Discussion
At the heart of digital computers and calculators is the arithmetic unit.
Depending on the system, this unit can range in complexity from very simple
such an adder, to a unit that possesses the capabilities of computing values
for special functions such as trigonometric and logarithmic functions.
No matter how complex the unit is, its most important functions is still
addition.
The primary building blocks in adder circuits are half adders and full
adders.
Full Adder
The Truth table for a full adder that shows all the input and output values
for all possible case is in FIGURE 1.
Augend bit input
A
|
Addend bit input
B
|
Carry bit input
CIN
|
Sum bit output
S
|
Carry bit output
COUT
|
0
0
0
0
1
1
1
1
|
0
0
1
1
0
0
1
1
|
0
1
0
1
0
1
0
1
|
0
1
1
0
1
0
0
1
|
0
0
0
1
0
1
1
1
|
FIGURE 1
Truth table and block diagram for a Full Adder circuit.
FIGURE 2
Complete circuitry for a full adder.