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Add-on Components

The Keyboard

In 1872, an inventor called C. L. Sholes devised an arrangement of letters for a typewriter that slows typing, encourages errors and fatigue and places convenient keys in inconvenient places. This was necessary as the mechanical typewriting machines that were state of the art then were klunky and jammed often if you typed too fast. The arrangement of letters that starts with QWERTY is C. L. Sholes invention and it is still in use on most computer and typewriter keyboards today. There are many alternatives to the QWERTY layout - in 1930 August Dvorak devised an alternative that has proved in several tests to allow much faster speeds with a higher degree of accuracy.

What sort of marketing campaign would be necessary for the world to convert to DVORAK given they are already familiar with QWERTY? I doubt even Microsoft could successfully market that idea (even though the idea is actually a good one - unlike many from that American company).

Every key on the keyboard generates at least one 7 bit SCAN CODE when you press it. Some special keys (the function keys, for example) generate two such codes.

These scan codes are used by the computer to differentiate your keyboard intentions. Additionally, a 1 is placed in the MSB of the scan code when it is pressed (MAKE), and a 0 is placed in the MSB when it is released (BREAK) - this way the computer can tell when you press and release (or indeed hold down) a key.

   eg.  "a"  = 30(dec)  =  10011110    make
                        =  00011110    break on release

   eg.  "A"             = 10110110     make right-shift
                        + 10011110     make "a"
                        + 00011110     break "a"
                        + 00110110     break right-shift

There are also in primary memory 2 KEYBOARD STATUS BYTES which store the current state of the keyboard (ie. whether the numlock or capslock is on, if the ctrl is pressed... etc). The state of the Keyboard status bytes determine the overall effect of the keypress.

Scan codes are bussed to the CPU. If the CPU is told (by an intermediary chip) that the keypresses are of a higher priority than the CPU's current task, it first pushes its registers onto the stack. The CPU then checks the keyboard status bytes to determine the overall effect of the keypress, then looks up the key combination to turn the code into ASCII if appropriate - this character is then placed in the keyboard buffer, break scan codes are discarded. If there was a delay (hardware/software configurable) before a corresponding break scan code is received, the CPU generates multiple copies of the ASCII sending them to the keyboard buffer as well.

The keyboard buffer is a circular queue (FIFO structure). When the keyboard buffer is full, extra key presses are discarded (along with a warning beep).

Software interrupts allow the CPU time to inspect the keyboard buffer and process ASCII codes waiting there - if they are printing characters they may be sent to video RAM which then appears on the screen.

Characters are displayed in 2 byte form (in text mode) - 1 byte for the ASCII, and one byte for the attributes (color, blinking, bold...

Some Common Peripherals

A PERIPHERAL is a device that can be attached to a computer (and therefore be used by it). Most peripherals require particular INTERFACES (ie. connection types) and use specialised signalling and fast protocols to allow rapid data interchange

  • Cassette Recorder

    This device used to be used as a STORAGE unit for early personal computers. To SAVE a program or stream data out, you needed to have RECORD on the tape player, to LOAD programs or data back in to the PC, you PLAYed tape. This was SLOW, SEQUENTIAL ACCESS and ANALOG (ie. Digital data was converted into SOUND on record, and back again on PLAYBACK) and often whole programs were lost by accidentally recording over the start or end sequences on poorly marked tapes.

  • Joy Stick

    Used mainly for interacting with GAMES, the joystick of today allows the recording of relative movements in any direction, along with the transmission of game switches (like fire, jump...)

  • Mouse

    Allows the recording of relative positions, used to control a number of graphical pointing devices, One, two or three buttons are common.

  • Digitising/Graphics Tablet

    Graphic artists usually find a Mouse to be an unnatural drawing tool. A pressure sensitive surface that can be drawn on using a stylus is a popular alternative. Digitising tablets are able to translate the relative position of the stylus into a set of XY Coordinates which can then control a pointing device on a graphical system.

  • Scanner

    Flat-Bed and Hand Held scanners are commonplace now, available in MONOCHROME (B&W) and FULL COLOUR. MONO scanners sample the image they are given at pre-determined resolutions, producing a stream of BINARY data that breaks the image into scan lines of pixels. Colour scanners typically SCAN the image taking 3 or 4 readings at a time, each pass detecting the relative intensities of each of the colours RED, GREEN, BLUE, then combining that data into a colour image bit stream for display.

  • Plotter

    A Plotter is typlically used for producing high definition technical drawings, and harnesses a series of pens that are moved across the surface of the paper in all directions - an image is built up by drawing sections of lines, curves and text in the order specified by the graphics program.

