This document discusses memory organization and different types of memory. It begins with an introduction to memory and memory cells. It then discusses how memory cells can be organized into various memory structures like registers and memory arrays. Next, it describes different types of memory like read-only memory, serial access memory, and their applications. Finally, it covers external storage devices like magnetic disks, optical disks, and their characteristics including storage capacity, access mechanisms, and read/write technologies.
2. Agenda
• In this unit we will see,
• Introduction
• Memory Cell
• Memory Organization
• Read Only Memory
• Serial Access Memory
• Physical Devices Used to Construct Memories
• Magnetic and Optical Disk
• Virtual Memory
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3. Introduction
• Memory is required in a computer to store a program and the
data processed by the program
• It is made up of large number of cells, with each cell capable of
storing one bit
• Cell may be organized as a set of addressable words, each word
storing sequence of bit
• Time to store or retrieve a word is independent of the address
of word – called RAM(Random Access Memory)
• Another one is SAM(Serial Access Memory) – Linear sequencing
for storing and retrieving word
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4. Memory Cell
• It may be defined as a device which can store a symbol
selected from a set of symbols.
• It may characterized by the following properties:
• 1) The number of stable state in which it can be place
• Cell can be placed to determine the number of distinct symbols it
can store.
• Each stable state may be assigned to represent a symbol.
• If a cell may have 10 stable state , each state must be used to
represent one symbol-cell store decimal digit
• If cell can be placed only one out of two stable state- cell store
binary digit
• 2) Whether a cell can store a symbol indefinitely even when
power is turned off.
• Volatile cell: Symbol will disappeared in a cell when no energy
supplied
• Non Volatile cell: Symbol stored in a cell when no energy supplied
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5. Cont…
• 3) Whether, after reading a symbol from a cell, the stored symbol
is engaged in the memory cell or disturbed.
• Non-destructive cell: Ones read symbol should not disturbed(Not
erase).
• Destructive cell: Ones read the symbol is erased from cell.
• 4) The time taken to read a symbol from a cell and time taken to
write a new symbol in it.
• Read-time: Time taken to read symbol from cell.
• Write-time: Time taken to write symbol to cell.
• 5) Whether a symbol, ones written, can only be read and not
changed.
• Read only cell: A symbol is permanently written and can only be
read, not to modify.
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6. Memory Organization
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• Current technology of memory cell- Symbol can be placed in one out of
two stable state (i.e. Binary cell)
• Storage cell:
• Input data line: Symbol to be written is sent to the cell through this line
• Write line: In order to write particular symbol in cell “write control
signal” sent to this line
• Read line: If content of a cell is to be read from cell “read control signal”
sent to this line
• Output data line: Content of the cell may sensed on this line
7. Cont…
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• Appropriation variation in the interconnection of binary
memory cell it is possible to organized different type of
memory.
• For example:
• Here we assume that, individual cells are non-volatile and
reading is non-destructive
• Simplest organizations of set of cell is given below,
A 3-bit Register
8. Cont…
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• In this organization(Above figure), three cells are interconnected in a
such way that,
• The write and read control lines of all the cells are connected together.
• The bits to be written in each cell is fed to the appropriate input data
lines.
• When write signal is applied to the write-control line, these bits are
written in the individual cells.
• The previous content in the cell are automatically erased when the new
information is written.
• To read a content from cell, read signal is applied to read line.
• The content of the cell appears on the respective output line.
• The content of individual cell are not erased by the read operation as
reading from these cell. (As we assumed that- non destructive cell)
• This interconnection of cells is called register.(Here in this example it
store 3 bits)
10. Cont…
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• Input data line of the first register of all four registers are connected
together, similarly for second and third register also.
• Same for output data line
• Four write line and four read line, one for each 3 bit register
• To read or write data to particular register is possible with putting
specific signal with read and write line
• Each register having unique identification, so appropriate register may
be selected for writing or reading
• At a time we may either read from or write in a register
• Identification code of each register is corressponding to a word in the
memory is called address.
