Transcript Data Storage Technology

Chapter 5
Data Storage Technology
Storage Device Characteristics
Storage Medium
Device or substance that holds the data
Electrical circuits (memory)
Polymers and metal oxides (tapes)
Read/Write Mechanisms
The method of reading/writing data to/from a
storage medium
Electrical circuits (memory)
Electro-mechanical (tapes, disks)
Device Controller
(communicates with the software driver)
Interface between the system bus and the storage
device
Electronic hardware unit
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Storage Device Characteristics
CPU
I/O Device
Operating
System
DRIVER
Device
Controller
Read/Write
Mechanism
Storage
Medium
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Storage Devices
Distinguishing Characteristics
Speed
Volatility
Access method
Portability
Cost and capacity
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Storage Devices
Distinguishing Characteristics
Speed
Most important distinguisher between Primary
(memory) and Secondary storage
• Extends capacity of CPU register
• CPU and execution cycle are short compared
to secondary device transfer speed
• Secondary storage speed
• Effects overall computer performance
• Starting program execution
• Transfer of data
• Access Time: time required to
execute one read or write operation
completely
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Speed
Total Access Time determined by
Access time
Time required to execute one read or write
operation completely
Data transfer unit
Vary from one storage device to another
CPU: usually a word
I/O devices:
Blocks: larger than words
Block size
Device dependent
Sector: magnetic and optical
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Speed
Data Transfer Rate Computed
1
Unit of
------------
X
Total access time
(seconds)
data transfer = data transfer rate
(bytes)
Access time = 15 nanoseconds
Unit of data transfer = 4 bytes (32 bits)
1.
1
/ 1,000,000,000
second
2.
billion
.000000001 x
one nanosecond
3.
1
second
=
.000000001
one nanosecond
15
= .000000015
access time
total access time
/ .000000015
total access time
15 nanoseconds
=
66,666,666.666667
# of accesses times
per second
4. 66,666,666.666667 x 4
= 266,666,666.66667 #
# of access times
bytes
bytes/second
per second
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Speed
Milliseconds: thousandths of a second
1/1,000 second = .001
Microseconds: millionths of a second
1/1,000,000 second = .000001
Nanoseconds: billionths of a second
1/1,000,000,000 second = .000000001
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Storage Devices
Distinguishing Characteristics
Volatility
Volatile: the device cannot reliably hold data
for a long period of time
Primary storage (memory)
Video cards
Non-volatile: a device holds data without loss
over a long period of time
Hard drives
Floppy disks
Tapes
CDs
DVDs
Device may become volatile
Magnetic decay
Obsolescence
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Storage Devices
Distinguishing Characteristics
Access Methods
Serial Access
Random Access
Parallel Access
Serial Access
Stores and retrieves data in a linear,
sequential order
Access time is dependent upon where
the read/write mechanism and the
desired data are located
Not used for frequently required data
due to slow access times
i.e.: tape units
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Storage Devices
Distinguishing Characteristics
Access Method
Random Access
Not as restricted to location of read/write
mechanism or data
Primary storage (memory): access time is
constant as it is electronic and not
hampered by physical movements
Disk assess time varies as there is a
physical dependency on the location of the
read/write mechanism and the data
Parallel Access
Simultaneously accessing multiple storage
locations
Primary storage (memory)
Operating systems (some)
Store a single file on different
secondary storage (disk) devices
at the same time. Segment the file.
