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Chapter 5

Data Storage Technology

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Systems Architecture Chapter 5

Chapter Goals

• Describe the distinguishing characteristics of primary and secondary storage

• Describe the devices used to implement primary storage

• Describe the memory allocation schemes• Compare and contrast secondary storage

technology alternatives

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Chapter Goals

• Describe factors that determine storage device performance

• Choose appropriate secondary storage technologies and devices

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Chapter Topics

• Explore storage devices and their technologies

• Outlines characteristics common to all storage devices

• Explains the technology strengths and weaknesses of primary storage and secondary storage

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Storage Device Characteristics

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• Speed• Volatility• Access method• Portability• Cost and capacity

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Speed

• The delay between a user request for program execution and the first prompt for the user input depends on the speed of primary and secondary storage devices.

• Primary storage speed is typically greater than secondary storage speed by a factor of 105 or more.

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Speed

• Secondary storage device speed is called access time.

• The access time for reading and writing is assumed to be the same unless otherwise stated.

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Volatility

• A storage device or medium is non-volatile if it holds data without loss over long periods of time.

• A storage device or medium is volatile if it cannot reliably hold data for long periods of time.

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Volatility

• Primary storage devices are generally volatile.

• Secondary storage devices are generally non-volatile.

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• Access Method

– Serial Access– Random Access– Parallel Access

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Storage Devices Characteristics

• Serial Access – stores and retrieve data items in a linear, or sequential order. (Magnetic tape)

• Random Access (Direct Access) – access device is not restricted to any specific order when accessing data. (Hard Disk)

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• Parallel Access – a device that is capable of simultaneously accessing multiple storage locations. (Random Access Memory)

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Portability

• Data can be made portable by storing it on a removable storage medium or device.

• Portable devices typically have slower access speed than permanently installed devices and those with non-removable media.

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Cost and Capability

• An increase in speed, permanence or portability generally comes at increased cost if all other factors are held constant.

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Memory-Storage Hierarchy

• Cost and access speed generally decrease as one moves down the hierarchy.

• Due to lower cost, capacity tends to increase as one moves down the hierarchy.

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Primary Storage Devices

• Storing Electrical Signals• Random Access Memory• Read-Only Memory• Memory Packaging

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Storing Electrical Signals

• Data is represented as electrical signals.• Digital signals are used to transmit data to

and from devices attached to the system bus.• Storage devices must accept electrical

signals as input and output.

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Random Access Memory

Random Access Memory describes primary storage devices with these characteristics:

– Microchip implementation using semiconductors– Ability to read and write with equal speed– Random access to stored bytes, words, or larger

data units

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Random Access Memory

Two types of memory:– Static RAM (SRAM)– Dynamic RAM (DRAM)– Synchronous DRAM (SDRAM)– Ferroelectric RAM

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Static RAM• Implemented with transistors.• Basic unit of storage is a flip-flop circuit.• A flip-flop is an electrical circuit that

remembers its last position.• One position represents 1, the other position

represents 0.

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Dynamic RAM

• Uses transistors and capacitors.• Lose their charge quickly.• Require a fresh infusion of power thousands

of times per second.• Each refresh operation is called a refresh

cycle.

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Primary Storage Operations

Synchronous DRAM

• Read-ahead RAM that uses the same clock pulse as the system bus.

• Read and write operations are broken into a series of simple steps and each step can be completed in one bus clock cycle.

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Ferroelectric RAM

Embeds iron or iron compounds within a microchip to store bits in much the same manner as old-fashioned core memory.

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Read-Only Memory

• Electronically Erasable Programmable Read-Only Memory (EEPROM)

• Flash Memory

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Read-Only Memory – a random access memory device that can store data permanently or semipermanently.

Instructions that reside in ROM are called firmware.

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Electronically Erasable Programmable Read-Only Memory – can be programmed, erased, and reprogrammed by signals sent from and external control source, such as a CPU.

Flash Memory – can be erased and rewritten more quickly.

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Memory Packaging

• Dual In-line Packages (DIPs)• Single In-line Memory Module (SIMM)• Double In-line Memory Module (DIMM)

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Memory Packaging

Memory circuits are embedded within microchips and groups of chips are packed on a small circuit board that can be installed or removed easily.

