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) TFT Liquid Crystal Display. The type of display found on digital watches, lap-top 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 surround-sound cinema style speakers.

More Backing Storage CD-R CD-RW DVD-R DVD-RW 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 CD-RW 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 DVD-R 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. C:/ D:/     Computer 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 Random 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 random-access 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 ALU Registers MDR MAR ACC IR X Y 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 Print x 50. Next x 60. … 70….. 80……. Source code Compiler for Processor 1 Compiler for Processor 2 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 Processor 1 Processor 2

More on Input Devices Voice Recognition Handwriting 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 on-screen 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.