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Engineering Digital Design
book

Engineering Digital Design

by Richard F. Tinder
January 2000
Intermediate to advanced content levelIntermediate to advanced
884 pages
29h 39m
English
Academic Press
Content preview from Engineering Digital Design
8.4 THE CARRY LOOK-AHEAD ADDER 345
sum of two positive numbers only if C
n
= C
n1
. Thus, a sign-bit overflow error detector
can be implemented by
S
Error Det
= C
n
C
n1
, (8.4)
requiring that the sign-bit carry-in C
n1
be accessible, which it may not be for IC chips.
Another approach permits a detector to be designed that depends only on the external
inputs to and sum bit from the sign-bit stage. A further inspection of truth table for an FA
in Fig. 8.2c indicates that a sign-bit overflow error can occur only if A = B when C
out
= S
for the (n 1)th stage. Shown in Fig. 8.11a is the truth table for the sign-bit overflow error
conditions based on this fact. From this truth table there results the expression
S
Error Det
=
¯
S
n1
(A
n1
B
n1
) + S
n1
(
¯
A
n1
¯
B
n1
), (8.5) ...
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Publisher Resources

ISBN: 9780126912951