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CPU Memory Access

• Physical Memory Organization• Memory Allocation and Addressing

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CPU Memory Access

Physical Memory Organization

Main memory can be regarded as a sequence of contiguous, or adjacent memory cells.

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Physical Storage Devices

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Memory Allocation and Addressing

Memory Allocation – describes the assignment of specific memory addresses to system software, application programs and data.

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Memory Allocation and AddressingMemory Addressing:

• Absolute Addressing – describes memory address operands that refer to actual physical memory locations.

• Relative Addressing (Indirect Addressing) – automatically computes physical memory addresses.

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Magnetic Storage

Magnetic Storage• Exploit the duality of magnetism and

electricity.• Electric current is used to generate a

magnetic field.• A magnetic field can be used to generate

electricity.

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Magnetic Storage

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Magnetic Storage

Disadvantages of Magnetism• Magnetic decay• Magnetic leakage• Minimum threshold current for read

operations• Storage medium coercivity• Long-term storage medium integrity

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Magnetic Storage

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Magnetic Storage

Magnetic Decay and Leakage

Magnetic Decay – the tendency of magnetically charges particles to lose their charge over time.

Magnetic Leakage – a decrease in the strength of individual bit charges.

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Magnetic Storage

Storage Density

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Magnetic Storage

Magnetic Integrity

Depends on the nature of the storage medium’s construction and the environmental factors.

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Magnetic Storage

Magnetic Storage Devices:

• Magnetic Tape• Magnetic Disk

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Magnetic Storage

Magnetic Tape

• Ribbon of plastic with a coercible surface coating.

• Mounted in a tape drive for reading and writing.

• Compound the magnetic leakage problem by winding the tape upon itself.

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Magnetic Storage

Magnetic Tape

Approaches to recording data:• Linear recording• Helical recording

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Magnetic Storage

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Magnetic Storage

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Magnetic Storage

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Technology Focus

Magnetic Tape Formats and Standards

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Magnetic Storage

Magnetic Disk

• Flat circular platters with metallic coatings that are rotated beneath read/write heads

• Multiple platters can be mounted.• Once concentric circle is a track.• A fractional portion of a track is a cylinder.

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Magnetic Storage

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Magnetic Storage

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Magnetic Storage

Magnetic Disk Types:• Hard Disk – magnetic disk media with a rigid

metal base.

• Floppy Disk (Diskette) – uses a base of flexible or rigid plastic material.

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Magnetic Storage

Magnetic Disk

Disk access time depends on several factor including:

– Time required to switch among read/write heads– Time required to position the read/write heads– Rotational delay

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Technology Focus

Whither the Floppy Disk

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Optical Mass Storage Devices

• Advantages:– Higher recording density– Longer data life– Retain data for decades– Not subject to problems of magnetic decay and

leakage

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• Optical storage devices store bit values as variations in light reflection.

• Storage medium is a surface of highly reflective material.

• The read mechanism consists of a low-power laser and a photoelectric cell.

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• CD-ROM• WORM (CD-R)• Magneto-optical• CD-RW• DVD

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CD-ROM

• Originally developed for storing and distributing music (CD-DA).

• Includes additional formatting to store the directory and file information.

• Holds approximately 650 MB.

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WORM

• Manufactured with all bit areas in a highly reflective state.

• When a bit area is changed to low reflectivity, the process is irreversible.

• Use high powered lasers to burn holes in the reflective layer.

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CD-R

• Cheaper technology than WORM.• Use a laser that can be switched between

high and low power.• Uses a laser-sensitive dye embedded in the

CD-R disc.

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Magneto-Optical

• Uses a laser and reflective light to sense bit values.

• Applies a magnetic charge in a bit area. • The magnetic charge shifts the polarity of the

reflected laser light.

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Phase-Change Optical Discs (CD-RW)

• Allows optical storage media to be written non-destructively.

• Based on materials that can change state easily.

• The difference can be detected by newer optical scanning technologies.

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Summary

• A typical computer system has primary and secondary storage devices.

• The critical performance characteristics of primary storage devices are their access speed and the number of bits that can be accessed in a single read or write operation.

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Summary

• Programs generally are created as through they occupied contiguous primary storage locations starting at the first location.

• Magnetic storage storage devices store data bits as magnetic charges.

• Optical discs store data bits as variations in light reflection.

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systems architecturechapter

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