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Computer data storage, often called storage or memory, refers to computer components andrecording media that retain digital data used for computing for some interval of time. Computer datastorage provides one of the core functions of the
modern computer, that of information retention. It isone of the fundamental components of all modern
computers, and coupled with a central processingunit (CPU, a processor), implements the basiccomputer model used since the 1940s.
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Memory consists of a number of
storage locations, each ofwhich is identified by a uniqueaddress.
Each location stores a word i.e.the number of bits that can be
processed by the CPU in asingle operation. Word lengthmay be typically 16, 24, 32 or asmany as 64 bits.
A large word length improves
system performance, thoughmay be less efficient onoccasions when the full wordlength is not used
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The amount of information that can be
stored on a medium.
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Unit Description ApproximateSize
1 bit 1 binary digit
1 nibble 4 bits
1 byte 8 bits 1 character
1 kilobyte 1,024 bytes page, double
spaced
1 megabyte 1,024 kilobytes 500,000 pages
1 gigabyte 1,024 megabytes 5 million pages
1 terabyte 1,024 gigabytes 5 billion pages
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Computer MemorySystem
Location
Internal
External
PhysicalCharacteristic
Volatile/ NonVolatile
Erasable/ NonErasable
Physical Type
Magnetic Type
Semiconductor
Optical
Method ofAccess
Sequential
Random
Direct
Performance
Access Time
Transfer Rate
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Location refers to whether the memory is internal orexternal to the computer. Internal memory is oftenequated with the main memory. External Memory
consists of peripheral storage devices such as disk ortape, that are accessible to processor via I/o
controllers
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Volatile /Non Volatile- Volatile Memory is the memorythat requires power to maintain the storedinformation. As soon as the power is switched off, it
loses all the information stored in it. Examples areRAM. Non Volatile memory retains the stored
information when the power is not supplied. Exampleare ROM, flash memory ,optical disks.
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Magnetic core memory is an early form of randomaccess computer memory. It uses small magneticrings, the cores, through which wires are threaded to
store information via the polarity of the magnetic fieldthey contain. Such memory is often just called core
memory
Semiconductor memory-In this memory
semiconductor chips are used to make mainmemory.
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Sequential
Start at the beginning and read through in order
Access time depends on location of data andprevious location
e.g. tape
Direct
Individual blocks have unique address
Access is by jumping to vicinity plus sequentialsearch
Access time depends on location and previouslocation
e.g. disk
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Random
Individual addresses identify locations exactly
Access time is independent of location orprevious access
e.g. RAM Associative
Data is located by a comparison with contentsof a portion of the store
Access time is independent of location or
previous access e.g. cache
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Access time-Access time is the time taken to performa read or write operation.
Memory cycle time-this concept is primarily added tothe random access memory and consists of the
access time plus any additional time required beforea second access can commence.
Transfer Rate-Transfer rate is the rate at which the
data can be transferred into or out of a memory unit.
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In general
Slow memory is cheap and has more storagecapacity
Fast memory is expensive and has less storage
capacity
Users want
Faster memory, which will ensure faster data fetching,less wait for the processor, in turn shortens responsetime. Never the less we want to have more storage atan affordable price
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The cache is a smaller, faster memory which storescopies of the data from the most frequently used mainmemory locations. Sits between normal main memory
and CPU. May be located on CPU chip or module
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1. Cache fetches data
from next to current
addresses in main
memory
2. CPU checks to see
whether the next
instruction it requires is in
cache
3. If it is, then the
instruction is fetched from
the cache a very fast
position
4. If not, the CPU has to
fetch next instructionfrom main memory - a
much slower process
Main
Memory
(DRAM)
CPU
Cache
Memory
(SRAM)
= Bus connections
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Random access memory or RAM most commonly refers to
computer chips that temporarily store dynamic data to
enhance computer performance. By storing frequently used
or active files in random access memory, the computer can
access the data faster than if it to retrieve it from the far-larger hard drive. Random access memory is also used inprinters and other devices.
The original name of RAM comes from the fact that anyportion on the memory can be accessed in a constant
time. (Randomly)
Today it used to refer to main memory.
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RAM
SRAM
DRAM
Fast PageMode DRAM
SDRAM
DDR SDRAM RDRAM
EDO RAM
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Bits stored as charge in capacitors
Charges leak
Need refreshing even when continuouslypowered
Simpler construction & Smaller per bit
Less expensive Slower
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Bits stored as on/off switches
No charges to leak
No refreshing needed when powered
More complex construction
Larger per bit
More expensive
Faster
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o It is non volatile in nature.
o It is created like an integrated chip, with the dataactually wired in chip as a part of fabrication process.
o Used to store system programs, library routines forfrequently used functions.
o Contains a permanent pattern of data that can notbe changed. This is in the strict sense Masked ROM.(MROM)
ROM
MROM PROM EPROM EEPROMFlash
Memory
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It is a type of ROM that can be programmed.
It comes without instructions and the user program
it according to his requirements.
Once PROM programmed, it is not possible tomake any changes or reprogram it.
There are other types of ROM memories such asEPROM (can be erased and programmed again)
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Each bit consists of a floating FET.
Have clear window in centre of top of the chip
Chips erased by focusing high-intensity UV light
through window, so care has to be used
UV radiation can erase entire contents of a chip
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Contains an array of FETs.
Use higher than normal voltage to erase contents
Dont need to be removed from socket to
reprogram
Can be manipulated at byte level, so individual
bytes can be erased without erasing entire chip
Common for storing setup parameters for printers
and other peripherals
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Essentially same as EEPROM except voltage required are at
levels normally found inside PC
Flash ROM bulk-erase typically, but newer ones have multiple
independently erasable blocks (size 4K to 128K) Two styles
Sectored-erase flash
Boot block
Usually one or more blocks are specially protected from normal
erase operations (firmware).
Support random reading and writing
Current generations have on-board logic to handle operations
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Secondary storage (also known as external memory orauxiliary storage), differs from primary storage in that it isnot directly accessible by the CPU. The computerusually uses its input/output channels to accesssecondary storage and transfers the desired data usingintermediate area in primary storage. Secondarystorage does not lose the data when the device ispowered downit is non-volatile.
Types:
Magnetic Storage Devices
Optical Storage Devices
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Magnetic disks: A form of secondary storage on amagnetized disk divided into tracks and sectorsthat provide addresses for various pieces of data.
Examples include Hard Disk Drives, Floppy Disks.
Each disk consists of a number of invisibleconcentric circles called tracks. Information isrecorded on tracks of a disk surface in the formof tiny magnetic spots.
The presence of a magnetic spot representsone bitand its absence represents zero bit. Theinformation stored in a disk can be read manytimes without affecting the stored data.
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Hard disk: A form of secondary storage that
stores data on platters divided into concentric
tracks and sectors, which can be read by a
read/write head that pivots across the rotating
disks.
Floppy disk: A form of easily portable secondarystorage on flexible disks; also called floppy disks.
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Optical storage devices: A form of secondary storagein which a laser reads the surface of a reflectiveplastic platter.
Compact disk, read-only memory (CD-ROM): A form
of secondary storage that can be only read and notwritten on.
Digital versatile disk (DVD): An optical storage deviceused to store digital video or computer data.
Other types include: HD-DVDs, Blu-Ray Disks, etc.
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