1.1 Bits and Bit Patterns
CS11102 Introduction to Computer Science
Data Storage
1.1 Bits and Their Storage
1.2 Main Memory
1.3 Mass Storage
1.4 Representation of information as bit patterns
• Bit: Binary Digit (0 or 1)
• Bit Patterns are used to represent information.
– Numbers
– Text characters
– Images
– Sound
– And others
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Figure 2.1 CPU and main memory
connected via a bus
Boolean Operations
• Boolean Operation: An operation that
manipulates one or more true/false values
• Specific operations
– AND
– OR
– XOR (exclusive or)
– NOT
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Figure 1.1 The Boolean operations AND, OR, and
XOR (exclusive or)
As result (for an operation):
• Registers hold data immediately related to the
operation being executed.
• Memory is used to store data and programs
required in the near future.
• Auxiliary memory is used to store data and
programs required later.
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How the CU finds instructions and Data:
1.2 Main Memory
Memory:
It’s called also:
* Primary memory
* Main storage
• Internal storage
* Primary storage
* Main memory
* Internal memory
– It holds instructions and data of the program being
executed.
– Memory is Volatile M data is lost if the computer is turned
off.
– There is an address for each location in memory
(sometimes called numerical designator), CPU will
refer to this address to store / retrieve data.
– In computer programs, symbolic addresses are
used
Figure 1.7 The organization of a bytesize memory cell
Main Memory Cells
• Cell: A unit of main memory (typically 8 bits which is
one byte)
– Most significant bit: the bit at the left (high-order
end) of the conceptual row of bits in a memory cell
– Least significant bit: the bit at the right (low-order
end) of the conceptual row of bits in a memory cell
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Figure 1.8 Memory cells arranged by
address
Main Memory Addresses
• Address: A “name” that uniquely identifies one cell
in the computer’s main memory
– The names are actually numbers.
– These numbers are assigned consecutively starting
at zero.
– Numbering the cells in this manner associates an
order with the memory cells.
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Bits, Bytes and words:
0 or 1 is called a bit (Binary digit)
A group of bits is called a byte. A byte = 8 bits
210 = 1024 bytes = 1 kilobyte (KB)
210 x 210 bytes = million bytes = 1 megabytes.
(MB)
• 1 billions of bytes = 1 GigaByte (GB)
• 1 trillion of bytes = 1 TeraByte (TB)
• 1 quadrillion of bytes = 1 Petabyte (PB)
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RAM and ROM:
There are two types of memory chips:
• RAM (Random Access Memory): two categories:
– SRAM (Static RAM ):
• Static
• Faster
• No intervention from the CPU as long as power is maintained
– DRAM (Dynamic RAM) and SDRAM ( Synchronous DRAM)
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Dynamic
Must be constantly refreshed (recharged) by the CPU
Used for most PC memories because its size and cost advantages
SDRAM is faster than DRAM
• ROM (Read Only Memory) data cannot be changed.
1.3 Mass (Secondary) Storage
• Magnetic disk storage:
• Floppy Disks
• Hard Disks
• Tape
• Optical disk storage
• Compact disks
• DVD
• Blue-ray Disks
• Flash Drives
• Flash Drives
• Secure Digital (SD) Memory Card
Magnetic disk storage:
• Floppy Disks ; not in use
Made of flexible Mylar and coated with iron oxide
Magnetized spots on tracks on its surface
Composed of tracks and sectors
3.5-inch with 1.44MB
Portable
High capacity drives: up to 750MB
Magnetic disk storage:
• Hard Disks
Rigid platter coated with magnetic oxide that can be
magnetized
Varity of sizes
Several platters can be assembled into a disk pack
A Disk drive is a device that enables data to be read
from/written to a disk through a read/write head on
the end of an access arm
The read/write head does not match the surface,
otherwise data are destroyed (called head crash)
• Hard Disks (HD)
Removable HD are available
can be 100s of GB
Fast
Composed of sectors and tracks
A cluster is a fixed number of adjacent sectors that are
treated as one unit by OS.
A cylinder is track n on all platters
To reduce access time; Related data are stored on the
same cylinder (access arm movements are reduced)
How Data Is Organized in Hard Disks (HD)
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Track
Sector
Cluster
Cylinder
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Read/Write head
• the read/write heads do not touch the disk
but instead “float” just off the surface.
• head crash happens if the read/write touch
the surface of the disk
Track
• The circular portion of the disk
surface that passes under the
read/write head
– Floppy diskette has 80 tracks on
each surface
– Hard disk may have 1,000 or more
tracks on each surface of each
platter
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Cluster
Sector
• Each track is divided into
sectors that hold a fixed
number of bytes
• A fixed number of adjacent sectors that are
treated as a unit of storage
– Typically two to eight sectors, depending on the
operating system
– Typically 512 bytes per sector
• Zone bit recording assigns
more sectors to tracks in
outer zones than those in
inner zones
– Uses storage space more fully
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Cylinder
Disk formatting
• The location of tracks and sectors is not a
permanent part of a disk’s physical structure.
