2 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

3 Systems Architecture, Fifth Edition Chapter Goals (continued) Describe factors that determine storage device performance Choose appropriate secondary storage technologies and devices

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5 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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7 Characteristics of Storage Devices Speed Volatility Access method Portability Cost and capacity

8 Systems Architecture, Fifth Edition 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)

9 Systems Architecture, Fifth Edition 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

10 Systems Architecture, Fifth Edition Access Method Serial access (linear) Random access (direct access) Parallel access (simultaneous)

11 Systems Architecture, Fifth Edition Portability Removable storage media with standardized formats (e.g., compact disc and tape storage) Typically results in slower access speeds

12 Systems Architecture, Fifth Edition 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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14 Systems Architecture, Fifth Edition Memory-Storage Hierarchy

15 Systems Architecture, Fifth Edition Primary Storage Devices Critical performance characteristics –Access speed –Data transfer unit size Must closely match CPU speed and word size to avoid wait states

16 Systems Architecture, Fifth Edition 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)

17 Systems Architecture, Fifth Edition 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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19 Systems Architecture, Fifth Edition Random Access Memory To bridge performance gap between memory and microprocessors –Read-ahead memory access –Synchronous read operations –On-chip memory caches

20 Systems Architecture, Fifth Edition 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)

21 Systems Architecture, Fifth Edition 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

22 Systems Architecture, Fifth Edition 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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24 Systems Architecture, Fifth Edition 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

25 Systems Architecture, Fifth Edition 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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27 Systems Architecture, Fifth Edition 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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30 Systems Architecture, Fifth Edition 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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34 Systems Architecture, Fifth Edition 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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36 Systems Architecture, Fifth Edition 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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38 Systems Architecture, Fifth Edition 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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41 Systems Architecture, Fifth Edition 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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43 Systems Architecture, Fifth Edition To increase capacity per platter, disk manufacturers divide tracks into zones and vary the sectors per track in each zone.

44 Systems Architecture, Fifth Edition 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

45 Systems Architecture, Fifth Edition Optical storage devices read data by shining laser beam on the disc.

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47 Systems Architecture, Fifth Edition 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

48 Systems Architecture, Fifth Edition CD-ROM AdvantagesDrawbacks Standardized format High density Cheap to manufacture Cannot be rewritten Capacity limited to 700 MB

49 Systems Architecture, Fifth Edition 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

50 Systems Architecture, Fifth Edition 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

51 Systems Architecture, Fifth Edition 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

52 Systems Architecture, Fifth Edition 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

53 Systems Architecture, Fifth Edition Summary Storage devices and their underlying technologies Characteristics common to all storage devices Technology, strengths, and weaknesses of primary and secondary storage