1. MEMORY
DEVICES

2. INTRODUCTION
Memory is the most essential part of a computer.
Without memory there would be no computer, as we
know it today.
It is used for storing both instructions to be executed
and data.
This presentation has been developed after an intensive
research on Memory Devices.

3. TYPES OF MEMORY
MAIN MEMORY
o Random Access Memory (RAM)
o Read Only Memory (ROM)
CACHE MEMORY
SECONDARY MEMORY
o Optical Media Devices
o Magnetic Media Devices

4. MAIN MEMORY
Directly or indirectly connected to the CPU via a memory bus
Comprises of two buses: an address bus and a data bus
The CPU firstly sends a number through an address bus, a
number called memory address, that indicates the desired
location of data. Then it reads or writes the data itself using the
data bus.
Additionally, a memory management unit(MMU) is a small
device between CPU and RAM recalculating the actual memory
address, for example to provide an abstraction of virtual
memory or other task.
Broadly, the main memory is of two types-
o Random Access Memory (RAM)
o Read Only Memory (ROM).

5. RANDOM ACCESS MEMORY
 In RAM, the memory cells can
be assessed for information
transfer from any desired
location, that is, the processing
of a word in memory is the same
and requires an equal amount of
memory.
 It is the fastest main memory
technology.
 It requires constant power to
maintain the stored
information, therefore, it is
volatile.
A 1GB DDR RAM memory
module

6. BLOCK DIAGRAM OF RAM
Memory unit
2k words
N bits per word
K address line
read
write
n data input lines
n data output
lines
Control
lines

7. TYPES OF RAM
The RAM chips are of two types-
o Dynamic RAM(DRAM)
A form of volatile memory which also requires the stored
information to be periodically re-read and re-written,
or refreshed, otherwise it would vanish.
o Static RAM (SRAM)
A form of volatile memory similar to DRAM with the
exception that it never needs to be refreshed.

8. Dynamic RAM
 A type of RAM that stores
each bit of data in a
separate capacitor within an
integrated circuit.
 Since real capacitors leak
charge, the information
eventually fades unless the
capacitor charge is refreshed
periodically.
 Its advantage is its structural
simplicity: only one transistor
and a capacitor are required per
bit, compared to four transistors
in SRAM. This allows DRAM to
reach very high density.

9. DRAM STORAGE

10. DRAM STORAGE
0 1 2 3 4 5 6 7Row address
0 0000
1 0001
2 0010
3 0011
4 0100
Array of storage cell circuits
5 0101
6 0110
7 0111
Regenerator circuits
0000 0001 0010 0011 0100 0101 0110
0111
Column
address
Read/write
circuitry to
control
input/output

11. TYPES OF DRAM
DRAM chips are available in various designs:
 EDODRAM (Extended Data Out DRAM)
 SDRAM (Synchronous DRAM)
 RDRAM (Rambus DRAM)
 DDRDRAM (Double Data Rate DRAM)

12. EDODRAM (Extended Data Out DRAM)
Its cells keep the data valid
until it receives an
additional signal.
It has a dual-pipeline
architecture that allows the
memory controller to
simultaneously read new
data while discharging the
old.
A pair of 32 MB EDO
DRAM modules

13. SDRAM (Synchronous DRAM)
 SDRAM has a synchronous
interface, meaning that it waits
for a signal before responding
to control inputs and is
therefore synchronized with the
computer's system bus.
 This allows the chip to have a
more complex pattern of
operation than asynchronous
DRAM which does not have a
synchronized interface.
 Pipelining means that the chip
can accept a new instruction
before it has finished
processing the previous one

14. RDRAM (Rambus DRAM)
It is a type of
synchronous DRAM,
designed by
the Rambus Corporation.
It is fairly fast and has tried
to address some of the
complex electrical and
physical problems involved
with memory.

15. DDRDRAM (Double Data Rate DRAM)
Unlike SDRAM, it can do
two operations per cycle
thereby doubling the
memory bandwidth over
the corresponding single-
data-rate SDRAM

16. Static Random Access Memory (SRAM)
It is a type of memory in
which, memory refreshing is
not required.
It uses flip-flops to store
binary information.
As it takes up more space
than DRAM, it is used for
specialized applications.
It is much easier to use and
has shorter read-write cycles
compared to DRAM.

17. READ ONLY MEMORY
 It performs only read function
not write function. So the data
stored in ROM cannot be
modified.
 It comes with special internal
electronic fuses that can be
programmed for a specific
configuration. Once this pattern
is established it stays in the unit.
Thus, ROM is non-volatile.

