Outcome 1 - Contents. 1 Data Representation 2 Computer Structure 3 Computer Performance 4 Peripherals 5 Networking 6 Using Networks 7 Computer Software 8 Supporting Software. 1 Data Representation 1.1 Introduction. Lowest level in computer only binary numbers can be used.
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Computers work in number base 2 which uses 2 symbols, 0 and 1 to represent a value.
In computing systems, large numbers are expressed in terms of powers of 2 and use the following abbreviations:
21 has a decimal equivalent of 2
22 has a decimal equivalent of 4
23 has a decimal equivalent of 8
24 has a decimal equivalent of 16
25 has a decimal equivalent of 32
26 has a decimal equivalent of 64
27 has a decimal equivalent of 128
28 has a decimal equivalent of 256
29 has a decimal equivalent of 512
210 has a decimal equivalent of 1024 and is abbreviated to 1 kilo
220 has a decimal equivalent of 1,048,576 and is abbreviated to 1 Mega
230 has a decimal equivalent of 1,073,741,824 and is abbreviated to 1 Giga
240 has a decimal equivalent of 1,099,511,627,776 and is abbreviated to 1 Tera
Long binary numbers can be difficult to read correctly.
Computers have memory addresses of 2 or 4 bytes long which give addresses of 16 or 32 bits.
Hexadecimal is base 16 and organises the bits into groups of four.
The conversion between base 2 and base 16 is very simple. Hex needs the digits 0-9 and letters A-F.
E.g. 11010100010110010011001010010110 becomes
1101 0100 0101 1001 0011 0010 1001 0110 which in Hex is D459 3256
Most displays use Raster graphics – same as TV.
Displays store images as a matrix of pixels in the refresh buffer.
Separate images now stored in VRAM (Video RAM).
VRAM represents the entire screen area and the term bit map is used to describe the one-to-one mapping of pixels in VRAM to pixels on the screen.
No of bytes is
There are many different ways of arranging the bytes that hold image information, but one way is to map them so that the first byte represents the top left pixels, the second byte represents the pixels to the right of the first pixel, until the end of the first row is reached, when the next byte holds the information for the left hand end of the second row. For a small (24 pixel by 4 rows ) display the layout would look like this:Data Representation1.4.3 Arranging The Bytes
The image is saved as a series of bytes to a storage device, such as memory or disk.If we wish to review the image then it is a simple matter to transfer the image data back into the video memory as a direct copy. As this image is also in a bit mapped format, we can still move it to and from other storage devices without any translation.
A rudimentary greyscale effect provides a ’black’, ’white’ and two levels of ’grey’. As this comprises four different values we need two bits to represent each pixel (00 for black, 01 for darker grey, 10 for lighter grey and 11 for white ).
As each pixel now requires twice as many bits, we will require twice as much memory for a given screen size as a black and white image.
We can provide more levels of grey by allocating more bits to each pixel. By the time we have eight bits (one byte) to one pixel we can represent 256 different intensities.
Monochrome displays are often clearer, especially for text than colour display. The requirement to use colour for such items as colour pictures and user interface issues, dictates that colour displays are more likely to be purchased.
One colour can be represented by one byte giving 256 colours (GIF format).
Monitors etc. have 3 primary (additive) colours, Red, Blue and Green. Other colours obtained from adding light.
We use 8 bits for Red, 8 for Blue and 8 for Green which give us 256 x 256 x256 colours – over 16 million.
We need 3 bytes to describe RGB coded colours.
Codes can be used by a programmer to describe colours in Hex code.
This unit on Computer Structure describes in detail the function of the component parts of a processor in the manipulation of data.
This is extended to the methods of transferring data within a processor and between a processor and memory.
The concept of a stored program is considered and the steps in the fetch-execute cycle to access and run programs. Memory types are considered, from registers to backing storage and how memory is defined and addressed.
RAM & ROM
Data bus – 2 way
Registers, A, MAR, MDR, PC, SP
Address bus – 1 way
All computers based on same basic design, known as the Von Neumann Architecture.
Computers carry out tasks by executing machine instructions. A series of these instructions is called a machine code program held in main memory as a stored program, a concept first proposed by John Von Neumann in 1945.
Central Processing Unit (CPU) fetches, decodes and executes the machine instructions.
By altering the stored program it is possible to have the computer carry out a different task.
To execute a machine code program it must first be loaded, together with any data that it needs, into main memory (RAM). Once loaded, it is accessible to the CPU which fetches one instruction at a time, decodes and executes it at electronic speed.
Fetch, decode and execute are repeated until a program instruction to HALT is encountered. This is known as the fetch-execute cycle.
1. The contents of the PC are copied into the MAR;
2. The contents of memory at the location designated by the MAR are copied into the MDR;
3. The PC is incremented;
4. The contents of the MDR are copied into the IR.
18.104.22.168 The Execute phase
1. Decode the instruction in the IR;
2. Execute the instruction in the IR.
The components of the CPU and the connections to devices that are external to it are shown.
