More Input Devices
Microphone
A device that converts sound
waves into audio signals.
These audio signals can then be
converted into digital values and
stored in the computer.
Microphones are often used with
webcams for video conferencing.
More Output Devices
Liquid Crystal Display (LCD)
Liquid Crystal Display. The type of
display found on digital watches, laptop computers and some flat-screen
monitors. Their major advantage is low
power consumption, leading to long
battery life for portable devices.
TFT
Abbreviation of thin film transistor, a
type of LCD flat-panel display screen
where each pixel is controlled by its
own transistor. This technology
provides the best resolution of all the
flat-panel techniques. Also sometimes
called active-matrix LCDs.
More Output Devices
Loudspeakers
Loudspeakers are audio
output devices which take
the output from a
computers soundcard and
covert it back into audio
waves.
Loudspeakers can be
anything from small
headphones to surroundsound cinema style
speakers.
More Backing Storage
CD-R
recordable compact disk. Disk can be written to by a user
with the proper kind of CD drive. Cannot be erased to be
written on again. One time write only!
CD-RW
similar in virtually all respects to a CD-R, except that a CDRW disc can be written and erased many times. This
makes them best suited to many backup tasks. CD-RW
drives can also create CD-R discs.
DVD-R
A write-once, recordable format. DVD-R drives can write
DVD-R discs, which can be written only once.
DVD-RW
DVD-RW (recordable/rewritable) drives can write both DVDR discs and DVD-RW rewritable discs, which can be written
to and erased many times.
More Backing Storage
USB Flash Drive
Universal Serial Bus is an
interface which allows various
devices to be attached to a
computer.
A flash drive is a device which
contains memory chips which
can be used to store data
permanently.
These devices are small and
portable. They have
capacities up to 2Gb (the
most common being 512 Mb
(0.5 Gb).
Comparison of printers in terms of:
Speed (ppm)
Pages per minute is used as a means of comparing
printers. General the more pages a printer can
print in a minute the better for the user (the user
has less time to wait for the printouts).
Capital Costs
The initial cost of the printer and it’s installation
are capital costs. Printers can range in price from
a few tens of pounds to thousands or tens of
thousands of pounds.
Some printers require network cabling, special
power supplies, additional computers etc. All of
this adds to the capital cost of the printer.
Comparison of printers in terms of:
Running costs
The running costs are the costs to operate the
printer. These include the cost of:
•
•
•
•
Paper and other printing media
Power
Ink or toner for the printer
Service contacts or support for hardware failure
Resolution
Is an indication of the sharpness of images on a
printout. It is based on the number and density of
the dots used. The more dots used in an image,
the more detail can be seen and the higher the
image's resolution.
Resolution for printers is measured in dots per
inch (dpi). The higher the dpi the better quality
the image.
Comparison of backing storage
Speed (data transfer)
Backing storage devices can be compared in terms of the time
it takes to transfer data from the disc to the computer.
Speeds are most commonly measured in Mbits (megabits – i.e.
thousands of bits per second)
A typical floppy disc drive transfers data at 1 Mbits/sec
A typical 32x CD-ROM drive transfers data at 4.8 Mbits/sec
A typical 16x DVD-ROM drive transfers data at 16.7 Mbits/sec
A typical Hard disc drive transfers data at 150 Mbits/sec
•
These are all typical statistics and given as a rough guide
Cost
Floppy drives are cheaper than CD-ROM drives which are
cheap than DVD-ROM which are cheaper than Hard-discs
All devices can be compared on a cost basis.
Comparison of backing storage
Capacity
Refers to how much data a medium can store.
Floppy disc is 1.44 Megabytes
Most common USB Flash Drive size is 512
Megabytes
CD-ROM is 673 Megabytes
DVD-ROM is a maximum of 17 Gigabytes
The most common hard disc size if currently 120
Gigabytes
OS - Different modes of processing
Interactive systems with
background job capability
Some operating systems
allow a background process
to occur at the same time as
a foreground interactive
process.
The foreground process is
the one that accepts input
from the keyboard, mouse,
or other input device.
