Transcript Systems Architecture, Fifth Edition

Chapter Goals
• Describe the distinguishing characteristics of
primary and secondary storage
• Describe the devices used to implement primary
storage
• Describe memory allocation schemes
• Compare and contrast secondary storage
technology alternatives
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Chapter Goals (continued)
• Describe factors that determine storage device
performance
• Choose appropriate secondary storage
technologies and devices
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Storage Devices
• Consist of a read/write mechanism and a storage
medium
– Device controller provides interface
• Primary storage devices
– Support immediate execution of programs
• Secondary storage devices
– Provide long-term storage of programs and data
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Characteristics of Storage Devices
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Speed
Volatility
Access method
Portability
Cost and capacity
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Speed
• Primary storage speed
– Typically faster than secondary storage speed by a
factor of 105 or more
– Expressed in nanoseconds (billionths of a second)
• Secondary storage speed
– Expressed in milliseconds (thousandths of a second)
• Data transfer rate = 1 second/access time (in seconds)
x unit of data transfer (in bytes)
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Volatility
• Primary storage devices are generally volatile
– Cannot reliably hold data for long periods
• Secondary storage devices are generally
nonvolatile
– Hold data without loss over long periods of time
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Access Method
• Serial access (linear)
• Random access (direct access)
• Parallel access (simultaneous)
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Portability
• Removable storage media with standardized
formats (e.g., compact disc and tape storage)
• Typically results in slower access speeds
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Cost and Capacity
• Cost increases:
– With improved speed, volatility, or portability
– As access method moves from serial to random to
parallel access method
• Primary storage - expensive (high speed and
combination of parallel/random access methods)
• Capacity of secondary storage devices is greater
than primary storage devices
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Memory-Storage Hierarchy
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Primary Storage Devices
• Critical performance characteristics
– Access speed
– Data transfer unit size
• Must closely match CPU speed and word size to
avoid wait states
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Storing Electrical Signals
• Directly
– By devices such as batteries and capacitors
– Trade off between access speed and volatility
• Indirectly
– Uses energy to alter the state of a device; inverse
process regenerates equivalent electrical signal
• Modern computers use memory implemented with
semiconductors (RAM and NVM)
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Random Access Memory
• Characteristics
– Microchip implementation using semiconductors
– Ability to read and write with equal speed
– Random access to stored bytes, words, or larger
data units
• Basic types
– Static RAM (SRAM) – uses transistors
– Dynamic RAM (DRAM) – uses transistors and
capacitors
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Random Access Memory
• To bridge performance gap between memory and
microprocessors
– Read-ahead memory access
– Synchronous read operations
– On-chip memory caches
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Nonvolatile Memory
• Random access memory with long-term or
permanent data retention
• Usually relegated to specialized roles and
secondary storage; slower write speeds and limited
number of rewrites
• Generations of devices (ROM, EPROM, and
EEPROM)
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Nonvolatile Memory
• Flash RAM (most common NVM)
– Competitive with DRAM in capacity and read
performance
– Relatively slow write speed
– Limited number of write cycles
• NVM technologies under development
– Ferroelectric RAM
– Polymer memory
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Memory Packaging
• Dual in-line packages (DIPs)
– Early RAM and ROM circuits
• Single in-line memory module (SIMM)
– Standard RAM package in late 1980s
• Double in-line memory module (DIMM)
– Newer packaging standard
– A SIMM with independent electrical contacts on
both sides of the module
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CPU Memory Access
• Critical design issues for primary storage devices
and processors
– Physical organization of memory
– Organization of programs and data within memory
– Method(s) of referencing specific memory
locations
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Physical Memory Organization
• Physical memory
– Actual number of memory bytes that physically are
installed in the machine
• Most and least significant bytes
• Big endian and little endian
• Addressable memory
– Highest numbered storage byte that can be
represented
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Memory Allocation and
Addressing
• Memory allocation
– Assignment of specific memory addresses to
system software, application programs, and data
• Absolute addressing
• Indirect addressing (relative addressing)
– Offset register
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Magnetic Storage
• Exploits duality of magnetism and electricity
– Converts electrical signals into magnetic charges
– Captures magnetic charge on a storage medium
– Later regenerates electrical current from stored
magnetic charge
• Polarity of magnetic charge represents bit values
zero and one
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Magnetic Tape
• Ribbon of plastic with a coercible (usually
metallic oxide) surface coating
• Mounts in a tape drive for reading and writing
• Relatively slow serial access
• Compounds magnetic leakage; wraps upon itself
• Susceptible to stretching, friction, temperature
variations
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Magnetic Tape
• Two approaches to recording data
– Linear recording
– Helical scanning
• Several formats and standards (e.g., DDS [DAT],
AIT, Mammoth, DLT, LTO)
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Magnetic Disk
• Flat, circular platter with metallic coating that is
rotated beneath read/write heads
• Random access device; read/write head can be
moved to any location on the platter
• Hard disks and floppy disks
• Cost performance leader for general-purpose
on-line secondary storage
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Magnetic Disk Access Time
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Head-to-head switching time
Track-to-track seek time
Rotational delay
Most important performance numbers
– Average access time
– Sequential access time
– Sustained data transfer rate
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To increase capacity per platter, disk manufacturers
divide tracks into zones and vary the sectors per track
in each zone.
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Optical Mass Storage Devices
• Store bit values as variations in light reflection
• Higher areal density and longer data life than
magnetic storage
• Standardized and relatively inexpensive
• Uses: read-only storage with low performance
requirements, applications with high capacity
requirements, and where portability in a
standardized format is needed
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Optical storage devices read data by shining laser
beam on the disc.
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CD-ROM
• Read-only; data permanently embedded in durable
polycarbonate disc
• Bit values represented as flat areas (lands) and
concave dents (pits) in the reflective layer
• Data recorded in single continuous track that
spirals outward from center of disc
• Popular medium for distributing software and
large data sets
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CD-ROM
Advantages
Drawbacks
• Standardized format
• High density
• Cheap to manufacture
• Cannot be rewritten
• Capacity limited to 700
MB
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CD-R
• Uses a laser that can be switched between high
and low power and a laser-sensitive dye embedded
in the disc
• Relatively cheap
• Common uses: create music CDs on home
computers, back up data from other storage
devices, create archives of large data sets, and
manufacture small quantities of identical CDs
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Magneto-Optical
• Uses a laser and reflected light to sense bit values
• Technology peaked in the mid 1990s
• Advantages over CD-RW in access speed and
capacity
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Phase-Change Optical Discs
• Enables nondestructive writing to optical storage
media
• Materials change state easily from non-crystalline
(amorphous), to crystalline, and then back again
– Reflective layer is a compound of tellurium,
selenium, and tin
• Example: CD-RW
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DVD
• Improves on CD and CD-RW technology
– Increased track and bit density: smaller wavelength
lasers and more precise mechanical control
– Improved error correction
– Multiple recording sites and layers
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Summary
• Storage devices and their underlying technologies
• Characteristics common to all storage devices
• Technology, strengths, and weaknesses of primary
and secondary storage
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