Arithmetic Logic Unit (ALU)

ALU is a fundamental building block of Central Processing Unit (CPU). ALU stands for Arithmetic Logic Unit. It is a combinational logic circuit that performs arithmetic and logic operations. Even the simplest microprocessors contain one for purposes such as maintaining timers. 

The two operations, arithmetic and logic, depend upon the number of selection lines to select a particular operation in the unit. Mainly ALU contains two inputs which are controlled by the select line for the multi-operation.

When, 

S2=0: Arithmetic operations (total 8). Which one – is specified by S1, So and Cin.

S2=1: Logic operations (total 4). Which one – specified by So and Cin.

Schematic Symbol of ALU

A and B: the inputs to the ALU (i.e. operands)

R: Output or Result 

F: Code or Instruction from the Control Unit (i.e. as op-code) 

D: Output status; it indicates cases such as:

• carry-in
• carry-out,
• overflow,
• division-by-zero
• And

Major Parts of ALU

Arithmetic block: This block is used to perform arithmetic operations such as addition, subtraction and comparison. The core of the arithmetic block is an adder. The adder uses carry look-ahead and sum-select techniques. The arithmetic operations performed by ALU are given below:

Logic block: This block is used to perform simple bitwise logic operations such as AND (masking), OR and XOR, XNOR, NAND, NOT and etc.The logic operations performed by ALU are given below:

Multiplexers(i.e. the selector to choose between the two circuits): These blocks are used to select the appropriate inputs for the arithmetic and logic blocks. Usually more than two buses arrive at the inputs of the ALU. Sometimes these multiplexers are used to perform some simple logic operations. The 2:1 MUX can be programmed to invert one of the operands (this can be used to execute a subtraction using just an adder).

Design of ALU

    The basic design of an Arithmetic Logic Unit is given below: