Hardware
We now need to consider more closely what makes up the
computer's processor and how it interacts with the other components
of the computer system. The processor is purely electronic
circuits. It will be mounted on a motherboard which has connectors
to various buses used to communicate with input and output
devices.
A bus
A bus a pathway along which data travels and consists of a set
of parallel wires (lines). Each line can transmit a single bit. The
bus is made up of a number of lines so be would described as (for
example) a 16-line bus. 16 lines means 16 bits can be sent along
the bus at the same time, so a 16-line bus therefore can transmit a
16-bit number.
The address bus
All components of the system - including the primary memory -
will be referenced by the processor using an address. Since it is
the only data that the processor understands an address will be a
binary number.
The only component which can load an address onto the address
bus is the processor. This address is then passed to the relevant
component such as the main memory or an input or-output device.
Device controllers
The processor has a fundamental problem. Manufacturers will want
a particular processor to be able to communicate with thousands of
different devices which are manufactured by various companies. It
would be impossible for these connections to be hard-wired for all
possible devices and so communication between the processor and the
various devices is done through an intermediary device
controller.
The data bus
Data will be continually passed to and from the processor, the
main memory and the devices. Since data needs to be both read and
written, the data bus will be bi-directional.
The control bus
Both the address bus and data bus use all the lines together to
form a binary member. The control bus works very differently where
each line is used for a different control signal. It is the task of
the control bus to coordinate the various actions and send
appropriate timing signals to indicate something has happened or
been completed.
Consider when the processor has to read a value
from main memory. The sequence of events would be as given
here.
• Address bus is loaded with the address
• Data bus is loaded with the contents of this address and
copies the value to a register in the processor
• Control bus sends a signal to confirm the 'data read'
operation has been completed.
Other control bus signals include:
• memory write completed
• output completed - data on the data bus has been copied to the
output device
• input completed - data has been copied from the input device
to the data bus
• reset - the user has pressed the Reset button on the
computer.
Inside the processor
The processor will be continually receiving program instructions
in sequence. Each time it must make sense of the instruction before
it is able to execute the instruction.
A typical instruction could require the processor
to:
• load the contents of a memory location
• add/subtract a number
• add/subtract the contents of a memory location
• manipulate the bits in some way, for example, shift them one
place to the left.
Each instruction must be loaded from memory onto
the data bus and it is one of the main roles of the control unit to
manage the sequencing of instructions.
Arithmetic is done by circuits inside the processor
called the Arithmetic Logic Unit (ALU). The ALU is made up oflogic
circuits which are designed for various computations required.
Processor
CIR
Accumulator
| ↑
↓ ↓
Instruction
decoder ALU
Figure 4.1 Inside the processor
Figure 4.1 assumes there is only one general
storage location available inside the processor called the
Accumulator.
Primary memory (or Main
memory)
Primary memory
Operating system
Word processor
Document
Unused
Figure 4.2 Primary memory
Consider a session of word processing on a PC. The user will
load the word processing software (from the secondary storage) -
type the document and later save the document onto the secondary
storage.
For this computer session the primary memory Will
contain both program instructions (the OS and the word processing
software) and what the computer system will consider as 'data' (the
document) (Figure 4.2) .
The memory consist of a number of memory cell Which
are each referenced with a unique memory address.
Each cell of main memory on a PC will contain one
byte (i.e. 8 bits); hence modern computers are described as
'byte-addressable'.
It is a major job for the operating system to
manage which memory cells are being used and which cells are
'free'. When the word processing session finishes both the memory
used for the program and the document will become free and
available.
It is a fundamental concept that the computer
system when reading/writing values to and from memory will treat
the contents of all memory cells as a 'data' value. It is what it
then does with that data value that will give the byte a
context.
Random Access Memory (RAM)
'Random access' because the processor is able to go directly to
a particular memory location (assuming it knows the address) to
retrieve the data value from that cell.
For the most popular form of RAM - Dynamic RAM
(DRAM) - each bit is formed from a capacitor and a transistor
working together. The capacitor stores the charge representing the
value 0 or 1. The transistor is the switch to this component which
allows the value to be written or read.
RAM is volatile - that is its contents will be lost
when the power to the computer system is switched off. Also the
contents of the RAM can continually change (think back to our word
processing session scenario).