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Storage Devices
Distinguishing Characteristics
Access Method
Portability
Removable storage medium
Slower access times
Lack of environmental control
Cost and Capacity
Increased cost
Increase in speed
Permanence (volatility)
Portability
Access method
Serial
Random
Parallel
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Primary Storage Devices
Performance Characteristics
Access time
Data transfer unit size
CPU memory (registers)
Fastest memory access
Less physical memory
More costly memory
Primary storage
Slower access than the CPU memory
Less costly
Greater amount of memory
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Primary Storage Devices
Random Access Memory (RAM)
Read and Write with equal speed
Random access to stored data
• Static RAM (SRAM)
Flip-flop circuits suing two transistors
165
Remains in one of two states
(0 or 1 state)
Volatile
More expensive
• Dynamic RAM (DRAM)
Transistors and capacitors
Capacitors must be continually refreshed
Slower than SRAM due to refresh cycle
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Primary Storage Devices
Performance increase
Read ahead memory access
Synchronous DRAM (SDRAM)
Read (anticipate) the next
instruction location
Synchronous read operations
Enhanced DRAM (EDRAM) (Cached DRAM)
When data is requested, the words
around the requested data are also
fetched and stored in the SRAM
On-chip caching
Ferroelectric RAM
Iron chips, similar to core, stored
on the microchip
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Primary Storage Devices
Read-Only Memory
(ROM)
Random access
Storage of data (firmware)
Permanent
semi permanently (volatile)
System boot program and BIOS
Electronically Erasable Programmable Read-only
Memory (EEPROM)
Programmed, erased, and reprogrammed
Require high voltages
Flash Memory
Erased and re-written more quickly
Limited life span
Programs and data that are not frequently
updated
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Primary Storage Devices
Memory Packaging
Circuits embedded within microchips
Groups of chips are packed on circuit boards
Easily installed and removed
Single In-line Memory Module (SIMM)
Memory chips on single boards
Boards have electrical contacts which
incorporate into slots on the
motherboard
Double In-line Memory Module (DIMM)
Double sided SIMM
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CPU Memory Access
Primary Storage (memory)
Contiguous memory cells (bytes)
Each byte is addressable
Starting at address 0 through the end
Numeric values
Highest position weight left to right
(what we have been doing)
Most significant byte
Left most byte
High order byte
Least significant byte
Right most byte
Low order byte
Big endian
Stores the most significant byte at the
lowest address
Little endian (most common)
Stores the least significant byte at the
lowest address
Addressable memory
Determined by size of address register
Physical memory
The actual memory available for use
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168
CPU Memory Access
Memory Allocation and Addressing
170
Memory Allocation
The assignment of specific memory
addresses
Operating system occupies lowest memory block
Programs begin following OS memory
Program Offset
The difference between where a program
instruction is located and address
zero (0)
Offset Register
Segment Register
171
Used with
Indirect addressing
Relative addressing
Address =
Segment Register + Offset Register
Absolute Addressing
Memory address that refers to an actual
physical memory location
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Magnetic Storage
172
Write data
Electrical signals are converted to magnetic
charges
Read data
Magnetic charge is converted to an electrical
signal
Polarity of the charge determines the data value
one (1) or zero (0)
Problems
Magnetic Decay
Loss of charge over time
Magnetic Leakage
Polarity of the magnetic charges may affect
surrounding charges of other bits
Storage Density
Coercivity
The ability of a substance to accept and hold
a magnetic charge
Varies among elements and compounds
Recording density
The amount of surface area allocated to a bit
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Magnetic Storage
Media Integrity
Depends upon construction and environment
Problems
Age
Environmental stress
Physical stress
Temperature
Humidity
Magnetic Tape
177
Ribbon of plastic with metallic oxide
Slow
Serial access
Physical wear and tear
Tape stretching
Physical contact with R/W head
Primarily used for backups
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Magnetic Storage
Magnetic Disk Drives
180
Circular platters
Magnetic coatings
Rotate beneath read/write heads
Track
One concentric circle of a platter
Data is written
Sector
Fractional portion of a track
Is the data transfer unit of the device
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Optical Mass Storage Devices
187
Higher Recording density
Tightly focused lasers
Access very small storage medium area
Longer data life
Not magnetic medium
Data stored as variations in light reflection
Storage medium is a highly reflective material
Read mechanism
Low-power laser
Photoelectric cell
188
Laser focused onto one bit at an angle
Photoelectric cell at a complementary angle
A highly reflective spot is a one (1)
A non reflective spot is a zero (0)
Reflectivity
Dints
Burned areas
Dyes
Change from crystalline (fixed shape) to
amorphous (shapeless, vague) states
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