Last updated on June 13th, 2020 at 08:31 pm
The following diagram is representing the addressing of 5 ROM chips using 74LS138 where each chip is of 4 KB.
215
A15
|
214
A14
|
213
A13
|
212
A12
|
211
A11
|
210
A10
|
29
A9
|
28
A8
|
27
A7
|
26
A6
|
25
A5
|
24
A4
|
23
A3
|
22
A2
|
21
A1
|
20
A0
|
||
ROM
0
|
Start
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
End
|
0
|
0
|
0
|
0
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
ROM
1
|
Start
|
0
|
0
|
0
|
1
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
End
|
0
|
0
|
0
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
ROM
2
|
Start
|
0
|
0
|
1
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
End
|
0
|
0
|
1
|
0
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
ROM
3
|
Start
|
0
|
0
|
1
|
1
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
End
|
0
|
0
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
ROM
4
|
Start
|
0
|
1
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
End
|
0
|
1
|
0
|
0
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
So, from the above table it can be found that, by setting the value of A15 to 0 we are enabling 74LS138, by the values of A12, A13, A14 we are selecting that which ROM will be read, then for each location of that ROM we are having 12 digit binary value where the value for first location is 000000000000 and value for the last location is 111111111111.
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