Integrated circuit technology has progressed at
such a rate that PCs with a memory size of 4GB are now common and
this has implications for programming. In the early days of
computing memory was at a premium and programming techniques were
specifically designed to maximise the use of the available memory.
One quote Bill Gates will want to forget is "640K ought to be
enough for anybody!" (1981).
Read Only Memory (ROM)
'Read only' implies that the contents of this type of memory
cannot be changed. The bit pattern in each cell of ROM is burned at
the time of manufacture of the memory component. ROM also allows
random access to cells. Because the contents of this memory cannot
be changed this makes it suitable for storing the program
instructions the computer needs for its start-up process (called
booting the computer). The software on the ROM memory is called the
bootstrap loader. The computer will be hard-wired to run the
bootstrap loader software when the power is switched on and its
main task will then be to load parts of the operating system from
the secondary storage.
Secondary storage
Also called auxiliary storage or backing storage.
If the RAM is volatile the computer system must use
secondary storage for the permanent storage of programs and
data.
Issues concerned with secondary storage will be:
• Is the device internal or external to the computer?
• Does the medium allow for direct access to particular
files?
• Is the technology based on optical or magnetic methods of
storage?
• Is the data effectively online or off-line to the computer
system
Magnetic devices
Hard disk
A magnetic medium which has a number of surfaces (or disk
platters) each divided into tracks and sectors. Data is
read/written to and from the surface of a platter by a read/write
head which floats above the surface of the disk. The disk rotates
at high speed. To read a particular sector from the disc requires
that:
• the head moves to a position over the correct track
• the head must wait until the disk rotates with the start of
the sector under the head
• the head reads/writes the sector data.
Convention
♦ 'Disk' for magnetic media
♦ 'Disc' for optical media
The hard disk is the device for secondary storage on all PCs and
is an internal unit. External hard drives are available and are
often used for archiving or the backing up of data.
The hard disk is an electro-mechanical device. Read
and writing the data is electrical but the positioning of the head
to the correct track and waiting for the disc to rotate (the
latency time) are all the result of a mechanical movement. The
mechanical aspect has implications for the speed with which data
can be retrieved and written to the device.
Optical devices - General operation
Like a hard disk, an optical disc is rotating at high speed. The
disc consists of a protective plastic layer on top of a reflective
layer (that looks like aluminium foil). The surface of the disc
alternates between flat areas (called 'lands') and tiny depressions
(called 'pits'). These two surfaces effectively encode the pattern
of Os and 1s.
A laser is shone onto the disc and light that
strikes a land it reflected back to a detector - light that strikes
a pit is scattered. Unlike a hard disk, the data is written onto a
single track divided into areas called sectors all of the same
size. The speed of rotation of the disc is continually
changing.
CD-ROM (Compact Disc-Read Only Memory)
Typically used for the commercial distribution of media files -
software, audio and video/film. Typical capacity 600-800MB.
CD-R
A CD-R can be written to once and read many times. Capacity the
same as for a CD-ROM. It is useful for the backup/archiving of data
.
CD-RW
A CD-RW can be written to many times. Capacity the same as for a
CD-ROM. It is useful for the backup/archiving of data.
DVD
Rapidly replacing the CD-ROM. Has a capacity approximately 13
times that of a CD-ROM.
Technology is similar to a CD-ROM but the DVD disc
is multi-layered hence the increased storage capacity.
DVD-RW
A DVD-RW can be read and written to many times.
Blu-ray
Designed to eventually replace the DVD, Blu-ray discs store 25GB
of data in each layer and are dualed. Hence they are used for High
Definition (HD) media and games machines such as the Playstation 3.
A Blu-ray player is becoming a standard household item to connect
to the television monitor.
The media use a blue laser (replacing the red laser
used with DVDs) which allows data to be stored with a higher
packing density.
Until 2008 there had been two competing optical
formats Blu-ray and HD DVD and it was the announcement from Warner
Bros that it was to adopt the Blu-ray format that made it become
the de-facto standard.
Solid-state Storage
A Solid-state Storage Device (SSD) uses solid-state technology
to store and retrieve the data. Unlike a hard disk (which is an
electro-mechanical device) the SSD is only electrical. The benefits
which result from this are that the device is silent and not
susceptible to shock (such as a hard disk).
SSDs are increasingly being used as the main
secondary storage in laptops, netbooks and other mobile
devices.
