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A ROM is a combinational component for storing data. The data might be a truth table or the data might be the control words for a microprogrammed CPU. A ROM that implements two functions does not require twice the number of gates as a ROM that implements one function. (The decoder is shared by every output function.)

we will design Read only memory using the circuits cloud simulator

http://circuits-cloud.com/

if you does not used the circuits-cloud before, you can see this intractable:

https://www.instructables.com/id/Circuits-Cloud/

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## Step 1: The ROM Design

Because it is 16*8 ROM, it needs:

- 4 digital inputs

- 8 leds ( as output )

So, drag and drop the components from the digital components panel and connect them

## Step 2: Store the Value in ROM Locations

1. Double click in ROM parameter.

2. write an 8-bit value in each location of ROM.

3. click on ( save parameter)

## Step 3: Saving and Running the Circuit

save your circuit then running it.

check the output value in each address (location):

if input ( A0, A1, A2, A3 ) =

0000 , output = the value stored in location 0

1000 , output = the value stored in location 1

0100 , output = the value stored in location 2

1100 , output = the value stored in location 3

0010 , output = the value stored in location 4

and so on.

- you can check the circuit:

http://circuits-cloud.com/circuit/details/gdd1FJeK...

- also you can see how the circuit works.. in this video

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## 2 Discussions

An eight x eight ROM contains 8 words with 8 bits in each word. Addressing 8 locations takes only 3 bits, A0, A1 and A2. Why does your ROM require 4 address bits?