EE200(class 81)
Prof. M.M. Dawoud
72
EE200 DIGITAL LOGIC CIRCUIT DESIGN
Class Notes
CLASS 81
The material covered in this class will be as follows:
⇒
Overflow.
⇒
BCD adder.
⇒
Binary multiplier.
⇒
Magnitude comparator.
Overflow
Overflow is defined as the situation when two Ndigit numbers are
added and the sum occupies (N+1) digits. This situation occurs
when adding binary numbers as follows:
1.
An end carry is generated when adding two Nbit unsigned
numbers.
2.
The carryin and carryout bits are different when adding two
Nbit signed binary numbers.
Example: Suppose we are adding +70 and +80 using 8bits in
signed 2’s complement form.
+70
Æ
01000110
and +80
Æ
01010000
The carry
0 1
+70
0 1 0 0 0 1 1 0
+80
0 1 0 1 0 0 0 0
+150
1 0 0 1 0 1 1 0
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Prof. M.M. Dawoud
73
The same overflow occurs when adding (70) + (80)
The carry
1 0
70
1 0 1 1 1 0 1 0
80
1 0 1 1 0 0 0 0
150
0 1 1 0 1 0 1 0
An overflow can be detected by observing the carryin and the
carryout of the sign bit. If we apply them to an exclusiveor gate
then the output is one when overflow occurs.
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 Spring '09
 prof.ahmed
 Binarycoded decimal, Prof. M.M. Dawoud

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