ICT in Orgs Chpt3 Input Output Devices

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  • 7/28/2019 ICT in Orgs Chpt3 Input Output Devices




    Unit Structure

    3.1 Overview

    3.2 Learning Objectives3.3 Data Representation

    3.3.1 Binary Representation

    3.3.2 Binary Representation within the computer

    3.3.3 Data vs. Information

    3.3.4 Memory Units

    3.3.5 Coding Scheme

    3.4 Primary Memory

    3.4.1 Random Access Memory (RAM) Types of RAM

    3.4.2 Read-Only Memory (ROM) Types of ROM

    3.4.3 Cache Memory

    3.4.4 Virtual memory

    3.5 Data Organisation on Secondary Storage

    3.5.1 Data Organisation on Magnetic Disk

    3.5.2 Data Organisation on Magnetic Tape

    3.5.3 Data Organisation on Optical drives

    3.6 Summary

    3.1 OVERVIEW

    Section Outline:

    Data Representation Types of Primary Memory Data organization on secondary storage


    Upon completion of this section students should be able to:

    List and describe the four data coding schemes Explain the RAM and ROM technologies Illustrate the difference between Cache Memory and Virtual Memory Explain data organization on magnetic disk, magnetic tape and optical drive

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    Data representation is mainly concerned with the way data is represented within the computer.

    Computer uses the binary system to represent data.

    3.3.1 Binary Representation

    The binary system uses the base or the radix 2; which means that only 2 symbols or digits can be

    used to represent data. The digit can be thought of as a box to hold a number. In the binary

    system, this number can be either a 0 or a 1. Data is thus represented in a computer as a

    sequence of 0s and 1s.


    A binary digit is called a bit after the term binary digit. A bit is the smallest unit of data a

    computer can recognize. A collection of 8 bits forms a byte. Any English character or symbol can

    be represented in a byte of data (8 bits).

    For Example:

    The alphabet A represented as a byte (8 bits) is as follows:

    3.3.2 Binary Representation within the computer

    Computers can only understand binary language. The Computer Processing Unit (CPU) is a chipmade up of transistors. The transistor is simply a tiny switch that can be on or off. On is equal to

    binary 1 and Off is equal to binary 0. The CPU consists of several million of transistors.

    3.3.3 Data vs. Information

    Data in the computer is represented as 0 and 1. For example, switch off is 0 and switch on is 1.

    Data is raw numbers and text. Information is processed data which is meaningful.

    3.3.4 Memory Units

    1 byte (octet) = 8 bits

    1 Kilobyte (KB) = 1024 bytes

    1 Megabyte (MB) = 1024 x 1024 bytes

    1 Gigabyte (GB) = 1024 x 1024 x 1024 bytes

    1 Terabyte (TB) = 1024 x 1024 x 1024 x 1024 bytes

    1 Petabyte (PB) = 1024 TBs

    1 Exabyte (PB) = 1024 PBs

    1 Zettabyte (ZB) = 1024 EBs

    0 1 0 0 0 0 0 1

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    3.3.5 Coding Scheme

    The Text codes is an agreed upon system which allow computers and programmers to represent

    letters of the alphabet, punctuation marks and other symbols. Text codes allow the same

    combinations of (binary) numbers to represent the same individual pieces of data, which make

    exchange of data possible between computers.

    Standard text (alphanumeric) code systems are as follows:


    Binary Coded Decimal (BCD) provides a method for coding decimal numbers in which each digit

    is represented by its own binary sequence.

    For example:

    The decimal number 2345 in BCD is as follows:

    2 3 4 5

    0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 EBCDIC

    Extended Binary Coded Decimal Interchange Code (EBCDIC), pronounced as ebb-se-dick, was

    created to extend the BCD. It was designed by IBM for early computers and is still used in IBM

    mainframe and midrange systems. The EBCDIC uses 8 bit code to define 256 symbols.

    Example of EBCDIC Scheme

    Representation of A in different notations

    Notations Representations

    Denary A

    Hexadecimal C1

    Octal 301

    The Hexadecimal notation is base 16 and uses the following symbols:


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    The Octal notation is base 8 and uses the following symbols:

    0,1,2,3,4,5,6,7 ASCII

    American Standard Code for Information Interchange (ASCII), pronounced as aeski, is by

    far the most common code used on computers of all types. It is a character encoding based on the

    English alphabet. The ASCII uses an 8 bit code and can define up to 256 characters which cover all

    Western European languages.

