Chapter 5 Data Storage Technology

1
Chapter 5Data 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
2
Storage Device Characteristics
CPU
I/O Device
OperatingSystem
3
Storage Devices Distinguishing
Characteristics Speed Volatility Access
method Portability Cost and capacity
4

• 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

5
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
6

• Speed
• Data Transfer Rate Computed
• 1 Unit of
• ------------ X data transfer
  data transfer rate
• Total access time (bytes) (seconds)
• Access time 15 nanoseconds
• Unit of data transfer 4 bytes (32 bits)
• 1 / 1,000,000,000
  .000000001second billion
  one nanosecond
• 2. .000000001 x 15
  .000000015 one nanosecond access time
  total access time
• 3. 1 / .000000015
  66,666,666.666667second total access time
  of accesses times 15
  nanoseconds per second
• 4. 66,666,666.666667 x 4
  266,666,666.66667 of access times
  bytes bytes/secondper second

7
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
8
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 Obsolescenc
e
9
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
10
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.
11
Storage Devices Distinguishing
Characteristics Access Method Portability Removab
le 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
12
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
13

• 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
• 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

165
14
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 Ferroelect
ric RAM Iron chips, similar to core, stored
on the microchip
15
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
16
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
17
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
168
18
CPU Memory Access Memory Allocation and
Addressing 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 Used with Indirect
addressing Relative addressing Address
Segment Register Offset Register Absolute
Addressing Memory address that refers to an
actual physical memory location
170
171
19
Magnetic Storage 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
172
20
Magnetic Storage Media Integrity Depends upon
construction and environment Problems Age Env
ironmental stress Physical stress Temperature
Humidity Magnetic Tape 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
177
21
Magnetic Storage Magnetic Disk Drives Circular
platters Magnetic coatings Rotate beneath
read/write heads Track One concentric circle
of a platter Data is written Sector Fractiona
l portion of a track Is the data transfer unit
of the device
180
22
Optical Mass Storage Devices 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 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
187
188