  • Printers

    • Daisy Wheel

      A print wheel is fitted to a stepping motor, which can with great accuracy (and usually a lot of noise) rotate the wheel to the correct character prior to an electromagnetic HAMMER pounds tha wheel leaving a carbonised ribbon imprint on the paper. By varying the WHEEL, different fonts are attainable. A daisywheel printer is typically unable to reproduce graphics.

    • Dot Matrix

      The print head is a line of 9 or more metal PINS that can be programmed to extend or retract whilst the print head moves left to right and back again. A Dot matrix printer builds up it's image by repeatedly passing left to right, varying the pin positions as it moves. The image is transferred to paper by a carbonised ribbon that the pins strike when extended. Dot matrix printers are able to render graphics with a number of factors depending the quality of the resultant image (closeness of pin strikes, size of pins, responsiveness of ribbon etc.). Colour Dot Matrix printers use special ribbons with each of the colors CYAN, MAGENTA and YELLOW (and usually black) and overprint rows with varying mixes of these colours.

    • Ink-Jet

      Inkjet printers are similar to Dotmatrix printers with the main difference being they 'spit' tiny droplets of ink directly on to the paper using a set of high precision nozzles.

    • Laser

      Laser printers harness a number of technologies to render images on paper- typically, a fine lazer is used to 'draw' the image on an electrostatically charged print drum (such that the parts of the image that are to be coloured are statically charged and the background is not). Toner powder is then sprinkled on the drum, adhering to the charged areas. The drum is then rolled onto a page of paper, transferring the toner pattern to the paper, this paper passes through a FUSER (which essentially bonds the toner to the paper), the stable printed page is then ejected from the printer.

    • Print Buffers

      Most printers use BUFFERS to store the data they are working on at the time - this allows the computer to stream a job to the printer and then carry on doing something else whilst the printer (usually one of the slowest peripherals attached to it) gets around to completing the job. Network printers usually have print queue buffers also to allow more than one person to print to the printer at the same time.

  • Microphone

    Soundcards these days are capable of sampling audio data at very high rates, converting analogue soundwaves into digital soundbytes. Microphones can be used in the place of mice and other input devices for those with mobility problems, for dictation and music mixing etc.

  • Robotic Devices

    Movements of robotic devices (and other control technologies) can be though of as OUTPUT peripherals, translating instructions into cues for movement. Control technology is gaining wide use in industry, medicine and electronics.

Secondary Storage - Disks and Drives

Note: we will only consider MSDOS conventions for info storage (some, however are cross-platform standards)


a floppy disk

An UN-FORMATTED disk has no sensible magnetic patterns recorded on its surface. FORMATTING a disk involves setting up many concentric rings of organised magnetic information called TRACKS. The disk surface is also subdivided (pie like) into wedges called SECTORS. Depending on the system (and the version of DOS, one or more contiguous track-sector combinations is termed a CLUSTER or ALLOCATION UNIT


  • Common sizes - 8", 5.25" and 3.5" disks
  • Composition: flexible circular plastic disk with magnetic coating then polymer overcoat on both sides of the disk, in plastic jacket. Magnetic coating can be altered by magnet (disk read/write head).
  • Speed: Rotate 100 - 200 revs/min (any faster and the disk distorts like a pizza)
  • Typically, 1 sector = 512 bytes

5.25" 80 tracks, 15 sectors, 2 sides = 1.2 Mb
40 tracks, 9 sectors, 2 sides = 360 Kb
3.5: 80 tracks, 18 sectors, 2 sides = 1.44 Mb

sector/track arrangement on a floppy disk

Retrieval speeds:

'access time' determined by

  • seek time = time to move heads to specified track
  • latency = time to move platen to specified sector
  • transfer = time to transfer data into input/output channel

'Floppies' are relatively slow, compared to other media (except audio tape). Retrieval speeds may also greatly be affected by the disjointed nature of the information on the disk (fragmentation of file)



    typically glass or metal alloy platen with magnetic coating then polymer overcoat. May be more than one disk in unit, sealed to be dust free.


    typically rotate 3600 revs per minute or more

Each disk has sectors and tracks, with each side of the disk having its own read/write head. Since all heads are attached to one 'arm', all tracks and sectors are aligned down through the stack of disks.

Sectors at the same distance from the centre spindle are called CYLINDERS. Groups of adjoining cylinders are often termed PARTITIONS.

a hard disk stack

a 10 Mb Hd might have 17 sectors, 306 cylinders, 4 heads (ie. 4 platen surfaces)

= 17 sectors x 306 cylinders x 4 sides x 512 bytes/sector
= 1065369 bytes (= 10 Mb ish).

