Systems Architecture, Fifth Edition

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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

• 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-purposeon-lin
  e secondary storage

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Magnetic Disk Access Time

• 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
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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