MA251 Computer Organization and
Architecture [3-0-0-6]
Lecture 7: Read-Only Memories (ROM)
Spring 2011
Partha Sarathi Mandal
Read-Only Memories (ROM)
• Memory is like an array of mailboxes
(registries of binary data)
• A stored array of binary data which
can be read only, but not changed.
• A block diagram of a ROM
– For each combination of inputs on the n
input lines, the corresponding pattern
of 0’s and 1’s (a word) is generated on
the m output lines.
– typical sizes range from (32 words x 4
bits) to (512K words x 8 bits) or larger.
n inputs
2n X m
ROM
m inputs
2n × m ROM
n inputs
n inputs
lines
n–to-2n
2n X m
ROM
m inputs
m inputs lines
Logical construction of a 32 X 4 ROM
Address input
Minterms
Combinational-circuit implementation with a 4 X 2 ROM
ROM Realization of Logic Functions
F0= A’B’ + AC’
F1= B + AC’
F2= A’B’ + BC’
F3= AC + B
ROM Realization of Logic Functions
• ROM consists of a decoder and a memory array.
• When a particular input sequence is applied to the n decoder inputs, exactly
one of the 2n outputs is set to 1.
• This output line selects one of the words in the memory array.
F0
F1
F2
F3
Exercise: Design a combinational circuit using a ROM. The circuit accepts a 3bit number and generates an output binary number equal to the square of
the input number.
• First step is to derive the truth table for the
combinational circuit
– In some case, we can fit a small truth table for ROM
by using certain properties in the truth table
Block diagram
ROM truth table
ROM Types
• Mask-programmable (ROM)
– Permanent programming done at fabrication time
– Fabrication take place at factory as per customer order
– Very expensive and therefore feasible only for large quantity orders
• Programmable ROM (PROM)
– User programmed after purchase, called field-programmable ROM (FPROM)
– Reprogrammable by user, erasable by UV emission, called erasable,
programmable ROM (EPROM)
• Electrically erasable, programmable ROM (EEPROM)
– User can erase individual words; switching elements can be enabled/disabled
– Can be erased and reprogrammed limited number of times, typically 100 to
1000 times
• Flash memory
– Like EEPROM, but all words (or large blocks of words) can be erased
simultaneously
– Become common relatively recently (late 1990s)