Chapter Goals
description
Transcript of Chapter Goals
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2Systems 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
• 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
• 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