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7/28/2019 ICT in Orgs Chpt3 Input Output Devices
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CHAPTER 3: DATA STORAGE
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)
3.4.1.1 Types of RAM
3.4.2 Read-Only Memory (ROM)3.4.2.1 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
3.2 LEARNINGOBJECTIVES
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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3.3 DATA REPRESENTATION
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.
0111100001110101011111111000000111100..111111100001110101011111110000
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:
BCD EBCDIC ASCII Unicode
3.3.5.1 BCD
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
3.3.5.2 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:
0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F
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The Octal notation is base 8 and uses the following symbols:
0,1,2,3,4,5,6,7
3.3.5.3 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.
3.3.5.4 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
addresses.
3.4 PRIMARY 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:
ROM EPROM EEPROM Flash ROM (BIOS)
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
instructions.
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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3.4.1.1 Types of RAM
SRAM
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.
DRAM
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
milliseconds).
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.
SDRAM
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.
DDR SRAM
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.
3.4.2.1 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.
PROM
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
manufacturer.
EPROM
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.
EEPROM
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
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 like the cache memory. However, the
algorithm used by the virtual memory for swapping files from and to the main memory requires
some space from the secondary storage, i.e. the hard disk.
3.5 DATA ORGANISATION ON SECONDARY STORAGE
We should differentiate between the way data is storedand organizedon a storage medium and
how it is being accessed. Data is organized in similar manner on magnetic disk and optical disk but
differently on magnetic tapes
There are 2 methods for accessing data: Random Access
Random access refers to reading and writing of data in any order.
Sequential AccessSequential access refers to reading or writing data records in sequential order, that is, one
record after the other. For example, to read record 10, records 1 through 9 must be read.
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3.5.1 Data organization on Magnetic Disk
Data is organized and stored in concentric circular tracks on the disk. Each track is divided into
sectors. Each track has the same number of sectors and each sector stores the same number of
bits. Formatting involves the creation of tracks and sectors on a disk. Formatting wipes the disk.
Devices can also be classified as sequential access or random access.
3.5.2 Data organization on Magnetic Tape
Each bit is stored by magnetizing a small region of the tape surface. It is reliable, cheap, and
provides high capacity (many GB). Access time is relatively long since the tape must be read
sequentially. Magnetic tape is commonly used for backup of data files for the simple reason that
past updates in master files can be easily traced.
3.5.3 Data organization on Optical drives CDR/DVD
Data is organized in single spiral track which is read from the centre outwards. The bit density
along track is constant. The track is divided into sectors of approximately 2 KB and provides a
total capacity of around 10 GB. Each bit is stored as a mark or bump on the surface, and is read
using laser light. When an optical drive shines light into a pit, the light cannot be reflected back.
This represents a bit value of 0 (off). A land reflects light back to its source, representing a bit
value of 1 (on).
3.6 SUMMARY
Data used in computers is represented using binary notation symbolizing an on state with a
binary digit 1 and an off state with a binary digit 0.
To encode alphanumeric characters there are commonly 4 coding schemes BCD, EBDIC, ASCII
and Unicode. There are other schemes to encode multimedia files.
Cache memory as well as virtual memory speeds up the performance of computers. Cache is
physical and real whereas virtual memory makes us of a swapping algorithm and part of the hard
disk to enable multitasking.
Magnetic disk is a random access device whereas a magnetic tape is a sequential access device.