    Example of ASCII scheme:

    Note that the alphabet a and the alphabet A in the table above are symbols with different

    ASCII code. Unicode

    The Unicode is a Worldwide Character set standard, designed to allow text and symbols from all

    the writing systems of the world to be consistently represented and manipulated by computers.

    The Unicode uses a 2 byte (16 bit) coding scheme. The Unicode extends far beyond ASCII and can

    be used to represent 65,536 characters.

    Example of Unicode scheme:

    The Euro sign is represented by 20AC16. The alphabet A represents 10 and the alphabet C

    represents 12 in the hexadecimal (base 16) notation which is generally used for memory



    Memory, also known as Primary memory, Primary storage, Main memory or Internal Storage, is

    the part of the computer that holds data and instructions for processing. Memory is separated

    from the CPU, although closely associated with it. Memory has the advantage of faster access

    compared to the backing storage (for example, hard disk).

    Primary Memory is basically of two types:

    Random Access Memory (RAM) volatile or non permanent

    Alphabet ASCII Binary

    A 65 01000001

    A 97 01100001

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    Read Only Memory (ROM) non-volatile or permanent

    There are different RAM and ROM technologies. They are classified according to their data access

    (read & write) speed and storage capacity.

    RAM can be classified as: SRAM DRAM SDRAM DDR SRAM

    ROM can be classified as:


    3.4.1 Random Access Memory (RAM)

    Whenever software is installed on a computer, the latter is placed on the hard disk of the

    computer. However, when that software is run (e.g. when you double-click on its icon), the latter

    has to be transferred to the memory of the computer for the CPU to be able to execute it. RAM can

    be purchased in the form of RAM sticks and it is normally measured in terms of its size (256 MB,

    512MB, 1GB, 2GB...). The physical components of memory are called the memory chips. One

    important feature about the memory chip is the capacity of data it can hold. For example, a

    capacity of 512MB means that the memory chip can hold 512 millions of characters of data or


    RAM requires current to retain values. It is said to be volatile, meaning that, information stays

    here as long as the power supply is on but as soon as it is turned off, the information inside the

    RAM disappears. Data and instructions can be read and modified in RAM. Since the data stored in

    memory is volatile, it can be lost during a power failure or when the computer is switched off. It is

    therefore a good practice to save your work in hard disk or pen drive every 10 minutes.

    Generally, the more RAM a computer has, the more capacity the computer has to hold and process

    large programs and files. The amount and type of memory in the system can make a big difference

    in the system performance.

    RAM is used to hold the following:

    i) Operating Systemii) Program currently runningiii) Data needed by the programiv) Intermediate results waiting to be output

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    6 Types of RAM


    Static Random Access Memory (SRAM) is a very fast, relatively expensive RAM. The SRAM uses

    more power than other types of RAM. The word static indicates that the memory retains its

    contents as long as power remains applied, unlike dynamic RAM (DRAM) that needs to be

    periodically refreshed. However, data are lost when the circuit gets powered down, which makes

    SRAM a volatile memory as opposed to read-only memory and flash memory. Nowadays, SRAM is

    currently used in digital cameras and cell phones, onboard cache in computers and data buffers in

    hard disks.


    Dynamic RAM (DRAM) is the most common form of RAM used for main memory storage. It

    requires frequent refreshes, as data stored in them deteriorates over time (typically within a few


    DRAM, is rated in nanoseconds (time needed to read or write one word of data) and ranged from

    30-100+ ns. A higher number mean slower DRAM.


    Synchronous Dynamic RAM (SDRAM) is DRAM that is synchronized with the system bus, i.e. it

    runs at the same speed as the motherboard Front Side Bus speed. It requires higher tolerances

    and faster architecture. Speed measured in MHz, from 66 MHz to current maximums of 400 MHz.


    Double Data Rate Synchronous Dynamic Random Access Memory (DDR SDRAM) is a type of

    memory currently used in computers. It achieves greater bandwidth than ordinary SDRAM by

    transferring data on both the rising and falling edges of the clock signal. The current types of RAM

    memory used in computer systems are referred to as DDR2 and DDR3 RAM. It has replaced the

    old SDRAM which was used some time back and it offers improved performance. DDR4 is the new

    type of RAM technology that is very likely going to take over from DDR3 RAM. It will be out in the

    market beginning 2014.

