chapter 5 data storage technology 2005 is112. chapter goals describe the distinguishing...
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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
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)
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.
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
Primary Storage Devices
Critical performance characteristics Access speed Data transfer unit size
Must closely match CPU speed and word size to avoid wait states
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)
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
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
Random Access Memory
To bridge performance gap between memory and microprocessors Read-ahead memory access Synchronous read operations On-chip memory caches
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)
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
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
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
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
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
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
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.
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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