• Address usually specified as a binary number and is placed in register
called Memory Address Registers(MAR)
• Data read from or written to memory is placed in register called
Memory Data Registers(MDR)
13. Read Only Memory
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• Ones a word written in memory, later it can be read from
memory by specifying its address.
• Characteristics of ROM:
• Content of the word cannot be altered
• Reading from ROM should be non-destructive
• Memory should be non-volatile
• Application of ROM:
• Trigonometric function
• Washing machine functions (sequencing of operation of washing
machine may be stored in ROM and interpreted by processing unit)
14. Cont…
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• Factory programmed ROM:
• A ROM which has information written in it during manufacture in a
factory
• It is feasible only in cases where the demand for such programmed
ROM is large.
• Programmable ROM (PROM):
• For more specialized uses, user may like to store his special
function or program to ROM
• Here information will only installed ones
• Programming is done using special writing circuits
• Time taken to write information is long while read time is small
• Not flexible because it can not be alter
15. Cont…
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• Erasable Programmable ROM (EPROM):
• Information in ROM is erased by shining ultra-violet light
• After the ROM is exposed to UV lights all bits are erased and
become 0
• Then ROM may reprogrammed, this PROM is known as EPROM
• Electrically Erasable Programmable ROM (EEPROM):
• Electrical pluses are used instead of ultra-violet line to erase the
PROM
• Erasing PROM with electrical voltage is convenient compared to UV
rays to erase it
• Now it is dominant (Leading technology)
16. Flash Memory
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• Variant of EEPROM
• It is Random Access Memory
• It use one transistor switch per memory cell
• Capacities ranging from 32 KB to 1 GB
• It is non-volatile: Do not required power to preserve the data
• Read time of flash memories is tens of nano second while write times is
several micro seconds
• It is compact and came in various shape
• Example: Pen drive, micro card etc.
• So, Advantages of Flash Memory:
• Random access
• Non-volatile
• Slow in write – fast in read memories
• Overwrite the data
• Compact
• Price/byte is rapidly reducing
17. Serial Access Memory
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• Serial access memory is non-addressable memory
• That is, set of bits stored in the memory cannot be selectively retrieved by
specifying their location in the memory
• The bits stored can be retrieved in strict serial order
18. Cont…
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• Here in above organization of memory cells, output of cell is input to the next
cell
• A read-signal places the content of each of the cell on the respective output
lines
• A write-signal following this read signal will stores these bits in the respective
“next” cell
• One read-write pair of signal would thus “shift” the content of the cell
right by one cell position
• And the bit stored in right-most cell will appear on the output line
• As the bits stored in the cell appear serially at the output, this memory is
called serial access memory
• This structure(In above figure) is also known as “Shift register”
20. Cont…
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• “Read head” reads the content of the cell placed below it and places it on
output line
• Cells are moved physically from left to right and each cell appears below
the read-head its content is placed on the output
• This bits appear on the output as shown in fig(b)
• Write Mechanism:
23. Magnetic Disk
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• magnetic disk is a thin, circular plate/platter made of nonmagnetic
material called substrate
• Substrate coated with magnetizable material (iron oxide)
• Traditionally substrate used to be aluminium
• But now glass substate is used
• Increases reliability
• Better stiffness
• Better shock/damage resistance
• Recording & retrieval done via conductive coil called a head
• Two head : Read Head and Write Head
• During read/write, head is stationary (fixed), platter rotates
24. Cont…
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• Data Organization and Formating:
• Head will gives rise to organization of data on the platter in a concentric set
of rings (Having common centre), called track.
• Data are transferred to and from the disk in Sectors. (i.e. Track are divided
in to sector)
• Thousand of track per surface.
• Adjacent tracks are separated by gap called Intertrack gap.
• Hundreds of sectors per track.
• Adjacent sectors are separated by gap called Intersector gap.