    Figure 2-8 SD, DDR and DDR2 RAM

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    3.4.2 Read-Only Memory (ROM)

    The ROM chips permanently hold programs and instructions, typically recorded at the factory, for

    booting the computer. Unlike the RAM, the data and instructions in ROM chips can be read, but not

    modified (non-volatile). Chips retain their contents even when the computer is powered down

    (non-volatile memory). On a PC, ROM contains essential information for computer start-up like the

    BIOS (Basic Input/Output System). BIOS contains the instructions and data in the ROM chip that

    control the boot process and the computer hardware. After testing the hardware (POST), it fetches

    the boot sector from hard disk. The instructions inside a ROM are generally called as the

    firmware. A set of chips on the Motherboard working jointly with CPU called the chipset is an

    example of ROM memory. Types of ROM

    Read Only Memory (ROM) I used for BIOS and other applications where code or data is fixed by

    manufacturer and must not be overwritten during normal operations.


    Programmable ROM (PROM) are standardised blank chips like the Programmable Logic Arrays

    (PLA) where data or code can be stored once, but never erased afterwards (write-once). They are

    permanent memory chips programmed by the customer rather than by the chip manufacturer.

    They differ from a ROM chip, which is created and coded at the time of manufacture by the



    Erasable PROM (EPROM) is a special type of programmable read-only memory (PROM) that can

    be erased using a specific frequency of UV light. Usually it has a glass window on top of the chip to

    facilitate erasing.


    Electrically Erasable PROM (EEPROM) are erased using voltage higher than standard operating

    voltage instead of ultraviolet light. It must usually be removed from socket and transferred to an

    EEPROM programmable device.


    Flash ROM, is EEPROM that can be quickly erased and rewritten using standard operating voltage.

    It is used in smaller devices as non-volatile read-write secondary storage. EEPROM can be found in

    the following:

    MMC cards Compact Flash cards for cellphones and digital cameras Sony Memory stick PCMCIA Flash cards for laptops and handhelds USB pocket drives

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    3.4.3 Cache Memory

    Caching is a method used to improve performance when transferring data to and from a fast

    device to a relatively slow one. The purpose of cache memory is to speed up a computer while

    keeping its price low. Cache exists in different forms in computer but it primarily refers to the

    small memory locations found on the processor. The fastest cache RAM is found inside the CPU.

    Cache can also be found as small SRAM chips on the Motherboard. A quantity of memory is

    installed between the CPU and RAM, for example, and frequently accessed data from RAM is

    temporarily stored in cache. The next time the CPU tries to access the same data, the data is

    retrieved from cache, instead of the relatively slower RAM.

    There are several levels of cache in the processor of a computer these are Level 1 cache (L1),

    Level 2 cache (L2) and Level 3 cache (L3).

    When a program is being run, it will take data from the hard disk and place it in the RAM. RAM is

    closer to the CPU and is also faster to access but yet, it is still slower than the processor. To speed

    things up even more, the data that are going to be worked with are transferred from the RAM to

    the cache which means that when a program is being executed by the CPU, the latter will look for

    information in the cache first and not in the RAM.

    The problem is that the cache is very small (we do not have a lot of space on the processor to

    have a big amount of cache) and so cannot store a lot of information. Therefore before trying to

    access data from the cache, we must first check if it is there a cache hit. Else, we have a cache

    miss and we need to go retrieve the data from the RAM.Cache Size and processor performance

    The size of the cache is an important aspect that will determine the overall performance of

    processor. The cache size usually quoted is that of the Level 2 and Level 3. For example, on a

    Pentium Core i7, the size of the L2 cache is 1MB and L3 cache is 8MB.

    Cache memory is arranged in a hierarchy as shown below:

    Figure 2-9 Cache Hierarchy

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    The table below shows the approximate size of each component.

    Memory Type Size

    L1 Cache ~ 16 - 64 KB

    L2 Cache ~ 512 KB 2 MB

    L3 Cache ~ 8 MB

    RAM 256 MB 4 GB

    Hard Disk 50 GB 10 TB

    Table 2-2 Memory Type and SpeedAnother domain where the term cache is often used is the web. Connecting to the Internet

    (especially on a 56 KB modem) is very slow and also the data that we are trying to access might

    be for example in Hawaii making data retrieval an even slower process. So to try to improve the

    speed of accessing the page, that page can be stored in a web server in Mauritius the first time it

    is accessed and the next time another user wants access to the same page, the latters browser

    will not have to fetch the page from Hawaii but can instead get it directly from the web server in

    Mauritius, thus speeding up data access.

    3.4.4 Virtual Memory

    In practice a CPU executes multiple processes concurrently. Virtual Memory (VM) provides the

    basis for multi process operation. VM supports the illusion that multiple programs are running

    concurrently. Virtual Memory is not a physical memory l...