Theory

Design of ALU

ALU or Arithmetic Logical Unit is a digital circuit to do arithmetic operations like addition, subtraction,division, multiplication and logical oparations like and, or, xor, nand, nor etc. A simple block diagram of a 4 bit ALU for operations and,or,xor and Add is shown here :

The 4-bit ALU block is combined using 4 1-bit ALU block

Design Issues:

The circuit functionality of a 1 bit ALU is shown here, depending upon the control signal S1 and S0 the circuit operates as follows:
for Control signal S1 = 0 , S0 = 0, the output is A And B,
for Control signal S1 = 0 , S0 = 1, the output is A Or B,
for Control signal S1 = 1 , S0 = 0, the output is A Xor B,
for Control signal S1 = 1 , S0 = 1, the output is A Add B.

The truth table for 16-bit ALU with capabilities similar to 74181 is shown here:

The L denotes the logic low and H denotes logic high.