• Reduce gap to increase capacity
26. Characteristics
Head Motion Platter
- Fixed head (one per track) - Single platter
- Moveable head (one per surface) - Multiple Platter
Disk Portability Head Mechanism
- Non removable disk - Contact (Floppy)
- Removable disk - Fixed gap
- Flying (Winchester)
Sides
- Single sided
- Double sided
27. Characteristics : Head Motion
• Fixed head
• One read write head per track
• Heads mounted on fixed ridged arm
• Movable head
• One read write head per side
• Mounted on a movable arm
28. Characteristics : Disk Portability
• Removable disk
• Can be removed from drive and replaced with another disk
• Provides unlimited storage capacity
• Easy data transfer between systems
• Non removable disk
• Permanently mounted in the drive
29. Characteristics : Sides
• Single sided
• Magnetizable coating applied on one side
• Less expensive
• Double sided
• Magnetizable coating applied on both side
30. Characteristics : Platters
• Single platter
• Only single plate is there
• Multiple platter
• One head per surface
• Heads are joined and aligned
• Aligned tracks on each platter form cylinders
• (Cylinders : The set of all the tracks in the same relative
position on the platter is referred to as cylinder.)
• Data is striped by cylinder
• reduces head movement
• Increases speed (transfer rate)
33. Characteristics : Head mechanism
• Fix gap
• Head has been positioned a fixed distance
• Floppy disk
• Head will physical contact with the medium during a read or
write operation
• FD is small, flexible platter and least expensive
• Flying (Winchester)
• Developed by IBM in Winchester (USA)
• Heads fly on boundary layer of air as disk spins
• Getting more robust
34. Speed
• Seek time
• Moving head to correct track
• (Rotational) latency
• Waiting for data to rotate under head
• Access time = Seek + Latency
• Transfer rate : Data transfer portion of operation
36. Optical Disk
• Consists of a circular disk,
• which is coated with polycarbonate coated with highly reflective
coat, usually aluminium
• for recording/reading of data on the disk : Laser beam technology is
used
• Also known as laser disk / optical laser disk,
• Data stored as pits
• Read by reflecting laser
• Proved to be a promising random access medium for high capacity
secondary storage
• Originally for audio
• 650Mbytes giving over 70 minutes audio
37. Cont…
• Has one long spiral track, which starts at the outer edge
and spirals inward to the center (To increase the density)
• Track is divided into equal size sectors
• Difference in track patterns on optical and magnetic
disks.
38. CD Operation
-> Pit having rough surface, so density is low
-> Land having smooth surface, so density will be high
-> Difference between pit and land has been detected by photosensor and
it will convert into digital signal
39. Cont…
• Optical Disk Products
• CD
• A non erasable disk storing digitized audio information only
• CD-ROM
• CD-R
• CD-RW
• DVD
• DVD-R
• DVD-RW
• Blu-Ray DVD (blue violet laser)
40. • CD-Recordable (CD-R)
• Now affordable
• Compatible with CD-ROM drives but user can write on disk only
once
• CD-RW
• Erasable
• Getting cheaper
• Mostly CD-ROM drive compatible but user can erase and rewrite
multiple times on disk
• Phase change
• Material has two different reflectivities in different phase states
Cont…
41. Cont…
DVD:
• Digital Video Disk
• Used to indicate a player for movies
• Only plays video disks
• Digital Versatile Disk
• Used to indicate a computer drive
• Will read computer disks and play video disks
• Multi-layer
• Very high capacity (4.7G per layer)
• Full length movie on single disk
• Using MPEG compression
• Double sided, capacity of up to 17 GB
• Basic DVD is read only (DVD-ROM)
42. • DVD-Recordable (DVD-R)
• Write only ones
• Only one sided
• DVD-Rewritable (DVD-RW)
• Erase and rewrite multiple times
• Only one sided
43. High Definition Optical Disks
• Designed for high definition videos
• Much higher capacity than DVD
• Shorter wavelength laser
• Blue-violet range
• Smaller pits
• HD-DVD
• 15GB single side single layer
• Blue-ray
• Data layer closer to laser
• Tighter focus, less distortion, smaller pits
• 25GB on single layer
• Available read only (BD-ROM), Recordable once (BR-R) and re-
recordable (BR-RE)
Cont…