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Data Storage Technology
By Maika
Introduction
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A storage device consists of a read/write
mechanism and a storage medium.
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The storage medium is the device or
substance that actually holds data.
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The read/write mechanism is the
device used to read or write data to and
from the storage medium.
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A typical computer system has many storage devices
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Primary storage: Static RAM (SRAM)
Dynamic RAM (DRAM)
Flash RAM
ROM
Secondary storage: Magnetic tape
Magnetic disk
Optical disk
Multiple devices are required because no one device
or technology can cost-effectively meet all storage
needs for a single computer system or user.
Primary Characteristics
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Speed
Volatility
Access method
Portability
Cost and capacity
Speed
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Is the most important characteristic that
differentiate primary and secondary storage
With current technology, primary storage
speed is typically greater than secondary
storage speed by a factor of 105 or more.
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Storage device speed is called access time.
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Is the time required to execute on read or write
operation completely.
Access time of primary storage devices generally
are expressed in nanoseconds.
Access time of secondary storage devices
typically are expressed in milliseconds.
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A complete measure of data access speed consists
of access time and the unit of data transfer to/from
the storage device.
Data transfer units vary from one storage device to
another:
 The data transfer unit for primary storage
devices is usually a word.
 The data transfer unit for secondary storage
devices is block.
 The date transfer unit for magnetic and optical
disk drives is sector.
A storage device’s data transfer rate is computed by
dividing 1 by the access time and multiplying the
result by the unit of data transfer.
Eg. The data transfer rate for a primary storage device
with 15 nanosecond access time and a 32-bit word
data transfer unit can be computed as:
(1second / 15 nanoseconds) * 32 bits
= 266,666,667 (bytes/second)
Volatility
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A storage device or medium is non-volatile if it holds
data without loss over long periods of time.
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A storage device or medium is volatile if it cannot
reliably hold data for long periods.
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Primary storage devices are generally volatile.
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Secondary storage devices are generally non-volatile.
Access Method
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The physical structure of a storage device’s read/write
mechanism and storage medium determines the
way(s) in which data can be accessed.
Three types of access methods:
 Serial access
 Radom access
 Parallel access
Serial Access

A serial access storage device stores and retrieves
data items in a linear or sequential order.
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Once written, data can be read back only in that same
order.
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Serial access time depends on the current position of
the read/write mechanism and on the position of the
desired date item within the storage medium.
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Serial access devices are not used for frequently
accessed data but for holding backup copies of data
stored on other storage devices.
Random Access

A random access device is not restricted to any
specific order when accessing data but directly access
any desired data item stored on the storage medium.

All primary storage devices and disk storage devices
are random access devices.
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Access time may or may not be a constant.
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It is a constant for most primary storage devices.
It is not a constant for disk storage.
Parallel Access
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A parallel access device is capable of simultaneously
accessing multiple storage locations.
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If one considers the unit of data access to be a bit,
then access is parallel.
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Parallel access also can be implemented by
subdividing data items and storing the component
pieces on multiple storage devices.
Portability
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Data can be made portable by storing it on a
removable storage medium or device.
External disk drives are portable if they can be
added or removed easily from a computer system
and transported without damage to the device or
its data content.
Portable devices typically sacrifice access speed to
compensate for the lack of environmental control.
Cost and Capacity
Each of the
storage
device
attributes is
directly
related to
device cost
Characteristics
Cost
Speed
Volatility
Cost increases as speed increases
For devices of similar type, cost
decreases as volatility increases
Serial is the least expensive,
Random is more expensive than serial,
Parallel is more expensive than nonparallel
For devices of similar type, portability
increases cost
Cost usually increases in direct
proportion to capacity
Access method
Portability
Capacity
Primary Storage Devices
Storing Electrical Signals
• Data are represented within the CPU as digital electrical
signals – the basis of data transmission among all
devices attached to the system bus
• Electrical power can be stored directly by various devices
including batteries and capacitors
• An electrical signal can be stored indirectly by using its
energy to alter the state of a device such as a
mechanical switch
Random Access Memory (RAM)
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RAM is a generic term describing primary
storage devices with the following
characteristics:
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Microchip implementation using semiconductors
Ability to read and write with equal speed
Random access to stored bytes, words, or larger
data units
There are two basic RAM types
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Static RAM (SRAM)
Dynamic RAM (DRAM)
Static RAM
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Is implemented entirely with transistors
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The basic unit of storage is a flip-flop
circuit
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A flip-flop circuit which contains two
transistors for bit storage is an electrical
switch that remembers its last position
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SRAM is volatile unless a continuous
supply of power can be guaranteed.
