Systems Architecture,
Sixth Edition
Chapter 5
Data Storage Technology
Chapter Objectives
•  In this chapter, you will learn to:
–  Describe the distinguishing characteristics of
primary and secondary storage
–  Describe the devices used to implement primary
storage
–  Compare secondary storage alternatives
–  Describe factors that affect magnetic storage
devices
–  Explain how to choose appropriate secondary
storage technologies and devices
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FIGURE 5.1 Topics covered in this chapter
Courtesy of Course Technology/Cengage Learning
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Storage Device Characteristics
•  Consist of a read/write mechanism and a
storage medium
•  Storage medium: device or substance that
actually holds data
–  Device controller provides interface between
storage device and system bus
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FIGURE 5.2 Primary and secondary storage and their component devices
Courtesy of Course Technology/Cengage Learning
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Storage Device Characteristics
(continued)
•
•
•
•
•
Speed
Volatility
Access method
Portability
Cost and capacity
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Speed
•  Most important characteristic differentiating
primary and secondary storage
•  Primary storage extends the limited capacity of
CPU registers
•  Secondary storage speed influences execution
speed
•  Access time
•  Blocks and sectors
•  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
–  Holds 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
•  Typically implemented in two ways
–  Entire storage device (USB flash drive)
–  Storage medium can be removed (DVDs)
•  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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TABLE 5.1 Storage device characteristics and their relationship to cost
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FIGURE 5.3 Comparison of storage devices in terms of cost and access speed
Courtesy of Course Technology/Cengage Learning
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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 with semiconductors
–  Capability to read and write with equal speed
–  Random access to stored bytes, words, or larger
data units
•  Basic types
–  Static RAM (SRAM): implemented entirely with
transistors
–  Dynamic RAM (DRAM): uses a single transistor
and capacitor
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FIGURE 5.4 A flip-flop circuit composed of two NAND gates: the basic component
of SRAM and CPU registers
Courtesy of Course Technology/Cengage Learning
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Random Access Memory
•  To bridge performance gap between memory
and microprocessors
–  Read-ahead memory access
–  Synchronous read operations
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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 (continued)
•  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
–  Magnetoresistive RAM (MRAM)
–  Phase-change memory (PCM)
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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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FIGURE 5.5 From left to right, 30-pin SIMM, 72-pin SIMM, DDR DIMM, DDR2 DIMM,
and DDR2 DIMM for a laptop computer
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Magnetic Storage
•  Exploits duality of magnetism and electricity
–  Electrical current can generate a magnetic field
–  Magnetic field can generate electricity
•  Polarity of magnetic charge represents bit values
zero and one
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FIGURE 5.6 Principles of magnetic data storage
Courtesy of Course Technology/Cengage Learning
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Magnetic Storage (continued)
•  Devices must control or compensate for some
undesirable characteristics of magnetism and
magnetic storage media
–  Magnetic decay
–  Magnetic leakage
–  Minimum threshold current for read operations
–  Storage medium coercivity
–  Long-term storage medium integrity
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Magnetic Decay and Leakage
•  Magnetic decay
–  Tendency of magnetically charged particles to
lose their charge over time
–  Constant over time and proportional to the power
of the charge
•  Magnetic leakage
–  If the polarity of adjacent bits is opposite
•  Their magnetic fields tend to cancel out the charge
of both areas
•  Strength of both charges falls below read threshold
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Areal Density
•  Surface area allocated to a bit
•  Coercivity: capability of a substance or magnetic
storage medium to accept and hold a magnetic
charge
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FIGURE 5.7 Areal density is a function of a bit area’s length and width (a); density
can be quadrupled by halving the length and width of bit areas (b)
Courtesy of Course Technology/Cengage Learning
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Media Integrity
•  Depends on its construction and the
environmental factors it’s subjected to
•  Hard disk drives and MRAM have high media
integrity
•  Magnetic tape and others have thin coatings of
coercible material layered over a plastic or other
substrate
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TABLE 5.2 Factors leading to data loss in magnetic storage devices
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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
•  Compounds magnetic leakage; wraps upon itself
•  Susceptible to stretching, friction, and
temperature variations
•  Two geometric approaches to recording data
–  Linear recording
–  Helical scanning
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FIGURE 5.8 Components of a typical cassette or cartridge tape
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TABLE 5.3 Comparison of modern tape formats
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FIGURE 5.9 Data recorded in linear parallel tracks (a) and with helical
scanning (b)
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Magnetic Disk
•  Flat, circular platters with metallic coating that is
rotated beneath read/write heads
•  Data normally recorded on both sides
•  Tracks, cylinders, and sectors
•  Hard disks and drive arrays
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FIGURE 5.10 Primary components of a typical magnetic disk
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FIGURE 5.11 Organization of tracks and sectors on one surface of a disk platter
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Magnetic Disk (continued)
•  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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TABLE 5.5 Hard disk drive performance statistics
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Magnetic Disk (continued)
•  To increase capacity per platter, disk
manufacturers divide tracks into zones and vary
the sectors per track in each zone
FIGURE 5.13 A platter divided into
two zones with more sectors per
track in the outer zone
Courtesy of Course Technology/Cengage
Learning
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Solid-State Drives
•  Expected to supplant magnetic disk as the
dominant secondary storage technology
•  Mimics the behavior of a magnetic disk drive but
uses flash RAM or other NVM devices as the
storage medium and read/write mechanism
•  Currently implemented with the same physical
size and interfaces as magnetic drives
•  Have an advantage in power consumption
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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 Mass Storage Devices
(continued)
•  Optical storage devices read data by shining
laser beam on the disc.
FIGURE 5.14 Optical disc read
operations for a 1 bit (a) and a
0 bit (b)
Courtesy of Course Technology/Cengage
Learning
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TABLE 5.7 Technologies
and storage formats for
optical and magnetooptical storage
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CD-ROM, DVD-ROM, and BD
•  CD
–  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
•  DVD: standard format for movies and
audiovisual content
•  Blu-ray disc (BD): update to DVD-ROM
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CD-ROM, DVD-ROM, and BD
(continued)
•  DVD and BD technologies improve on CD
technology
–  Increased areal density achieved with smaller
wavelength lasers and more precise mechanical
control
–  Higher rotational speeds
–  Improved error correction
–  Multiple recording sides and layers
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Recordable Discs
•  Use a laser that can be switched between high
and low power and a laser-sensitive dye
embedded in the disc
•  Dye is stable when scanned at low power during
a read operation
•  Write operation is destructive: recordable
formats only written once
•  Should be stored in dark location at room
temperature
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Phase-Change Optical Discs
•  Enables nondestructive writing to optical storage
media
•  Materials change state easily from noncrystalline (amorphous), to crystalline, and then
back again
•  Reflective layer loses its capability to change
state with repeated heating and cooling
•  Current rewritable media wear out after about
1000 write operations
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Magneto-Optical Drives
•  Uses a laser and reflected light to sense
magnetically recorded bit values
•  Reading is based on the polarity of the reflected
laser light
•  Technology peaked in the mid-1990s
•  Still a market for MO drives because many
organizations created data archives on
magneto-optical discs and still need access to
these archives
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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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