Explain Arithmetic Logic Shift Unit.

Arithmetic logic shift unit is a digital circuit that performs arithmetic calculations, logical manipulation and shift operation. It is often abbreviated as ALU. The above figure shows the one stage of arithmetic logic shift unit.

The block diagram of ALU includes one stage of arithmetic circuit, one stage of logic circuit and one 4*1 multiplexer. The subscript i designates a typical stage. Inputs Ai and Bi are applied to both the arithmetic and logic units. A particular microoperation is selected with inputs S1 and S0. A 4*1 mux selects the final output. The two inputs of the mux is received from the output of the arithmetic circuit and logic circuit. The other two is A_i-1 for the shift-right operation and A_i+1 for the shift left operation. The circuit is repeated n times for n-bit ALU. The output carry C_i+1 is connected to the input carry C_in. In every stage the circuit specifies 8 arithmetic operations, 4 logical operations and 2 shift operations, where each operation is selected by the five variables S₃, S₂, S₁, S₀ and C_in.

The operations of ALU can be summarized in table below:

Operation select					operation	Function
S3	S2	S1	S0	Cin
0	0	0	0	0	F=A	Transfer A
0	0	0	0	1	F=A+1	Increment A
0	0	0	1	0	F=A+B	Addition
0	0	0	1	1	F=A+B+1	Add with carry
0	0	1	0	0	F=A+	Subtract with borrow
0	0	1	0	1	F= A+ +1	Subtraction
0	0	1	1	0	F=A-1	Decrement A
0	0	1	1	1	F=A	Transfer A
0	1	0	0	╳	F=A˄B	AND
0	1	0	1	╳	F=A˅B	OR
0	1	1	0	╳	F=A⊕B	XOR
0	1	1	1	╳	F=	Compliment A
1	0	╳	╳	╳	F=shr A	Shift right A into F
1	1	╳	╳	╳	F= shl A	Shift left A into F