Dynamic RAM
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Uses transistors and capacitors.
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The capacitors are the dynamic element
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DRAM chips include circuitry that
automatically performs refresh operations
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Each refresh operation is called a refresh
cycle
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SRAM is more expensive to fabricate
than DRAM due to its more complex
circuitry.
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DRAM is slower than SRAM due to its
required refresh cycles.
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Neither RAM type can match current
microprocessor clock rates which range
from 500MHz to 1.5GHz.
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The fastest DRAM is at least 25 times
slower than modern microprocessors.
SRAM is at least 2.5 times slower.
Read-Only Memory (ROM)
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Is a random access memory device that can store
data permanently or semipermanently.
Is used to stored programs such as computer
system boot subroutines like the system BIOS.
Two ROM technologies are currently in use are:
Electrically Erasable Programmable Read-Only
Memory (EEPROM) and Flash Memory.
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EEPROM can be programmed, erased, and
reprogrammed by signals sent from an external control
source, such as a CPU.
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The primary drawback of EEPROM is that read and
erase operations require relatively high voltages
and repeated cycles to cause permanent change.
Flash Memory can be erased and rewritten much more
quickly and is used for programs and data that aren’t
frequently updated.
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It tends to wear out after 100,000 or more write
operations, making it unsuitable for primary
storage.
Memory Packaging
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Memory circuits are embedded within microchips and groups of
chips are packed on a small circuit board that can be installed
or removed easily from a computer system.
Early RAM and ROM circuits were packaged in dual in-line
packaged (DIPs)
Later, single in-line memory module (SIMM) is adopted
which incorporated multiple DIPs on a tiny printed circuit board.
The double in-line memory module (DIMM) is a newer
packaging standard. It is a double-sided SIMM with memory
DIPs and electrical contacts on both sides of the module.
CPU Memory Access
The physical organization of memory,
The organization of programs and data within memory,
And the method(s) of referencing specific memory locations
are critical design issues for both primary and secondary
storage devices and processors.
Physical Memory Organization
The main memory can be regarded as a sequence of
contiguous or adjacent memory cells.
Cell 0
Cell 1
Cell 2
Cell 3
Cell 4
In a byte sequence, the leftmost byte is called the
most significant byte and the rightmost byte is called
the least significant byte.
 Big endian describes architectures that store the most
significant byte at the lowest memory address.
 Little endian describes architecture that store the least
significant byte at the lowest memory address.
 The addressable memory of a CPU is the highest
numbered storage byte that can be represented.
 Addressable memory is determined by the number of
bits used to represent an address.
 Physical memory is the actual number of memory bytes
that physically are installed in the machine.
 Physical memory is usually less than addressable
memory.
Memory Allocation and Addressing
Memory allocation
describes the
assignment of
specific memory
address to system
software, application
programs, and data.
Unused
High memory
Program data
Program code
Program
offset
FFFFH
Operating
system
3000H
0000H
Low memory
 Absolute addressing describes memory address
operands that refer to actual physical memory locations.
 Usually, the operating system calculates and stores the
program offset in a register when the program first is
loaded into memory.
 During program execution, the CPU automatically adds
the program offset to all memory address operands
before accessing memory.
 The method of automatically computing physical
memory addresses is called indirect addressing.
 The register that holds the offset value is called offset
register.