Flash memory cards/Pens
In widespread use and capacities of 4GB now common. Fitted in
portable devices such as a smartphone or a digital camera and used
extensively for transferring files between two PCs.
Buffers
We have already described a hard disk as being an
electro-mechanical device - compared with solid state memory which
is purely electrical (there are no moving mechanical parts). This
has implications for the operation of the computer system.
The processor executes its program instructions on
a time scale of microseconds (i.e. millionth's of a second).
The mechanical parts of the disk drive operation
(rotation of the disk and the movement of the head) are on a time
scale of milliseconds (i.e. thousands of a second.
Consider these tasks:
• the output of data to a printer - this could take several
seconds to print an A4 document.
• the user keying in data at the keyboard - even slower!
This suggests that all stages of operation of the
computer, i.e. input, output and storing data are much slower than
the speed at which the processor can operate.
A buffer is a temporary storage area between the
device (for example, the hard disk) and the processor. Buffers are
used when there is a speed mismatch between the speed at which data
can be processed and input/output.
When data is read from the disk we can never read
just a byte or a couple of bytes. The operating system will
organise its available storage space into 'allocation units' which
will typically be two of the sectors on the disc - this is called a
physical record. An allocation unit which is read could contain
(for example) 10 of the customer records we are storing so we say
that one physical record from the disk is storing ten of the
logical records.
A read operation of the disk head will copy this
allocation unit to the disk buffer. And the operating system will
then sort out the logical records and make them available to the
application program (for example, the database).
Data transfer between the buffer and the processor
is only electrical and so the use of a buffer is effectively
speeding up the transfer of data between the secondary storage
device and the processor.
Double buffering
More sophisticated is the use of double buffering. When one of
the buffers is having its contents read by the application program
a second buffer can be filling up by reading data from the
disk.
Print buffer
The buffering technique can similarly be applied to the output
of data to a printer. Output is sent to a print buffer storage area
and then the printer receives the data from the buffer. Double
buffering would again speed up the operation.
Peripheral devices
A peripheral is any device which is not part of the Central
Processing Unit. This includes any external input, output and
storage device and also the internal hard disk.
Input devices
Keyboard
We expect to find a standard QWERTY keyboard on any PC. The
keyboard may have a numeric keypad on the right hand side. Each key
press generates the ASCII code for that character and the codes are
stored in a keyboard buffer before transmission to the
processor.
Concept keyboard
A keyboard device with a similar appearance to a touch screen.
Different areas on the keyboard can be represented as a picture and
each picture programmed to match a choice or action.
Mouse
Used by all computers with a graphical user interface. Original
mice had the movement detected by the physical movement of a
trackball, but newer mice use lasers and transit the movement
signal wirelessly (rather that through a connecting cable).
Scanner
Most scanners are a photo-diode system which works on the
following principle.
• A laser light is shone at the source document
• A moving mirror or prism moves the light across the source
• The light is reflected back; black areas reflect less light
than white areas/bands
• A light sensor - typically a photo-diode - measures the
intensity of the reflected light
• This level of reflected light is converted to an electrical
signal which is encoded as a digital measurement.
Barcode reader
The following group of devices - barcodes, OMR and OCR - are
given the description automatic data entry as the data capture is
done with some form other than the keying of data from the
user.
A barcode is a series of vertical black bars with
variable spacing of the width of the bars and the White space gap
between. The pattern of bars and gaps encodes the data value. The
two coding systems in widespread use are the Universal Product Code
(UPC), a variant UPC-E and the European Article Number (EAN).
A bar code is read by a peripheral device called a
bar code scanner and there are variations:
• a horizontal flatbed panel - as used at the EPOS in a
supermarket
• a hand held wand type scanner.
Optical Mark Reader (OMR)
Optical mark recognition uses 'marks' placed by the user at
certain positions on a document. For example, the shading in one of
a number of boxes coded (1 to 5) on a customer satisfaction survey
form. The positioning of the marks on the data capture form is the
method for encoding the data. The form once completed is scanned.
Some applications will have the ability to scan a whole batch of
completed forms at high speed.
Typical applications include
• the input of an entry into the National Lottery
• students complete a multiple choice examination where the
answer paper is designed for the OMR entry of the responses.