Hard disks suffer from a problem called interleaving - where the disk is moving so fast that the read/write head gets to the right place but the disk has moved before it can get there - therefore connected areas of data are 'staggered' on the disk surface to avoid having to wait for an extra revolution when reading contiguous sections of data.

Disk Organisation

Before a disk can store information, it may be partitioned (using fdisk.com if it is a fixed disk), then it must be divided into tracks, sectors (called formatting). There are two levels of format - a low level format (performed by bios - supporting a wide range of format options), and high level format (performed by the format.com file, or similar utility) supporting a relatively limited range of format options.

BOOT SECTOR May contain an initial program loader and a 'bootstrap' program (that can load the majority of the operating system into memory and transfer control to it), and a Boot Signature - this indicates that the sector contains the 'bootstrap' program and therefore is a 'bootable' disk.

FAT - (File Allocation Table) - this works with the directory, in keeping track of the clusters associated with the files stored on disk. The directory contains the address of the first cluster of a file, the FAT records each successive cluster (each of the 1 Kb 'chunks' of the file). Files are terminated with a special entry - called an EOF (end of file marker). The 'chunks' that constitute a file are termed a cluster chain.

ROOT DIRECTORY - The root directory is the parent of all other directory entries on the disk. It contains 32 byte entries recording information on all the files in it, namely:

    filename, extension, attributes, time and date of save, starting cluster address, and file size (total). Attributes may be whether the file is a sub-directory, if it has been backed-up, hidden, system, read only...
Hidden/system files do not appear on a normal directory listing

A Fixed disk must have a BOOT SECTOR, FAT's and a ROOT DIRECTORY for each of its PARTITIONS (if more than one)

Cluster Sizes

The size of a CLUSTER can vary due to a number of factors including Disk Capacity (more correctly PARTITION SIZE) and Operating System. Since a CLUSTER is the smallest size a file can be saved as on that system, the size of the cluster can greatly effect the storage capacity of the hard disk (or rather effect the WASTED SPACE on that media)

Under DOS and W95, Cluster sizes apply on the partition sizes listed:

  • less than 256Mb - 4Kb clusters
  • Between 256Mb and 511Mb - 8Kb clusters
  • Between 512Mb and 1023Mb(1gig) - 16Kb clusters
  • Between 1024Mb(1gig) and 2047Mb(2gig) - 32Kb clusters
  • Bigger than that....?

Not all operating systems are so restricted - Under NTFS (The Windows NT File System), all clusters are 4Kb regardless of the partition size.

When buying BIG (ie. 100-200Gb) Hard Disks, the balance between Partition Size and Cluster Size is worthy of thought, since, especially if you save lots of very small files, your wasted (unrecoverable) space is significant.


Although data transfer to and from OPTICAL drives are signifigantly slower than fixed media like Hard Drives or Flash Cards/ROMs, typically the storage capacities are huge.

Conventional CDROM (Compact DiskROM) can store approx. 640Mb of data (equating to approximately 70Mins of audio data. The reflective surface of a CDROM is orgainsed into a single SPIRALLING track made up of minute 'pits' (that act like light traps) and reflectors to represent 1's and 0's.

CDROM retrieval speeds are improving all the time, with 8 speed being common now. Single Speed CDROM can deliver data at approx. 150Kbps. a 4xCDROM can deliver data at approx. 600Kbps, 8xCDROM at 1200Kbps. Average seek time for a single speed CDROM ranges between 195-250ms.

WORM drives (Write Once Read Manytimes) are also optical (or Floptical) disks, and allow the storage/archiving of data without the ability to edit it later. CD-RW drives are also becoming popular, allowing multiple write sessions

DVD (Digital Video Disk or more recently Digital Versatile Disk) is an emerging standard (with access speeds comparable to a 9-speed CDROM), with DUAL SIDE, DUAL DEPTH technology using 2 read/write heads and a semi-transparent over a totally reflective layer. Each DVD layer has a capacity of 4.7Gb, with a 2 layer, 2 side DVD totalling 17GB of storage space. DVD Writable have a slightly diminished storage capacity (5.2Gb) due to the decreased data density possible with PC style read/write heads.

IOMEGA ZIP Disks can store up to 100Mb or 200Mb per disk cartridge, with access times approximately 3 times faster than a floppy, they are useful for transportation of multi-megabyte files between systems. JAZ Drives offer larger storage capacity (1Gb or more) and again offer removable storage with multi read/write capability.


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