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1. INTRODUCTION OF
ROM – Read Only
Memory
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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
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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.
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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.
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6. ROM Internal Structure
...
n Inputs
Lines
n bit
decoder
...
m Outputs Lines
.
.
.
Memory Array
2n words x m bits
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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
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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
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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
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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
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11. ROM Array Layout
Unit cell is 12 x 8 l (about 1/10 size of SRAM)
Unit
Cell
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12. Row Decoders
ROM row decoders must pitch-match with
ROM
Only a single track per word!
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13. Complete ROM Layout
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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
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