Transcript Chapter 1 Computer Technology: Your Need to Know

Chapter 5
Data Storage Technology
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Chapter 5
Chapter Goals
• Describe the distinguishing characteristics of
primary and secondary storage
• Describe the devices used to implement
primary storage
• Describe the memory allocation schemes
• Compare and contrast secondary storage
technology alternatives
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Chapter Goals
• Describe factors that determine storage
device performance
• Choose appropriate secondary storage
technologies and devices
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Chapter Topics
• Explore storage devices and their
technologies
• Outlines characteristics common to all
storage devices
• Explains the technology strengths and
weaknesses of primary storage and
secondary storage
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Storage Device Characteristics
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Storage Device Characteristics
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Speed
Volatility
Access method
Portability
Cost and capacity
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Storage Device Characteristics
Speed
• The delay between a user request for
program execution and the first prompt for the
user input depends on the speed of primary
and secondary storage devices.
• Primary storage speed is typically greater
than secondary storage speed by a factor of
105 or more.
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Storage Device Characteristics
Speed
• Secondary storage device speed is called
access time.
• The access time for reading and writing is
assumed to be the same unless otherwise
stated.
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Storage Device Characteristics
Volatility
• A storage device or medium is non-volatile if
it holds data without loss over long periods of
time.
• A storage device or medium is volatile if it
cannot reliably hold data for long periods of
time.
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Storage Device Characteristics
Volatility
• Primary storage devices are generally
volatile.
• Secondary storage devices are generally
non-volatile.
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Storage Device Characteristics
• Access Method
– Serial Access
– Random Access
– Parallel Access
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Storage Devices Characteristics
• Serial Access – stores and retrieve data
items in a linear, or sequential order.
(Magnetic tape)
• Random Access (Direct Access) – access
device is not restricted to any specific order
when accessing data. (Hard Disk)
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Storage Device Characteristics
• Parallel Access – a device that is capable of
simultaneously accessing multiple storage
locations. (Random Access Memory)
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Storage Device Characteristics
Portability
• Data can be made portable by storing it on a
removable storage medium or device.
• Portable devices typically have slower access
speed than permanently installed devices and
those with non-removable media.
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Storage Device Characteristics
Cost and Capability
• An increase in speed, permanence or
portability generally comes at increased cost
if all other factors are held constant.
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Storage Device Characteristics
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Memory-Storage Hierarchy
• Cost and access speed generally decrease
as one moves down the hierarchy.
• Due to lower cost, capacity tends to increase
as one moves down the hierarchy.
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Storage Device Characteristics
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Primary Storage Devices
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Storing Electrical Signals
Random Access Memory
Read-Only Memory
Memory Packaging
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Primary Storage Devices
Storing Electrical Signals
• Data is represented as electrical signals.
• Digital signals are used to transmit data to
and from devices attached to the system bus.
• Storage devices must accept electrical
signals as input and output.
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Primary Storage Devices
Random Access Memory
Random Access Memory describes
primary storage devices with these
characteristics:
– Microchip implementation using semiconductors
– Ability to read and write with equal speed
– Random access to stored bytes, words, or larger
data units
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Primary Storage Devices
Random Access Memory
Two types of memory:
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Static RAM (SRAM)
Dynamic RAM (DRAM)
Synchronous DRAM (SDRAM)
Ferroelectric RAM
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Primary Storage Devices
Static RAM
• Implemented with transistors.
• Basic unit of storage is a flip-flop circuit.
• A flip-flop is an electrical circuit that
remembers its last position.
• One position represents 1, the other position
represents 0.
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Dynamic RAM
• Uses transistors and capacitors.
• Lose their charge quickly.
• Require a fresh infusion of power thousands
of times per second.
• Each refresh operation is called a refresh
cycle.
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Primary Storage Operations
Synchronous DRAM
• Read-ahead RAM that uses the same clock
pulse as the system bus.
• Read and write operations are broken into a
series of simple steps and each step can be
completed in one bus clock cycle.
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Ferroelectric RAM
Embeds iron or iron compounds within a
microchip to store bits in much the same
manner as old-fashioned core memory.
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Primary Storage Devices
Read-Only Memory
• Electronically Erasable Programmable ReadOnly Memory (EEPROM)
• Flash Memory
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Primary Storage Devices
Read-Only Memory – a random access
memory device that can store data
permanently or semipermanently.
Instructions that reside in ROM are called
firmware.
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Electronically Erasable Programmable
Read-Only Memory – can be programmed,
erased, and reprogrammed by signals sent
from and external control source, such as a
CPU.
Flash Memory – can be erased and rewritten
more quickly.