Main memory (RAM and ROM) stores programs and data while the computer is operating.
Central Processing Unit. The CPU coordinates and controls the activities of all other units in the computer system. It executes program instructions and manipulates data in accordance with the instructions.
It uses a standard architecture composed of the following three components:
Arithmetic and logic unit (ALU);
All three components work together to form the processor.
We will now study the internal architecture of the microprocessor (CPU) itself. Because of the stored program concept, we must consider the relationship between the CPU and memory.
This is a diagram of a fairly typical microprocessor design, showing the internal structure of the CPU and its relationship to the memory of the computer.
The CPU has to access memory both for instructions and to receive and transmit data from or to memory.
Memory Address Register (MAR) - specifies the address in memory for the next read or write operation from or to memory;
The Memory Data Register (MDR) or Memory Buffer Register (MBR) - contains the data to be written to memory or receives the data read from memory.
The MAR and MDR registers have a large part to
play in the fetch-execute cycle.
To read data from memory, CPU places the address of the memory location into the MAR and activates the memory-read control line of the system bus. This will cause the required data to be transmitted from memory via the data bus to the MDR;
To write from the CPU to memory, the CPU places the data to be written in theMDR; the address of the memory location where they are to be written is placed in the MAR; and the memory-write control line is activated.
Data is transferred between memory and processor by buses. hold image information, but one way is to map them so that the first byte represents the top left pixels, the second byte represents the pixels to the right of the first pixel, until the end of the first row is reached, when the next byte holds the information for the left hand end of the second row. For a small (24 pixel by 4 rows ) display the layout would look like this:
Pinpoint memory location.
Transfers the data
Same size as Word size
Initiates and controls operations.
Inside the processor.2 Computer Structure 2.5 Buses
Sampler listens to sound repeatedly and stores a number representing the amplitude each time
To playback video on a standard computer it will need to be decompressed by hardware or software, usually on the card.
AVI – (Audio Video Interleave) or Video for Windows. Being replaced by Active Movie which will playback AVI, QuickTime and MPEG.
QuickTime – CODEC s/w developed by Apple but used by both Mac and PC.
MPEG – Video board uses hardware to make compression much faster.
Accuracy – Depends on Compression Technique, frame rate and resolution.
Speed – Hardware must be fast enough to cope with stream of data to memory and to the hard disk.
Cost – Not only card but good Multiscan Monitor required (17” and 19” nowadays)4.2 – Input & Output Devices4.2.4 Video
The CRT is the basis of most visual display technology.
The screen is arranged as a series of lines of dots and each dot is made up of three small areas of red, green and blue called a triad. The intensity of light shone on each triad determines the actual colour of the pixel.
The picture is redrawn between 50 and 100 times a second. This is the refresh rate.
A monitor which operate at different refresh rates is known as a multiscan or multisync monitor. The refresh rate is controlled by the video adapter.
Screen resolution is quantified by the dot pitch, the distance between the dots on the screen. Typically between 0.28 and 0.38mm, corresponding to 100 to 70 dpi.
When large amounts of data are to be sent to a peripheral device, or when the peripheral is shared across a network then spooling is a preferred method of compensating for the difference in speeds of the processor and the peripheral.
Spooling involves the input or output of data to a tape or a disk.
This, for example, allows output to be queued from many different programs and sent to a printer by a print spooler (special operating system software).
The print spooler stores the data in files and sends it to the printer when it is ready, using a print queue.
Magnetic storage devices include hard disks, floppy disks, Zip disks and magnetic tape.
They are called magnetic storage devices because their recording surfaces are coated with a material that responds to magnetic fields to enable data to be stored.
Storage devices can be fixed or removable. Removable storage devices allow the user to disconnect the device and physically transport data from one computer to another.
Varieties of removable devices include the Iomega and Syquest hard disks and Jaz cartridges.
All the sectors around the disk, equidistant from the centre, form a track. With multiple platters, the collection of tracks on each platter, equidistant from the spindle is called a cylinder. When data is to be read or written, the read and write heads are moved to the appropriate track, where they wait until the relevant sector spins past.
Rotational speed of hard disks has improved, from 3000 (rpm) of early disks, to current rotational speeds of 5,400 and even 7,200 rpm.
Performance is also measured in terms of the rate of data transfer from the disk.
SCSI - transfer rate 5Mb/sec
Ultra Fast SCSIIII transfer rates - 40 Mb/sec.
Hard disks have improved tremendously in their capacity to store data in the last 10 years. From the modest 10Mb disks of the early 80s to current 80 Gbyte disks on many of today’s PCs.
The hard disk is a direct access device, meaning that data can be directly read or written to any portion of the disk.
Storing data on tapes used to be the only solution to backing up hard disks of large capacity. Now, with large, removable magnetic disks and optical CR-RW technology, this is no longer the case.
However, removable storage media is comparatively expensive, costs 10 times tape. Tape, therefore, still has the edge in this market.
