Memory Organization | Computer Fundamental and Organization

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
22-10-2015Preparedby:ForamShah

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

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

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.

Memory Organization
• 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

Cont…
• 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

Cont…
• 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)

Cont…
A 3-bit per word, 4
word memory

Cont…
• 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)

Cont…
• Block diagram of Memory

Cont…

Read Only Memory
• 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)

Cont…
• 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

Cont…
• 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)

Flash Memory
• 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

Serial Access Memory
• 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

Cont…
• 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”

Cont…
• Another model of memory cell(for serial access memory) is as follow,
• Here one bit is stored in each cell
• Read Mechanism:

Cont…
• “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:

Cont…
• “Write-head” used to write content in memory
• Cell moves below the write-head, the bits fed to the write-head are
stored in the cell

External Storage

Magnetic Disk
• 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

Cont…
• 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

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

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

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

Characteristics : Sides
• Single sided
• Magnetizable coating applied on one side
• Less expensive
• Double sided
• Magnetizable coating applied on both side

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)

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

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

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

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.

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

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)

• 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…

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)

• DVD-Recordable (DVD-R)
• Write only ones
• Only one sided
• DVD-Rewritable (DVD-RW)
• Erase and rewrite multiple times
• Only one sided

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…

Any Questions…?

Memory Organization | Computer Fundamental and Organization

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