Magnetic Storage
►A
magnetic storage device
 converts electrical signals into magnetic charges,
 captures the magnetic charge on a storage medium,
 and later uses the stored magnetic charge to generate
an electrical current.
► Magnetic
storage devices must control or
compensate for some undesirable characteristics
of magnetism and magnetic storage media
Magnetic Decay and Leakage
Magnetic decay:
►
Is the tendency of magnetically charged particles to lose
their charge over time.
►
Is constant over time and proportional to the power of the
charge.
Will cause the stored charge power to fall below the
threshold required for a successful read operation
 the data content of the storage medium effectively is lost.
►
Magnetic leakage:
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The strength of individual bit charges also can decrease
due to magnetic leakage from adjacent bits.
Storage Density
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Coercivity is the ability of a substance or magnetic
storage medium to accept and hold magnetic charge. It
varies widely among elements and compounds.
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The amount of surface area allocated to a bit is referred
to as the recording density, which is expressed in bits,
bytes, or tracks per inch.
TIP: Designers and purchasers of magnetic media and
devices must find a suitable balance between high
recording density and the reliability of the media,
especially over relatively long periods of time.
Media Integrity
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The integrity of magnetic storage medium depends on
the nature of its construction and the environmental
factors to which it is subjected.
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Loss of coercible coating represent a loss of strength in
stored magnetic charges  data becomes unreadable
when the remaining charge falls below the threshold of
readability.
TIP: To extent the life of magnetic media, they must be
protected from physical abuse and temperature and
humidity extremes
Magnetic Tape
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Is a ribbon of plastic with a coercible surface
coating.
Tapes are mounted in a tape drive for reading and
writing.
Tape drive contains motors that wind and unwind
the tape.
Tapes primarily are used to make back up copies
of data stored on faster secondary storage devices
and to physically transport large data sets.
► There
are two basic approaches to record
data onto a tape surface:
 Linear recording places bits along parallel
tracks that run along the entire length of the
tape.
 Helical scanning reads and writes data to or
from a tape by rotating the read/write head at an
angle to the tape and moving from tape edge to
tape edge.
Magnetic disk
 Magnetic disk media are flat, circular platters with metallic
coating that are rotated beneath read/write heads.
 Data normally are recorded on both sides of a platter.
 A track is one concentric circle of a platter.
 A sector is a fractional portion of a track.
 A read/write head is placed for each side of each platter.
 Read/write heads are mounted on the end of an access
arm.
 Access arms are attached to a positioning servo.
 The servo moves the access arms so that the read/write
heads can be positioned anywhere between the outermost
track and the innermost track.
Hard disk:
 Is a magnetic disk media with a rigid metal base.
 Typical platter size is between three and five inches
in diameter.
 High speeds is up to 10,000 revolutions per minute.
 Drive capacity depends on the number of platters,
platter size, and recording density.
 Multiple hard drives often are enclosed in a single
storage cabinet and referred to as a drive array.
Floppy disk (diskette):
 Uses a base of flexible or rigid plastic material.
 Rang from 2.5 to 3.5 inches in diameter.
 Contains a single flexible plastic platter coated with an
iron or other metallic compound on both sides.
 The platter is mounted permanently in a plastic case
that can be removed from a diskette drive.
 The case has an access door that is closed when the
diskette is removed from the drive and opened
automatically when the diskette is inserted into the
derive.
Disk access time depends on several factors including:
 Time required to switch among read/write heads
 Time required to position the read/write heads
 Rotational delay – the time that the disk controller
must wait for the proper sector to rotate beneath the
heads
Optical Mass Storage Devices
Came of age in the 1990s
Primary advantages: higher recording density and
longer data life
Can retain data for decades
Not subject to problems of magnetic decay and
leakage
Popular due to standardized and relatively
inexpensive storage media.
Store bit values as variations in light reflection
Optical storage devices are available currently in
a wide variety of storage formats and write
technologies including:
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CD-ROM
WORM (CD-R)
Magneto-optical
CD-RW
DVD