Background processes
cannot accept interactive
input from a user, but they
can access data stored on a
disk and write data to the
video display unit.
For example, some word
processors print files in the
background, enabling you to
continue editing while files
are being printed.
Hierarchical filing system
A filing system in
which directories
have files and
subdirectories
beneath them
A hierarchical filing
system is one that
uses directories to
organise files into a
tree structure.
Directories (folders)
can store more
folders and/or files.
Computer
C:/
D:/
Drives
Folders
Files
A Hierarchical Filing Structure
Types of Access
There are two types of access
Random Access (also called Direct Access) devices retrieve the
data you want straight away e.g. CD-ROM drive, Hard disc drive,
floppy disc drive.
Sequential (also called Serial) devices have to move through all
the data, record after record, to find the data you need e.g.
Cassette tape, magnetic tape.
Types of access
Sequential Access
To go from file A to file Z in a
sequential-access system, you
must pass through all intervening
files.
Sequential access is sometimes
called serial access.
Random Access
Refers to the ability to access
data at random. In a randomaccess system, you can jump
directly to file Z. Disks are
random access media, whereas
tapes are sequential access
media.
Device Drivers
Computer operating systems are written by software
companies e.g. Apple, Microsoft etc.
It is impossible for these companies to know about
every single item of hardware that has been or might
be developed for their operating system.
To get round this problem the manufacturer of an
item of hardware writes a small program called a
“device driver” for their device. This program includes
instructions for the operating system about how to
communicate with the hardware device.
A device driver is software that controls how a
computer communicates with a device, such as a
printer. For example, a printer driver translates
information from the operating system into
information the printer can understand.
The Operating System
The operating system (OS) is the most
important program used by the computer.
The OS in most modern computers is stored on
hard disc and is loaded into RAM when the
computer starts up.
Some computers store their OS in ROM (e.g. some
palmtop computers) so that it cannot be erased by
accident. ROM operating systems load very
quickly.
The operating system controls and monitors the
operation of the computer system.
The filing system is part of the OS and it controls
the loading and saving of programs and data from
backing storage.
Modern Operating Systems
Examples of operating systems which are disc based
Windows XP for the PC
System X for Apple Macintosh
Linux for large fileservers and networks.
Examples of operating systems which are ROM based
JavaOS for Network Computers (NCs)
PalmOS or Windows Mobile for palm top and
handheld devices (computers, phones, MP3
players etc.).
Standard Functions of the OS.
User interface
the OS provides the means of communicating with
the computer system (Icons, commands etc. are
all provided by the operating system).
Manages memory
The way that memory is allocated to programs is
controlled by the OS.
Controls input/output
The access to all devices attached to the computer
is controlled by the OS. For example, the OS can
stop two programs trying to use the same device
at the same time.
More OS - Standard Functions
Filing System
The filing system manages backing storage
The access to all types of backing storage
devices is controlled by a special part of
the operating system called the filing
system.
The filing system will prevent two
programs trying to access the same files at
the same time.
Error Reporting
When an error occurs the operating system
provides information to the user about
what has happened and, sometimes, how
it can be fixed.
Resource allocation
When a computer is running the demands on the
systems resources (i.e. memory, input/output
devices, backing storage etc.) may be greater than
the resources available. To deal with this a
resource allocation system is built into the OS.
Various techniques are used to prevent two
programs demanding the same resource at the
same time (e.g. two programs can't both send files
to the printer at the same time).
The resource allocation system also ensures that all
programs running on the system receive as much
processing time as possible.
More about the Processor
Control
Unit
MDR
ACC
MAR
IR
X
Y
ALU
Registers
The processor is made up of three main parts.
The CONTROL UNIT fetches program instructions
from the memory storage locations, decodes and
then executes them.
The ALU (Arithmetic/Logic Unit) carries out all the
calculations as instructed by the control unit.
The registers are small storage locations inside the
processor where data can be stored while the
processor is executing instructions.
Word
All processors have a maximum number of
bits which they can process in one operation.
This number of bits determines the word size
of the computer.
An X-Box is a 32-bit computer system
because it can process 32 bits in one
operation, hence it’s word size is 32 bits.
