A computer uses a hierarchy of internal and external memory systems. Internal memory includes RAM, ROM, and cache, which provide fast access but are more expensive per byte. RAM allows independent access to each memory location and is used for main memory. ROM permanently stores data and is used for boot programs. Cached memory uses SRAM for faster access than RAM. External memory includes hard disks and USB drives, which provide large, inexpensive storage but are much slower to access.

1. Internal Memory

2. Introduction
Computer
memory exhibits different types
of technique, organization, performance,
size and cost.
But, no technology is optimal in memory
requirements for a computer system.
As a result, a computer equipped with a
hierarchy of memory subsystems.
Some are internal to the system – RAM,
ROM, CACHE
External memories are Hard disk, Optical
and Pen drive etc.,

3. RAM
Storage
cell unit consists of N cells each of
which can store 1 bit either 0 or 1.
Every location of RAM accessed
independently. So, the access and cycle
times can constant.
The address of memory location transferred
through MAR using address bus.
The address then proceed by the address
decoder.
Which select required location in storage
unit.

4. RAM
The
contd.,
contents of the selected location placed
in IR through MDR processor register.
The R/W control line specifies the type of
access to be performed.
Write operation, the word to be place in
MDR and the address specified in MAR
and then transferred to selected cell.
DDR RAM-Double Data Rate
synchronous dynamic random access
memory

5. RAM

7. ROM
Data
are written into a ROM when it is
manufactured.
ROM is mask programmed by the
manufacturer in the factory with the
contents ordered by the customers.
The contents are fixed by metal masks used
during chip fabrication.
Once programmed, the contents cannot be
erased.
Even a single bit wrongly programmed the
ROM chip is useless.

8. Application
Used
to store control programs such as
micro program.
Character generation, code conversion, etc.,

10. Boot screen

13. PROM-Programmable ROM
PROM
is a field programmable device.
The customer buy a blank PROM and store
desired data using PROM programmer(burner).
Programmability achieved by inserting a fuse at
point P.
Before programmed, the memory contains all 0s.
The user can insert 1 by burning out the fuse in the
particular cell using high current pulse.
The PROM chip can be programmed only once
and its contents cannot be erased.
PROM are flexible , faster and less expensive
because they can be programmed directly by the
user.

14. Application-PROM
Boot
program in micro computer

15. PROM

16. PROM

17. EPROM-Erase programmable ROM
A
rewritable chip that holds its contents
without power.
Previous data can be erased and new data
can be inserted
EPROM chips are written on an external
programming device before being placed on
the circuit board.
Capable of retaining stored information for
a long time.

18. EPROM
Eraser
contd.,
requires breakup the charges trapped
in the transistors of memory cell.[this is
done by break the chip to ultraviolet light].
This reason EPROM packaged with
transparent window.
Disadvantages:
Entire EPROM is erased as a whole and
selective erasing is not possible.
Should be removed from the chip for
reprogramming.

19. EPROM

21. EEPROM-Electrically Erased PROM
It
can be both programmed and erased
electrically( flashed back to Zero).
They do not need to removed when the chip
content erasure.
Also, erase selected content in the chip.
Erasing and programming dynamically
without removing the EEPROM from the
circuit.
Disadvantages:
Different voltages are required for erasing,
reading and writing the data.

22. Application
Used
as ROM
Critical system setup information in a nonvolatile way.

23. EEPROM

24. EEPROM

25. EEPROM

26. Flash Memory
Recent
kind of EEPROM is flash memory.
Which is erased by ultraviolet light.
EEPROM is byte erasable whereas, flash
memory is a block erasable.
Like a EEPROM when erasing the contents
of flash memory no need to remove from
the circuit.
In flash it is possible to read the contents of
a single cell, but is only possible to write an
entire block of cells.

27. Flash memory
It
contd.,
need single power supply and consume
less power.
Application:
Cell phones, hand held devices, digital
camera, washing machine, AC, Tablet etc.
NOTE: low power consumption of flash
memory makes it attractive for use in
portable equipment that is battery driven

28. Flash Card
Mount
flash chips on a small card.
That card simply plugged into a slot.
64 MB can store 1 hour songs.
Now a days, the device can equipped with
up to 32GB flash memory.

32. Speed, size and Cost
SRAM
chip is very fast memory in the computer
system. but it is expensive because their basic
cells have 6 transistors.
Which impossible to packaging a very large
number of cells onto a single chip.
Cost wise also expensive.
Cache memory can be implemented based on
SRAM.
Alternative is to use DRAM, which can have same
basic cells and thus much less expensive. But
significantly slower.
Still it is small in size compared to magnetic
drives.

33. Speed, size and Cost
Main
memory can be build with DRAM.
Secondary storage- large memory space with
reasonable price but much slower than
semiconductor memory unit.

34. conclusion
Huge
amount of cost-effective storage can
be provided by magnetic disk.
Main memory can be built with DRAM.
SRAM to be used in smaller unit where
speed is of the essence.(Cache)

35. SPEED

36. SPEED
Fastest
access is to data held in processor
registers.
There are often two levels of cache, the
primary cache inside processor refer to as
L1. secondary cache placed outside
referred as LEVEL 2(L2) usually SRAM
chip.
Next level in the hierarchy is called main
memory.
The typical main memory is about ten
times slower than the access for the L1

37. SPEED
The
bottom level in the hierarchy is called
the secondary memory such as magnetic
disk.
Disk provide a huge amount of
inexpensive storage. But they very slow.