EE 306  Introduction to Computing
Homework #2 Solutions
1.
3.24 (13 pts = 4+7)
a.
The role of X is to choose between two possible inputs as the second operand,
namely B and C. By setting X to either 0 or 1 the multiplexers choose either B
or C to be connected to the adder. If X is 0, the adder adds A and B while if it
is 1, it adds A and C (or vice versa depending on which of them is connected
to which input).
b.
This can be made by computing –B (B’s 2’s complement) on the fly. We set
all C
i
= NOT B
i
or as a whole C=NOT B; computing B’s 1’s complement then
we add 1 to it by setting carryin of the first adder module. The below diagram
illustrates the new circuit:
A3 A2 A1 A0
B3 B2 B1 B0
A3 A2 A1 A0
B3 B2 B1 B0
C3 C2 C1 C0
X
ADD/SUBTRACT
Carryin
S3 S2 S1 S0
S3 S2 S1 S0
2.
3.28 ad (8 pts = 2+2+2+2)
a.
For a two bit binary number, the maximum value is (11)
b
=(3)
10
.
b.
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 Fall '07
 Ambler
 pts, maximum value

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