Cis303a_chapt05.ppt

Chapter 5Chapter 5Data Storage TechnologyData 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

Cis303a_chapt05.ppt

OperatingSystem

CPUCPU

DRIVER Device Controller Read/Write

Mechanism

StorageMedium

I/O Device

Storage Device Characteristics

Cis303a_chapt05.ppt

Storage Devices Distinguishing Characteristics

Speed

Volatility

Access method

Portability

Cost and capacity

Cis303a_chapt05.ppt

Storage Devices Distinguishing Characteristics

SpeedMost 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

Cis303a_chapt05.ppt

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

Cis303a_chapt05.ppt

SpeedData 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 / 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 = 266,666,666.66667266,666,666.66667 # # of access times bytes bytes/secondbytes/secondper second

Cis303a_chapt05.ppt

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

Cis303a_chapt05.ppt

Storage Devices Distinguishing Characteristics

VolatilityVolatile: 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 drivesFloppy disksTapesCDsDVDs

Device may become volatile

Magnetic decay

Obsolescence

Cis303a_chapt05.ppt

Storage Devices Distinguishing Characteristics

Access MethodsSerial 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

Cis303a_chapt05.ppt

Storage Devices Distinguishing Characteristics

Access MethodRandom 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.

Cis303a_chapt05.ppt

Storage Devices Distinguishing Characteristics

Access MethodPortability

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

Cis303a_chapt05.ppt

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

Cis303a_chapt05.ppt

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

Cis303a_chapt05.ppt

Primary Storage Devices

Performance increase

Read ahead memory accessSynchronous DRAM (SDRAM)

Read (anticipate) the next instruction location

Synchronous read operationsEnhanced 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 RAMIron chips, similar to core, stored on the microchip

Cis303a_chapt05.ppt

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 reprogrammedRequire high voltages

Flash Memory

Erased and re-written more quicklyLimited life spanPrograms and data that are not frequently

updated

Cis303a_chapt05.ppt

Primary Storage Devices

Memory Packaging

Circuits embedded within microchips

Groups of chips are packed on circuit boardsEasily installed and removed

Single In-line Memory Module (SIMM)

Memory chips on single boardsBoards have electrical contacts which

incorporate into slots on the motherboard

Double In-line Memory Module (DIMM)

Double sided SIMM

Cis303a_chapt05.ppt

CPU Memory AccessPrimary Storage (memory)

Contiguous memory cells (bytes)

Each byte is addressableStarting at address 0 through the end

Numeric valuesHighest position weight left to right(what we have been doing)

Most significant byteLeft most byteHigh order byte

Least significant byteRight most byteLow order byte

Big endianStores the most significant byte at the

lowest address

Little endian (most common)Stores the least significant byte at the

lowest address

Addressable memoryDetermined by size of address register

Physical memoryThe actual memory available for use

168

Cis303a_chapt05.ppt

CPU Memory AccessMemory Allocation and Addressing

Memory AllocationThe assignment of specific memory

addresses

Operating system occupies lowest memory block

Programs begin following OS memory

Program OffsetThe difference between where a program

instruction is located and address zero (0)

Offset RegisterSegment RegisterUsed with

Indirect addressingRelative addressing

Address = Segment Register + Offset Register

Absolute AddressingMemory address that refers to an actual

physical memory location

170

171

Cis303a_chapt05.ppt

Magnetic StorageWrite data

Electrical signals are converted to magnetic charges

Read dataMagnetic charge is converted to an electrical signal

Polarity of the charge determines the data valueone (1) or zero (0)

Problems

Magnetic DecayLoss of charge over time

Magnetic LeakagePolarity of the magnetic charges may affect surrounding charges of other bits

Storage Density

CoercivityThe ability of a substance to accept and hold

a magnetic chargeVaries among elements and compounds

Recording densityThe amount of surface area allocated to a bit

172

Cis303a_chapt05.ppt

Magnetic StorageMedia Integrity

Depends upon construction and environment

ProblemsAgeEnvironmental stressPhysical stressTemperatureHumidity

Magnetic Tape

Ribbon of plastic with metallic oxide

Slow

Serial access

Physical wear and tearTape stretchingPhysical contact with R/W head

Primarily used for backups

177

Cis303a_chapt05.ppt

Magnetic StorageMagnetic Disk Drives

Circular plattersMagnetic coatingsRotate beneath read/write heads

Track One concentric circle of a platterData is written

SectorFractional portion of a trackIs the data transfer unit of the device

180

Cis303a_chapt05.ppt

Optical Mass Storage DevicesHigher Recording density

Tightly focused lasersAccess 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 mechanismLow-power laserPhotoelectric cell

Laser focused onto one bit at an anglePhotoelectric cell at a complementary angle

A highly reflective spot is a one (1)A non reflective spot is a zero (0)

ReflectivityDintsBurned areasDyesChange from crystalline (fixed shape) to

amorphous (shapeless, vague) states

187

188