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Chapter 5
Data Storage Technology
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
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
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
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)
Storage device characteristics
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
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
Access times Primary storage – expressed in
nanoseconds (billionths of a second)
Secondary storage – expressed in milliseconds (thousandths of a second)
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)
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
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
Volatility 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 Physical structure of storage
device’s read/write mechanism determines the way(s) data can be accessed Serial access Random access Parallel access
Serial access Stores and retrieves data items in
a linear or sequential order Slowest access method Tape typically used for backup
purposes
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
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.
Cost and capacity 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 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
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
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
Memory packaging
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
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
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
Memory allocation
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
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
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
Segmented memory
Each application has unique starting address
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
Read/write in magnetic device
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
Magnetic storage
Organization of tracks and sectors
Optical mass storage devices Advantages:
Higher recording density Longer data life Retain data for decades Not subject to problems of magnetic decay and
leakage
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.
Storing binary information
Examples of optical devices
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
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