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
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
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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
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!
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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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