. The larger the resolution of the picture to be displayed, the more memory is required. The memory that is on video cards is a lot faster then that of what is used but the computer itself. The reason for this is that a lot of processing has to be done by the card to keep the display updated i.e. moving the mouse cursor around. Due to this large amount of data movement it is necessary for the memory to operate at very high speeds so that the through put to the display is constant. Generally monitors have around 32mb of super fast memory on them now, this allows large images to be worked with and also allows 3D imagery to be produced and stored on the video card itself.
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A cylinder is the name given to the data area on a disk platter, a disk may have a few platters, these are like circular disks stacked on top of each other. If a hard drive has 4 platters, and each platter has 600 tracks then it can be said that there are 8 tracks in the cylinder. The reason these tracks are grouped into ‘cylinders’ is because each platter will have its own read and write head on each side of the disk surface, hence as the disk rotates it will be able to read and write to 8 places at the same time. These are simultaneously written on, on the top and underside of each of the four disks.
A sector on a disk is a data location that has its own specific reference number, this number is used to recall data stored at this sector. A sector is a singular part of a data track. An example of this maybe if a disk has 80 tracks and each of these data tracks maybe divided up into 20 sectors, a sector is the smallest size chunk of data that a hard disk can access. A track would look like the black circle show above, then this track would be further broken down into sectors by dividing up the disk, a bit like cutting a cake into equal parts (see below).
The problem with the cake slicing style method is that sectors near to the centre of the disk are fairly small, where as the outer tracks on the disk have much larger sectors. These larger outer tracks are wasting disk space as a lot more sectors could easily be fit in this space. It is however possible to use a different method to divide the tracks up into sectors, this is called zoned-bit recording, this method allows the outer tracks to contain more sectors then what is in the centre tracks.
File allocation unit
The file allocation unit is the smallest size ‘block’ of space allocated for saving too. The size of the file allocation unit varies between disks of different sizes. For example a disk that has a capacity of less then 32mb, then the allocated minimum size of the unit will be 512bytes per sector. For larger disk sizes the file allocation unit increases. A 64mb disk partition will have an allotted size of 1K, a 128mb disk will have a 2K unit and a 256mb disk, 4K. The problem of this is that when small files are saved, say less then 2K on the 256mb drive, whatever the remained of the 4K sector that is not used will be wasted. So in theory the same number of <2K files will be able to be saved on the 256mb disk as the 128mb one as the sectors are not being fully used.
When a file is stored the system will allocate certain sectors for the data to be stored at and then record these sectors reference number.
It can be said the smaller the allocation unit is, the less disk space will be wasted. But, it can also be said that the larger the allocated size is the smaller the TOC will be (as there will be less sectors to reference) thus leaving more of the disk space to be used for data storage.
Directory entry contents
The directory contents are used to store the where about of all files saved on the disk. The directory will house the path of where the file should be shown, i.e. what folder it should be located in. All files that are saved on the disk will be logged here so that the system knows where they are located on the disk and also what cluster to find them on.
Paths are routes that take the user to a certain folder or file. A path to a file may look like this: C:windowstempinstallAdobeinstall.exe. This path points at an executable file that is contained inside a series of folders. As the first folder is opened a few more folders will be available to the user, the user will have to visually find the next folder that contains the file and opened that then repeat this process until the required file is found. It is important to know where files are saved in order to access them again. If a file is saved without the user seeing where the file was saved it can often cause problems locating this file again.
File allocation tables
A file allocation table or FAT for short is used to keep a reference of where the data for a file is saved on a hard disk. The FAT table is a chart of numbers that correspond to cluster addresses on the disk. When the user or a program requests a file the FAT is searched to find the address of the clusters that the file is saved in. Once this is done the hard disk will be able to send the requested data to the OS.
So basically the file allocation table is like an address book that holds the location on the disk of all the data stored.
Fragmentation is when a file can not be saved on consecutive clusters and is forced to be saved in different parts of the disk. In other words the file is cut into fragments and saved in many different locations on a disk. Normally the problem with this is that is will slow down the performance of the disk as the read/write heads will have to move from one location to the next to reassemble the file in whole before it can be used.
