Data Storage Technology

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Chapter 5

• Data Storage Technology
• 2005 IS112

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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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Goals cont.

• Describe factors that determine storage device
  performance
• Choose appropriate secondary storage technologies
  and devices
• 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 types

• Primary storage memory or RAM
• Holds instructions and data for currently
  executing programs
• Volatile requires electricity to maintain data
• Secondary storage electromagnetic or optical
  devices
• Non-volatile storage devices with large
  capacities

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Storage device components

• Storage devices are comprised of
• Storage medium
• Read/write mechanism
• Device controller interface between the storage
  device and the system bus (discussed in chapter 6)

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Storage device characteristics
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Speed

• Speed of primary storage (RAM) directly impacts
  performance of entire system
• RAM extends the limited capacity of CPU registers
• The CPU continually moves data and instructions
  between registers and RAM
• If a read/write to RAM takes more than one CPU
  cycle, then CPU must wait for information
• RAM is faster than secondary storage by a factor
  of 105 or more

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Speed cont.

• Speed is also an issue for secondary storage
• Called access time or seek time
• Access time is defined as time to complete one
  read or write operation
• Access time for disk or tape storage can vary
  depending on location of information, therefore
  access time is expressed as an average

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Access times

• Primary storage expressed in nanoseconds
  (billionths of a second)
• Secondary storage expressed in milliseconds
  (thousandths of a second)

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Data transfer rate

• Complete measure of data access speed consists of
  access time and the unit of data transfer to/from
  the storage device
• Access time plus how much data is transferred
• Data transfer unit for primary storage is based
  on word size (usually 32 bit)

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Data transfer unit

• Data transfer unit (amount of data moved at a
  time) for secondary storage varies depending on
  the device
• Unit is called a block
• Block size is stated in bytes
• Sector is data transfer unit for magnetic and
  optical devices
• Common sector/block size is 512 bytes

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Data transfer rate

• Expressed in terms of bytes/second
• Access time combined with data transfer unit
• Data transfer rate describes how much data can be
  transferred between devices over a period of time

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Volatility

• Volatile storage device is volatile if it
  cannot reliable hold data for long periods of
  time
• Nonvolatile storage device is non-volatile if
  it can reliably store data for long periods of
  time
• Computer systems need a combination of volatile
  and non-volatile storage devices

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Access method

• Physical structure of storage devices read/write
  mechanism determines the way(s) data can be
  accessed
• Serial access
• Random access
• Parallel access

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Serial access

• Stores and retrieves data items in a linear or
  sequential order
• Slowest access method
• Tape typically used for backup purposes

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Random access

• Also called a direct access device
• Can directly access data stored on the device
• All primary storage and disk storage devices are
  direct access
• Parallel access with multiple read/write heads,
  can simultaneously access more than one storage
  location

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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 capacity

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

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Storage Device Characteristics
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Primary storage devices

• Random access memory (RAM) is a generic term for
  storage device that
• 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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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

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SRAM vs. DRAM

• Static RAM implemented with transistors
• Requires continuous supply of electricity to
  preserve data
• Dynamic RAM uses transistors and capacitors
• Require a fresh infusion of power thousands of
  times per second.
• Each refresh operation is called a refresh cycle

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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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SRAM vs. DRAM

• Static RAM implemented with transistors
• Requires continuous supply of electricity to
  preserve data
• Dynamic RAM uses transistors and capacitors
• Require a fresh infusion of power thousands of
  times per second.
• Each refresh operation is called a refresh cycle

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Read only memory

• ROM random access memory device that can store
  data permanently or semi-permanently
• Typically used to store BIOS (basic input output
  services)
• Instructions stored in ROM is called firmware

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Memory packaging
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CPU Memory Access

• Management of RAM is critical to performance of
  computer
• Organization, access, and management or RAM is
  done by the operating system
• How memory is accessed is large factor in
  performance of RAM

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Physical memory organization

• Main memory of any computer is a sequence of
  contiguous memory cells
• Addressable memory highest number storage byte
  that can be represented
• Determined by the number of bits used to
  represent an address
• If 32 bits used to represent and address, highest
  address is 232 4,294,967,296, or 4 GB
• Physical memory actual memory installed,
  usually less than addressable memory

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Memory addressing allocation

• Memory allocation is the assignment or
  reservation of memory segments for system
  software, application programs, and data
• Memory allocation is the responsibility of the
  operating system
• Common scheme is to place OS in low memory and
  applications in high memory
• This can be demonstrated with C program

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Memory allocation
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Absolute vs. relative addressing

• Some programming languages (C, C) allow
  instructions that reference explicit memory
  locations
• BRANCH to location
• STO to location
• Absolute addressing describes memory address
  operands that refer to actual physical memory
  locations

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Problems with absolute addressing

• If a program refers to a physical memory address
  in the code, then OS loses ability to re-arrange
  application locations in memory
• Instead, programs use relative addressing

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Relative addressing

• Instructions that refer to memory use a
  combination of registers to compute addresses
• When OS loads application into memory, OS loads
  starting point of application into one register
• Instruction in application that refers to memory
  location is using an offset (i.e. distance from
  beginning of application)
• OS adds offset to starting point to calculate
  physical memory location

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Segmented memory
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Each application has unique starting address
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Magnetic storage

• Uses magnetism to store binary information onto a
  storage medium that can store magnetic
  information
• Least expensive medium for secondary storage
• Can be portable
• Retains data without electricity
• Over longer periods of time will eventually lose
  information

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Read/write in magnetic device
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Magnetic decay and leakage

• Primary disadvantage is loss of data over time
• 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
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Organization of tracks and sectors
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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

• 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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Storing binary information
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Examples of optical devices
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Chapter 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 cont.

• 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