Tape is read and written on a tape drive. Data is written to tape in blocks with inter-block gaps between them. A single operation writes each block
Data is stored on magnetic tape as magnetised regions on the surface of the tape induced by the magnetic recording head. To read data, the tape passes under the read/write head and the stored magnetised regions produce very small voltages in the head, leading to a current in the head coil. This current can be analysed to give a representation of the stored binary data.
Magnetic tapes have large capacities, reaching up to several gigabytes and come in a variety of sizes and formats.
Since their introduction, tape drives have passed through many stages of improvement with extremely reliable Digital Audio Tape (44.1 kHz, 16-bit record and playback DAT) drives representing the current state of the art. A 4mm DAT tape can now store up to 24 Gbytes of data!
Tapes are sequential access devices. Accessing data on tapes is therefore much slower than accessing data on disks.
They are not suitable as storage media for applications where data needs be used regularly - where a disk is a more appropriate medium. Because tapes are so slow, they are generally used only for long-term storage and backup.
A plastic disk is scanned using a laser. It reflects off pits on the surface differently from lands (bumps)
Re-writeable CD-ROM becoming more common.
Capacity – About 650Mb
Speed – from single (150KB/sec) to 32x (or even 40x). The x refers to the times faster than CD Audio.
Cost – CD-ROM Drives fairly cheap.
Access – Always random
Based on a combination of magnetic and optical technologies.
Active layer is magnetic material.
Recording – magnetic material heated beyond a particular temperature by laser, allows magnetisation to be reversed.
Reading – laser operates at much lower temp and reflected beam rotated by magnetic field and detected by read head.
Capacity – 3.5” disks of 128, 230 and 384 Mb
Speed – Varies as multiple of standard single speed
Cost – decreasing with time with different formats and capacities becoming available.
Solid-state storage devices are made up entirely from electronic components i.e. they have no moving parts.
They are also called RAM disks, as they take the place of a magnetic disk as a mass storage device.
They can be in the form of a plug-in card or cartridge containing memory chips.
The chips of a SSSD are typically static RAM or Electrically Erasable Programmable ROM (EEPROM or Flash EPROM).
SSSD are used with devices where space is at a premium e.g. in a camera, or when portability is desirable e.g a USB flash drive.
Interfacing means making the hardware connections so that two devices can communicate effectively.
Data Format – data has to be consistent e.g. serial output to a serial device. Interface makes data consistent (also ADC)
Parallel/Serial – time & space division. Time separates transmission of actual bits and space can be used for multiple bits in parallel. Serial can be slow but use of fibre-optic cable very fast.
Voltage – different voltage levels between peripheral & computer need to be ironed out.
Protocols – rules that govern transmission of data. E.g. no of bits per packet, voltage levels etc.
Status signals from a device indicate what the device is doing at any given moment. E.g.if a device is unable to receive data, then a transmitting device can delay transmission and retry later.
Speed - Different devices send and receive data at different rates. The devices agree a rate prior to transmission by utilising a protocol.
Wireless communications can be achieved using WAP (Wireless Application Protocol) - a specification for a set of communication rules to standardise the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access, including e-mail, the World Wide web, newsgroups, and Internet Relay Chat (IRC). Interconnecting devices centred around an individual person is called a wireless personal area network (WPAN). Typically, a WPAN uses technology that permits communication between devices in a
short radius of about 10 metres. One such technology is Bluetooth.
has made networking easier and cheaper.
4 Factors Affecting Performance4.6. Network Topology - Bus
Data Security – data encryption methods used. High collision rates requiring re-transmission
Bandwidth – available bandwidth shared amongst all stations accessing network. Data compression used.
Reliability – fault in one station has no effect on rest. Cable fault will lose all that section.
Cost – relatively cheap
Bus Topology- easy to expand and cheap to set up.
e.g. Ethernet in school or college.
Data Security – higher security as data routed only to the computer that is to receive it. No collisions.
Reliability – if a link fails then only that station is off the network. Failure to central controller is fatal.
Cost – can be quite expensive due to high cost of cabling. Popular in small self-contained networks as not too expensive (small office).
All nodes connected to one central node that routes traffic to the appropriate place.
Similar to Bus in many respects with similar security problems.
Control system in charge of transmissions and stations guaranteed access to transmissions. Collisions avoided by use of a token
Additional expense for control s/w and system. May have to wait turn to transmit.
Network down to add station, but few if any crashes.
Fault in one cable does not affect network.
Lots of wiring
The technical factors which have led to the growthof computer networks have emerged in parallel with the economic factors which have driven the researchinto networking technology.
As the economic demandor networking technology has grown, the trend hasbeen for equipment prices to fall and performance toincrease.
Although still in its infancy, the development ofwireless networking is likely to follow the same pattern.
For each one, make a note of the following:
4. date discovered (find a recent one):
5. medium for infection (e.g. email, website):
6. cure (if any):