18. TYPES OF ROM
 Programmable read-only memory (PROM)
 This device uses high voltages to permanently destroy or
create internal links (fuses or antifuses) within the chip.
Consequently, a PROM can only be programmed once.
 Erasable programmable read-only memory (EPROM)
 It can be erased by exposure to strong ultraviolet light (typically for
10 minutes or longer), then rewritten with a process that again
requires application of higher than usual voltage.
 Electrically erasable programmable read-only
memory (EEPROM)
 It allows its entire contents (or selected banks) to be electrically
erased, then rewritten electrically, so that they need not be
removed from the computer (or camera, MP3 player, etc.).

19. CACHE MEMORY
 It is a high speed storage mechanism.
 Can be either a reserved section of main memory or an independent
storage device.
 It speeds up access to data and instructions stored in RAM.
 MEMORY CACHE-
 It is a portion of memory of SRAM instead of the slower DRAM. By
keeping as much of the information as possible in high speed SRAM,
it avoids accessing the slower DRAM.
 DISK CACHE-
 It works under the same principle, but uses conventional main memory
(DRAM) instead of high speed SRAM. It improves the computers
performance a lot as accessing data from RAM is much faster than
from hard-disk.

20. TYPES OF CACHE MEMORY
Level 1 (L1) cache
 Built inside the CPU.
 It works at half CPU clock speed.
Level 2 (L2) cache
 Built external to CPU, in the motherboard.
 It works at the motherboard bus speed.
Nowadays both L1 and L2 are integrated in the CPU to
reduce access time and further improve system
performance.

21. The whole idea of memory cache
is to keep staging more
instructions and data in a high-
speed memory closer to the CPU.
MEMORY CACHE

22. DISK CACHE
Disk
caches
are
usually
just a
part of
main
memory
made up
of
common
dynamic
RAM
(DRAM)
chips.

23. SECONDARY STORAGE DEVICES
These devices are used to store large amount of data permanently.
It differs from primary storage in that it is not directly accessible
by the CPU. So they need more access time and thus are much
slower.
Per unit, it is typically also an order of magnitude less
expensive than primary storage. Consequently, modern
computer systems have an order of magnitude more
secondary storage than primary storage and data is kept
for a longer time there (such as in hard disk).
It is broadly of two types- 1) MAGNETIC MEDIA and
2)OPTIC MEDIA.

24. MAGNETIC MEDIA DEVICES
Magnetic storage uses different patterns
of magnetization in a magnetizable material to store
data
It is a form of non-volatile memory.
The information is accessed using one or
more read/write heads.
HARD DISKS and FLOPPY DISKS are such devices.

25. HARD DISKS
It stores information on one
or more continuously
spinning disks which are
coated with magnetic
material.
Information is recorded by
magnetic heads called
access arms.
These days, hard disks
have storage capacity
between 80 to 300 GB.
ACCESS ARM

26. FLOPPY DISK (DISKETTE)
It is made of a flexible
substance called Mylar.
It has a magnetic surface
for recording data.
It stores upto 1.44 MB of
data.
It cannot include graphics
or pictures within it.
All floppy disks must be
formatted before data can
be written on it.

27. OPTIC MEDIA DEVICES
Optic devices are generally circular disc which can
contain data encoded in bumps on a special material on
one of its flat surfaces.
The encoding pattern follows a continuous, spiral path
covering the entire disc surface and extending from the
innermost track to the outermost track.
The data is stored on the disc with a laser or stamping
machine, and can be accessed when the data path is
illuminated with a laser diode in an optical disc drive
These are broadly of two types- 1) CDs and 2) DVDs.

28. COMPACT DISK (CD)
CDs are very cheap and
store upto 700 MB of data.
They are of three types-
 CD-ROM (CD Read
Only Memory)
 CD-R (CD Recordable)
 CD-RW (CD Rewritable)

29. DIGITAL VIDEO DISK (DVD)
It is of the same size as a
CD but stores 15 times as
much information, is 20
times faster than it.
It can hold 17 GB of data.
It comes in three varieties-
DVD-ROM (DVD Read
Only Memory)
DVD-R (DVD Recordable)
DVD-RW (DVD
Rewritable)

30. NEW AGE MEMORY DEVICES
FLASH MEMORY-
 is a non-volatile memory
 It is a specific type
of EEPROM
 primarily used in memory
cards and USB flash drives
BLU-RAY DISK-
 Supersedes DVDs
 Uses blue-violet laser to read
the disc
 stores almost six times more
data than on a DVD