This document discusses Read Only Memory (ROM) including its concept, characteristics, types, internal structure, implementation in Moore and Mealy machines, array layout, row decoders, and complete layout. ROM is read-only memory that does not lose its data when power is lost. It is commonly used to store firmware like the BIOS and can implement truth tables for combinational logic in finite state machines.

1. INTRODUCTION OF
ROM – Read Only
Memory
http://alltypeim.blogspot.in/

2. ROM
 Concept
 Characteristics of the ROM BIOS
 ROM Types
 ROM Internal Structure
 ROM Implementation
 ROM Implementation of a Moore Machine
 ROM Implementation of a Mealy Machine
 ROM Array Layout
 Row Decoders
 Complete ROM Layout
 Summary http://alltypeim.blogspot.in/

3. Concept
Is read-only memory.
Do not lose data when power is lost.
ROM memory is used to produce chips with
integrated CMOS BIOS program
http://alltypeim.blogspot.in/

4. Characteristics of the ROM BIOS
BIOS ROM containing the software configuration
and system diagnostics, and routine input / output
low level that DOS uses.
These programs are encoded in ROM and is called
firmware.
An important feature of the ROM BIOS is
detecting new hardware in the computer and
reconfigure the operating system as device driver.
http://alltypeim.blogspot.in/

5. ROM Types
 PROM (Programmable Read Only Memory):
 Type of ROM that information is only installed once.
 The CD can be called PROM.
 EPROM (Erasable Programmable ROM):
 Type of ROM that can erase and rewrite it.
 "CD-Erasable" can be called EPROM.
 EEPROM (Electronic Erasable Programmable ROM):
 It form enhence of EPROM, a difference compared to the
EPROM is able to write and remove the information again
and again by software rather than hardware.
 Example: "CD-Rewritable".
 Application specific EEPROM is "flash BIOS". ROM is the type
of information can install or upgrade software.
http://alltypeim.blogspot.in/

6. ROM Internal Structure
...
n Inputs
Lines
n bit
decoder
...
m Outputs Lines
.
.
.
Memory Array
2n words x m bits
http://alltypeim.blogspot.in/

7. ROM Implementation
• 16-word x 5 bit ROM ROM
L, R
S1:0
TL, TR, F
S'1:0
S1
' S0
' TR'TL' F'
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
4:16DEC
S1 S0 L R
http://alltypeim.blogspot.in/

8. ROM Implementation
• 16-word x 5 bit ROM ROM
L, R
S1:0
TL, TR, F
S'1:0
S1
' S0
' TR'TL' F'
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
4:16DEC
S1 S0 L R
http://alltypeim.blogspot.in/

9. ROM Implementation of a
Moore Machine
 ROMs implement combinational logic
 Note that ROMs do not hold state
 How would you determine the maximum clock
frequency of this circuit?
Look at the FF to FF path (NS to PS)
ROM ROM
Present
State
Next
State
Outputs
Inputs
http://alltypeim.blogspot.in/

10. ROM Implementation of a Mealy
Machine
 ROMs implement combinational logic
 Note that ROMs do not hold state
 How would you determine the maximum clock frequency of this
circuit?
 Look at the FF to FF path (NS to PS)
ROM
ROMPresent
State
Next
State
Outputs
Inputs
http://alltypeim.blogspot.in/

11. ROM Array Layout
Unit cell is 12 x 8 l (about 1/10 size of SRAM)
Unit
Cell
http://alltypeim.blogspot.in/

12. Row Decoders
ROM row decoders must pitch-match with
ROM
Only a single track per word!
http://alltypeim.blogspot.in/

13. Complete ROM Layout
http://alltypeim.blogspot.in/

14. Summary
 ROMs provide stable storage for data
 ROMs have address inputs and data outputs
ROMs directly implement truth tables
 ROMs can be used effectively in Mealy and Moore machines
to implement combinational logic
 In normal use ROMs are read-only
They are only read, not written
 ROMs are often used by computers to store critical
information
Unlike SRAM, they maintain their storage after the power is
turned off
http://alltypeim.blogspot.in/