Optical Character Reader (OCR)
Uses optical character recognition. A document contains a
numbers of characters which are printed in a stylised font which
can be read by a scanner.
Typical applications include
• utility billing - the invoice sent to the customer is a
document in two parts. The customer will return the bottom part of
the form with their payment. This returned form has data such as
the account number, printed in characters which can be scanned and
captured.
• The basis of any OCR software on a PC. A document is first
scanned and captured as an image. Software then processes the image
and converts its contents into separate text characters (which
could typically then be edited by word processing software).
Graphics tablet
The user draw on a glass plate with a digital pen and the
movement of the pen is captured by software and mirrored on the
screen.
Typically applications include
• capturing a handwritten signature
• tracing the outline of a shape from a paper copy (the paper
document would be first secured on the tablet).
Touch screen
The visual display can detect the touching of the screen and
translates the input as (for example) a menu choice and follows it
with some action.
The obvious benefit of this device is that it does
away with the need for an intermediary device such as a mouse.
Modern touch screens use one of two possible technologies:
Resistive - The screen consists of a number of layers, two of
which come into contact as the result of a 'touch'.
Capacitive - the glass screen is coated with a layer of
transparent conductor. Touching the screen causes the screen's
electro-static charge to change and so its capacitance.
Typical applications include the primary
communication for tablet PCs such as an iPad and smartphones such
as an iPhone.
Radio Frequency Identification Reader
(RFID)
An RFID chip or tag is embedded into an input device such as a
plastic card.
• The RFID reader transmits a radio signal
• When the RFID tag comes into close proximity to the reader the
signal from the reader activates (or 'energises') the tag
• The tag then transmits data to the reader by radio waves.
Typical applications include
• the identification of crates boxes of products in retail
distribution (or even individual items)
• a microchip tag under the skin of a pet - when the tag is
activated it sends data to a central database so allowing the
identification of the (lost?) pet
• tagging of new born babies in a hospital (to avoid them being
wrongly identified).
Output devices
Monitor or the Visual Display Unit (VDU)
Monitors using cathode ray tube technology are rapidly being
replaced by flat-screen LCD and TFT technology monitors. LCD
monitors have the benefitthat they use little power, but have the
drawback that they are susceptible to temperature changes.
Laser printer
A laser is a page printer, i.e. it prints a whole page at a
time. To print a page the following sequence is followed:
• the rotating print drum is negatively charged
• the printer generates a bitmap image of the page
• a laser beam is directed at the drum and removes the charge
corresponding to the black areas of the image
• the toner is given a negative charge and is attracted to the
positively charged areas of the drum
• toner is then bonded to the paper using heated rollers.
Typical application
The laser printer is the 'office workhorse'.
Dot matrix printer
A dot matrix printer has a print-head which moves laterally
across the page and so prints a line of characters before advancing
the paper to print the next line.
The print-head is made up of a matrix of pins and
various combinations of pins form the different characters. The
clarity of the printing will increase as the number of pins forming
the matrix is increased. The pins strike a carbon ribbon which then
impacts the character onto the paper.
Typical application
Although noisy and lacking in quality they are still in office
use as their impact way of working makes them suitable for
printouts which require one or more carbonised copies printed
concurrently.
Inkjet printer
A print-head squirts coloured ink- black, cyan, magenta and
yellow - onto the page from a piezoelectric crystal. Combinations
of these four colours produce the coloured output.
• A heater behind the ink reservoir warms the ink.
• This causes droplets of ink to vaporise and a small blob of ink
is forced out onto the paper
• The print-head moves laterally across the width of the page
and then prints the image line-by-line. The ink dries on the paper
before it is removed from the printer.
Typical application
Inkjet printers are in widespread commercial and home use for
producing photo-quality printouts.
Plotter
The most common type is a jlatbed plotter. A sheet of paper is
placed on the flat surface and a carriage mounted with pens of
different colours moves above the paper. The carriage can move both
horizontally and vertically so enabling the pens to be positioned
anywhere on the page.
Typical application
Production of CAD drawings.
Speakers/Headphones
A loudspeaker is an analogue device and so the digital signals
representing the sound wave must first pass through a
digital-to-analogue converter before they are received by the
loudspeaker.
Typical applications include
• the playback of media files
• signalling a warning message from a software application.
Storage devices
These can simply be summarised here as:
• an internal or external hard disk
• a variety of optical media storage devices