It is however possible to stop this file fragmentation happening by running special software that comes as standard with the windows operating system. The software finds all the broken up parts of the file and pieces them back together so that the whole file is saved on continuous clusters. 4 – Interface hardware and software
The BIOS or Basic Input Output System main function in a computer is to provide standard program interfaces to perform hardware-oriented functions such as writing to the screen and accessing disk drives. This happens when the computer is first powered on, the BIOS will locate all disk drives and video cards and configure them so that they are ready for any DOS program to call upon them or use them. The BIOS is stored on a non-volatile EPROM, this stops it erasing itself when power to it is lost after shut down. Once the BIOS has found and configured all the hardware, its next task is to execute the operating system that is stored on the system and load it.
The use of adapter cards is beneficial to anyone who wishes to make their PC do tasks that is currently can not do, this maybe connecting to a network or editing video footage. Adapter cards are used to add new features to a computer, this maybe such as adding a TV tuner or a network card to the system. Adapter cards plug directly into the motherboard through a small selection of ports, these are the ISA, PCI and AGP ports. Each has its own benefits and drawbacks. ISA ports are now old technology and are not used for many purposes as the through put of the bus that is runs on is very slow indeed, devices such as analogue modems or network cards may still use this port. PCI is a much faster and newer port; it runs on a 33mhz bus that is fast enough to cope with much more data throughput then the ISA bus. This makes the PCI port good for adapter cards such as sound cards, faster 100 base T network adapters, SCSI adapter cards and even video editing cards. The fastest of all the adapter card ports is the AGP port, this is used to connect very hi-spec video cards that require a lot of throughput (such as textures) to be sent to and from them.
Every piece of hardware connected to a computer from printer to video cards will need a device driver to work correctly. Device drivers are used to allow the hardware and the operating system to communicate together, if there was no device driver the system would not know what or how to use the attached device. The drivers are a set of commands that tell the device they are associated with what the system is asking them to do. In a printer’s case this maybe that it prints certain characters onto the paper or move the paper along to the next line. It is nice to think of the device driver as an interpreter that can speak both the languages of the operating system and that of the device it controls. The device drivers will also register the IRQ that the device needs to work correctly.
A buffer is a data store shared by hardware devices or program processes that operate at different speeds or with different priorities. A buffer is a “midpoint holding place” but exists not so much to accelerate the speed of an activity as to support the co-ordination of separate activities. The buffer allows each process or device to operate without being held up by other activities that are running at the same time. The buffers effectiveness is gauged by its size and speed of moving data into and out of it, if the buffer runs out then the process will have to be halted until the data flow resumes. So the faster the buffer is the less likely there is to be any stoppages. Buffers can be found in CD writers, this is because a constant flow of data needs to be sent to the drive to ensure there is always data to be written to the disk. When or if the buffer runs out, the writer will not have any data to write to the disk, the drive will write a large blank part on the disk, and the disk will be ruined. This is a good example of where a good quality buffer is required in order for a process to work correctly.
An interrupt is a signal from a device attached to the system, this signal is used to stop what the system is currently working on and to work out what to do next. Nearly all computers are now interrupt driven, the advantage of this is that multitasking is possible. Due to the fact that a processor will just follow instructions continually and never stop unless told, it is necessary interrupt this process so that other tasks maybe performed. The switching is so fast you do not even notice it, so because of this it appears that all the programs are running at the same time. An interrupt maybe sent from a sound card when it requires a new sound to be sent to it from storage. In this case an interrupt will momentarily halt the computer and the sound will then be sent to the sound card. Once this has occurred the previous task will then be undertaken again until another interrupt is received. There are also other software programs that aid in this, the first is called and ‘interrupt handler’. This is used to form a queuing system so that all the programs/ devices that have submitted an interrupt will have their turn in order. Also another program that aids this further is called a ‘scheduler’. A scheduler can help a program/ device with an urgent call for processor control skip the queue if it is urgent of the program has been given priority over other running.
Interrupt request (IRQ): hardware
An interrupt request is used like an addressing system for all hardware in the system. Like you would expect to find your friend Paul who lives at number 5 baker street at, number 5 baker street, the computer system uses the same sort of method to find its hardware components. Each piece of hardware is given an interrupt request such as 5, this is just like Paul’s address, its a location that the system can expect to find the piece of hardware it requires.