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Memory Packaging
• Dual In-line Packages (DIPs)
• Single In-line Memory Module (SIMM)
• Double In-line Memory Module (DIMM)
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Memory Packaging
Memory circuits are embedded within
microchips and groups of chips are packed
on a small circuit board that can be installed
or removed easily.
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CPU Memory Access
• Physical Memory Organization
• Memory Allocation and Addressing
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CPU Memory Access
Physical Memory Organization
Main memory can be regarded as a
sequence of contiguous, or adjacent memory
cells.
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Physical Storage Devices
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Memory Allocation and
Addressing
Memory Allocation – describes the
assignment of specific memory addresses to
system software, application programs and
data.
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Memory Allocation and
Addressing
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Memory Allocation and
Addressing
Memory Addressing:
• Absolute Addressing – describes memory
address operands that refer to actual physical
memory locations.
• Relative Addressing (Indirect Addressing)
– automatically computes physical memory
addresses.
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Memory Allocation and
Addressing
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Memory Allocation and
Addressing
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Magnetic Storage
Magnetic Storage
• Exploit the duality of magnetism and
electricity.
• Electric current is used to generate a
magnetic field.
• A magnetic field can be used to generate
electricity.
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Magnetic Storage
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Magnetic Storage
Disadvantages of Magnetism
• Magnetic decay
• Magnetic leakage
• Minimum threshold current for read
operations
• Storage medium coercivity
• Long-term storage medium integrity
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Magnetic Storage
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Magnetic Storage
Magnetic Decay and Leakage
Magnetic Decay – the tendency of
magnetically charges particles to lose their
charge over time.
Magnetic Leakage – a decrease in the
strength of individual bit charges.
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Magnetic Storage
Storage Density
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Magnetic Storage
Magnetic Integrity
Depends on the nature of the storage
medium’s construction and the environmental
factors.
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Magnetic Storage
Magnetic Storage Devices:
• Magnetic Tape
• Magnetic Disk
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Magnetic Tape
• Ribbon of plastic with a coercible surface
coating.
• Mounted in a tape drive for reading and
writing.
• Compound the magnetic leakage problem by
winding the tape upon itself.
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Magnetic Storage
Magnetic Tape
Approaches to recording data:
• Linear recording
• Helical recording
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Magnetic Storage
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Magnetic Storage
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Magnetic Storage
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Technology Focus
Magnetic Tape Formats and Standards
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Magnetic Storage
Magnetic Disk
• Flat circular platters with metallic coatings
that are rotated beneath read/write heads
• Multiple platters can be mounted.
• Once concentric circle is a track.
• A fractional portion of a track is a cylinder.
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Magnetic Storage
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Magnetic Storage
Magnetic Disk Types:
• Hard Disk – magnetic disk media with a rigid
metal base.
• Floppy Disk (Diskette) – uses a base of
flexible or rigid plastic material.
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Magnetic Storage
Magnetic Disk
Disk access time depends on several
factor including:
– Time required to switch among read/write heads
– Time required to position the read/write heads
– Rotational delay
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Technology Focus
Whither the Floppy Disk
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Optical Mass Storage Devices
• Advantages:
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Higher recording density
Longer data life
Retain data for decades
Not subject to problems of magnetic decay and
leakage
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Optical Mass Storage Devices
• Optical storage devices store bit values as
variations in light reflection.
• Storage medium is a surface of highly
reflective material.
• The read mechanism consists of a low-power
laser and a photoelectric cell.
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Optical Mass Storage Devices
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CD-ROM
WORM (CD-R)
Magneto-optical
CD-RW
DVD
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CD-ROM
• Originally developed for storing and
distributing music (CD-DA).
• Includes additional formatting to store the
directory and file information.
• Holds approximately 650 MB.
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WORM
• Manufactured with all bit areas in a highly
reflective state.
• When a bit area is changed to low reflectivity,
the process is irreversible.
• Use high powered lasers to burn holes in the
reflective layer.
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CD-R
• Cheaper technology than WORM.
• Use a laser that can be switched between
high and low power.
• Uses a laser-sensitive dye embedded in the
CD-R disc.
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Magneto-Optical
• Uses a laser and reflective light to sense bit
values.
• Applies a magnetic charge in a bit area.
• The magnetic charge shifts the polarity of the
reflected laser light.
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Phase-Change Optical Discs (CD-RW)
• Allows optical storage media to be written
non-destructively.
• Based on materials that can change state
easily.
• The difference can be detected by newer
optical scanning technologies.
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Summary
• A typical computer system has primary and
secondary storage devices.
• The critical performance characteristics of
primary storage devices are their access
speed and the number of bits that can be
accessed in a single read or write operation.
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Summary
• Programs generally are created as through
they occupied contiguous primary storage
locations starting at the first location.
• Magnetic storage storage devices store data
bits as magnetic charges.
• Optical discs store data bits as variations in
light reflection.
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