The old ZX Spectrum (a computer popular in
the early 1980’s) had a word size of 8 bits.
The next generation of Play Station (PS3) will
be 64-bit, a 64 bit word size.
Calculation of image storage
A black and white graphic uses 1 bit to stored
each pixel (picture element).
If the image is 300 pixels by 200 pixels then it
is stored using a total of 300 x 200 bits i.e.
60000 bits.
Divide this by 8 to give the number of bytes
60000/8 = 7500
Divide this by 1024 to give the number of
kilobytes
7500/1024 = 7.324
Round up to ensure you have enough storage
7.324 = 7.33 Kb
Types of translator
There are two types of translator for high level
languages.
Compiler
A program that translates source code into machine code.
The compiler gets its name from the way it works, looking at
all of the source code and collecting, reorganising and
converting the instructions into machine code.
Compilers require some time to create an executable
program. However, programs produced by compilers run
much faster than the same programs executed by an
interpreter.
The program produced by the compiler is executable. It
does not need the compiler or the source code.
Many compilers are available for the same language. For
example, there is a FORTRAN compiler for PCs and another
for Apple Macintosh computers. This makes the program
source code PORTABLE (able to run on a number of different
computer processors).
Types of translator
Interpreter
An interpreter translates high-level instructions into a
machine code line by line. Each HLL statement is converted,
in turn, into machine code and then executed.
An interpreter can immediately execute high-level programs
(unlike a compiler). For this reason, interpreters are
sometimes used during the development of a program, when
a programmer wants to add small sections at a time and test
them quickly. Also interpreters are often used in education
because they allow students to program interactively.
The advantage of an interpreter over a compiler is that it
does not need to go through the compilation stage during
which machine code instructions are generated.
Many interpreters are available for the same language. For
example, there is a BASIC interpreter for PCs and another
for Apple Macintosh computers. This makes the program
source code PORTABLE.
Portability of software
All programs must be translated into machine
code in order to run.
Different types of processors use different
types of machine code.
Translator programs (compilers and
interpreters) are written for specific processor
types.
The same source code (the name given to the
file containing the HLL program) can be
sometimes be translated by more than one
translator and therefore run on more than
one type of computer.
Portability of software
10. Print “Hello”
20. Let a= 20
30. For x = 1 to a
40.
Print x
50. Next x
60. …
70…..
80…….
Source code
Compiler for
Processor 1
Processor 1
The same source
code can be
translated by two
different programs to
run on two different
processors. This
means that the
source code is
portable
Compiler for
Processor 2
Processor 2
More on Input Devices
Voice Recognition
With voice recognition systems the computer can
understand the words spoken to it and can carry
out commands associated with the words. Using
voice recognition a letter can be dictated instead
of being typed at the keyboard.
Handwriting Recognition
Handwriting can be recognised and the computer
can convert the written text into typed text for
storing.
More on Output Devices
Voice Output
Voice output from a computer system is also know
as speech synthesis. Some voice output programs
are know as text readers. They take a text file and
recite it to the user.
Hello,
I am Hal!
Other speech synthesis
programs can tell the user
what they are doing (for
example floor
announcements in a
speaking elevator).
Input devices for disabled users
A wide range of input devices can be used to allow
people with a range of disabilities operating computer
hardware and software.
joystick and switch
combinations enable an individual lacking
sufficient mobility to use a full keyboard
to access a computer through an onscreen keyboard or other "virtual"
substitute
keyboard alternatives
allow individuals unable to use a standard
keyboard to input keystrokes with a
mouse, headmount, or other specialized
device.
Output devices for disabled users
screen magnifiers
enlarge the information displayed on the
computer screen in a range of magnifications
and a variety of fonts.
screen readers
are intended primarily to assist individuals
who are blind or otherwise visually impaired.
These solutions include magnification
software, screen reading programs, and
Braille translation devices. Screen readers, or
screen review programs, are highly
specialised applications that will read,
sometimes with the help of a speech
synthesizer, what is currently displayed on
the screen. Other packages are specialized to
read scanned documents, essentially turning
the computer into a reading machine for
printed documents.