Read Only Memory

1. READ ONLY MEMORY
PRADEEP GUPTA

2. ROM (READ ONLY MEMORY)
 ROM is a class of storage media used in computers
and other electronic devices.
 Data stored in ROM cannot be modified (at least
not very quickly or easily).
 The ROM is non-volatile memory as it retains
stored information even if the power is turned off
(unlike RAM).

3.  Processor uses ROM to store and execute
programs instructions.
 It is mainly used to distribute firmware.
 Firmware is software that is very closely tied to
specific hardware, and unlikely to require frequent
updates.
 It is permanent memory in a computer that cannot
be changed. Most memory is wiped clean when you
turn off your computer. If it lost all memory it would
not be able to start up again. So it needs some
instructions stored in memory that is there when the
computer starts. It then uses this to give further
instructions to the computer and help it boot up.

5. READ ONLY MEMORY
 Read
 The word line is activated.
 The transistor is closed and the
voltage on the bit line drops to
near 0 if there is a connection
between the transistor and the
ground.
 If there is no connection to
ground, the bit line remains high.
 Write
 Done when it is manufactured
Bit line
Word line
•Not connected to
store 1
•Connected to store
0
P

6. ROM INTERNAL STRUCTURE
n X N
decoder
Memory array
2^N words X m bits
n input lines
m output lines/data lines
Address
lines

7. Size of ROM to implement full adder :-)
Full adder
X
Y
S =∑(1,2,4,7)
Z
C=∑(3,5,6,7)
Address line= No. of variables=3
Data line= No. of functions to be implemented=2
CS
ZYX
A0
D1D0
A1 A2
Therefore, Size of ROM=2^3*2
Address location=2^3=8
2^3*2 ROM

8. Location A2 A1 A0 S C
0 0 0 0 0 0
1 0 0 1 1 0
2 0 1 0 1 0
3 0 1 1 0 1
4 1 0 0 1 0
5 1 0 1 0 1
6 1 1 0 0 1
7 1 1 1 1 1
ROM table
What is the data stored in 4th location?
Ans=10

9. APPLICATIONS
 A typical application of a ROM might be an
industrial machine that repeats the same sequence
over and over.
 ROM is used to store Basic Input-Output System
(BIOS).
 Used extensively in embedded system.
 The boot up software for a regular PC, which
performs the same task each time the power is
turned on.
 ROMs are used in constructing look-up tables for
computer systems. Look-up table are used to store
logarithms of number, sines and cosines of angles,
fixed physical constants and so on.

10. TYPES OF ROM
Programmable read-only memory
• Standard PROM can only be programmed once.
• PROMs are manufactured blank. The data is programmed
into them after manufacturing.
• PROM chips are manufactured with a series of fuses.
PROM chips have several different applications,
including cell phones, video game consoles, RFID tags,
medical devices, and other electronics.

11.  Programmability is achieved
by inserting a fuse at point P
 Memory contains all 0’s
before programming.
 The user can insert 1’s at the
required locations by burning
out the fuses at these
locations using high current
pulses.
 This process is irreversible.
 Provides flexibility and
convenience not
available in ROM.
Bit line
Word line
•Manufactured
connected (storing 0),
but the connection is
a “fuse” and can be
burned out with a high
current to change it to
1.
P(Fuse)

12. Erasable programmable read-only memory(EPROM)
 Content can be erased and reprogrammed.
 (EPROM) can be erased by exposure to strong ultraviolet
light.
 EPROM chip packages can often be identified by the
prominent quartz "window" which allows UV light to enter.
 Must be physically removed from the circuit for
reprogramming.
 Entire content are erased.

13. Electrically erasable programmable read-only memory
(EEPROM)
 Erased electrically.
 Need not be removed from the computer (or camera, MP3
player, etc.).
 Possible to erase the cell contents selectively.
 Disadvantages
 Different voltages are needed for erasing , writing and reading
the stored data.
 Writing or flashing an EEPROM is much slower (milliseconds
per bit) than reading from a ROM or writing to a RAM
(nanoseconds in both cases).

14. Electrically alterable read-only memory(EAROM)
 Only 1 bit can be modified at a time.
 Writing is a very slow process and requires higher
voltage (usually around 12 V) than is used for read
access.
 EAROMs are intended for applications that require
infrequent and only partial rewriting.
 EAROM is an expensive and complex technology.
 Most often used for industrial and military applications.

15. Flash memory
 Flash memory is a non volatile memory.
 They can be electrically erased and reprogrammed.
 It is an extension of EEPROM.
 It allows the stored data to be saved even when the
device is disconnected from the power source.
 Writing speed is much faster than EEPROM.
 Data capacity is more.
 E.g. pen drives, solid state drives, memory cards, etc.

16. DISADVANTAGES
 ROMs however also have some disadvantages.
 It takes a long time to manufacture a ROM.
 The unit price of a ROM is quite high.
 Once the ROM has been manufactured and if some
error has been discovered in the information store
you can’t change the contents.
 This means that making mistakes are very
expensive.

17. REFERENCES
 Computer Architecture And Organization JOHN P.
HAYES
 http://www.google.co.in
 http://www.worldofcomputing.net/memory/read-
only-memory.html
 https://www.slideshare.net/rohitladdu/romread-only-
memory
 http://www.homes.ieu.edu.tr/leren/ce302/lectures/R
OM%20(Read%20Only%20Memory)

18. THANK YOU