Instead, they are marked magnetically through a
process called formatting (or initializing) the disk.
• The track on each surface that is
beneath the read/write head at a
given position of the read/write
heads
– When file is larger than the capacity
of a single track, operating system
will store it in tracks within the same
cylinder
=> Formatting is the process of preparing the disk for
writing.
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measurements to evaluate a disk
performance
(1) seek time (the time required to move the read/write
heads from one track to another)
(2) rotation delay or latency time (the time required for
the desired data to rotate and to be under the
read/write head over the desired track);
(3) access time (the sum of seek time and rotation delay)
(4) transfer rate (the rate at which data can be
transferred to or from the disk).
• Magnetic Tape Storage
• Data is stored as extremely small magnetic spots
• Tape capacity: characters per inch (CPI) or Bytes per inch (BPI)
• Two heads: r/w head and erase-head
• Disk cache can be used to improve the performance
Secondary Storage
Disks vs. Magnetic Tapes
• Data on disks can be accessed directly or
sequentially, but tapes are only sequential
• tape systems have much longer data access times
than magnetic disk systems
• Optical disk storage:
• Metallic material spread over the surface of a disk
• Laser hits this surface to form spots that represent
0s and 1s
Digital Versatile Disk (DVD)
Compact Disks (CD)
• CDs are disks that are 12 centimeters
in diameter and consist of reflective
material covered with a clear
protective coating.
• Information on these CDs is stored on
a single track that spirals around the
CD, the track on a CD spirals from the
inside out
• CDs usually stores up to 700
• Examples on CD drives
• Short wavelength laser can read densely
packed spots
– DVD drive can read CD-ROMs
– Capacity up to 17GB
– Allows for full-length movies
– Sound is better than on audio CDs
– CD-ROM - drive can only read data from
CDs
– CD-R - drive can write to disk once
– CD-RW - drive can erase and record
multiple times
Copyright © 2003 by Prentice Hall
Blue-ray Optical Disks
• Several versions of writable and rewritable
DVDs exist
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Secondary Storage
Flash Memory
• 5 times as the capacity of DVDs
– Nonvolatile RAM
– Flash chips are used in cellular phones, digital
cameras, ..
– Requires less power and smaller that disk drives
– Examples on flash memory
Flash Drives
Secure Digital (SD) Memory Card (up to two GBs of storage)
SDHC (High Capacity) memory cards (up to 32 GBs)
the next generation SDXC (Extended Capacity) may exceed a TB.
Organizing data in Mass Storage
Files
File Storage and Retrieval
• Data file:
• File: A unit of data stored in mass storage
system
A Character
A Field: a group of characters
A Record ( Logical record): a group of fields
A File: a group of records
– A typical file may consist of a complete text
document, a photograph, a program, a music
recording, or a collection of data.
• Key field: an identifying field in a record. The value stored in a
key field is called key
• Database: a group of files
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Physical record versus Logical record
• A block of data that matches the size of a
sector is called a physical record. Thus, a large
file stored in mass storage will typically consist
of many physical records.
• Logical record corresponds to the natural data
record, eg. Student record. This may have a
size larger than the physical record, in which
case it will take more than one physical record
File Storage and Retrieval
1.4 Representation of information as
bit patterns
• Buffer: An area of memory used to hold data
on a temporary basis, usually during the
process of being transferred from one device
to another.
– Representing text
– Representing Numeric Values
– Representing Images
– Representing Sound
Figure 1.13 The message “Hello.” in
ASCII
Representing Text
• Each character (letter, punctuation, etc.) is assigned a
unique bit pattern.
– ASCII: Uses patterns of 7-bits to represent most
symbols used in written English text
– ISO developed a number of 8 bit extensions to ASCII,
each designed to accommodate a major language
group
– Unicode: Uses patterns of 16-bits to represent the
major symbols used in languages world wide
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Representing Numeric Values
Representing Images
• Binary notation: Uses bits to represent a
number in base two
• Limitations of computer representations of
numeric values
• Bit map techniques
– Pixel: short for “picture element”
– RGB
• Vector techniques
– Overflow: occurs when a value is too big to be
represented
– Truncation: occurs when a value cannot be
represented accurately
– Scalable
– TrueType and PostScript
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Figure 1.14 The sound wave represented by the
sequence 0, 1.5, 2.0, 1.5, 2.0, 3.0, 4.0, 3.0, 0
Representing Sound
• Sampling techniques
– Used for high quality recordings
– Records actual audio
• MIDI
– Used in music synthesizers
– Records “musical score”
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• Data manipulation