Chapter 5
Data Storage Technology
Chapter goals
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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
Goals cont.
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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
Storage types
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Primary storage – memory or RAM
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Holds instructions and data for currently
executing programs
Volatile – requires electricity to maintain
data
Secondary storage – electromagnetic or
optical devices
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Non-volatile storage devices with large
capacities
Storage device components
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Storage devices are comprised of
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Storage medium
Read/write mechanism
Device controller – interface between the
storage device and the system bus
(discussed in chapter 6)
Storage device characteristics
Speed
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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
Speed cont.
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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
Access times
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Primary storage – expressed in
nanoseconds (billionths of a second)
Secondary storage – expressed in
milliseconds (thousandths of a second)
Data transfer rate
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Complete measure of data access speed
consists of access time and the unit of
data transfer to/from the storage device
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Access time plus how much data is
transferred
Data transfer unit for primary storage is
based on word size (usually 32 bit)
Data transfer unit
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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
Data transfer rate
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Expressed in terms of bytes/second
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Access time combined with data transfer
unit
Data transfer rate describes how much
data can be transferred between
devices over a period of time
Volatility
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Volatile – storage device is volatile if it cannot
reliable hold data for long periods of time
Non–volatile – 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
Access method
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Physical structure of storage device’s
read/write mechanism determines the
way(s) data can be accessed
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Serial access
Random access
Parallel access
Serial access
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Stores and retrieves data items in a
linear or sequential order
Slowest access method
Tape typically used for backup purposes
Random access
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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
Portability
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Data can be made portable by storing it
on a removable storage medium or
device.
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Portable devices typically have slower
access speed than permanently
installed devices and those with nonremovable media.
Cost and capacity
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An increase in speed, permanence or
portability generally comes at increased cost
if all other factors are held constant.
Storage Device Characteristics
Primary storage devices
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Random access memory (RAM) is a
generic term for storage device that
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Microchip implementation using
semiconductors
Ability to read and write with equal speed
Random access to stored bytes, words, or
larger data units
SRAM vs. DRAM
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Static RAM – implemented with transistors
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Requires continuous supply of electricity to
preserve data
Dynamic RAM – uses transistors and
capacitors
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Require a fresh infusion of power thousands of
times per second.
Each refresh operation is called a refresh cycle
Read only memory
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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
Memory packaging
CPU Memory Access
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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
Physical memory organization
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Main memory of any computer is a sequence
of contiguous memory cells
Addressable memory – highest number
storage byte that can be represented
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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
Memory addressing &
allocation
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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
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This can be demonstrated with C++ program
Memory allocation
Absolute vs. relative
addressing
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Some programming languages (C, C++)
allow instructions that reference explicit
memory locations
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BRANCH to location #
STO to location #
Absolute addressing describes memory
address operands that refer to actual
physical memory locations
Problems with absolute
addressing
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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
Relative addressing
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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
Segmented memory
Each application has unique
starting address
Magnetic storage
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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
Read/write in magnetic device
Magnetic decay and leakage
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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
Magnetic storage
Organization of tracks and
sectors
Optical mass storage devices
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Advantages:
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Higher recording density
Longer data life
Retain data for decades
Not subject to problems of magnetic decay and
leakage
Optical storage
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Optical storage devices store bit values as
variations in light reflection.
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Storage medium is a surface of highly
reflective material.
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The read mechanism consists of a low-power
laser and a photoelectric cell.
Storing binary information
Examples of optical devices
Chapter summary
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A typical computer system has primary and
secondary storage devices
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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
Summary cont.
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Programs generally are created as through
they occupied contiguous primary storage
locations starting at the first location
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Magnetic storage storage devices store data
bits as magnetic charges
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Optical discs store data bits as variations in
light reflection