Arithmatic Unit:

• ALU is a single circuit, and this performs arithmetic and logical operations
• This is core of any processor
• A typical ALU will have two input ports and a result port. It will also have a control input telling it which operation
• To perform arithmetic and logical operations necessary operands are transferred from memory location to ALU where it is stored in the temporary registers
• It also has necessary hard ware to perform operations on signed, unsigned integers, floating point numbers, BCD numbers etc.

Let us consider the Circuit of Arithmetic Unit given below

• It has 4 bit parallel adder
• X and Y are the two input operands each of 4 bit
• There are 4 one of two multiplexers associated with the y operand of the parallel adder
• The multiplexer selects either y or Y' depending on the selection
• line S0 which is also carry in, input to the adder
• If S0=0 Output is F=X+Y
• If S0=1 Output is F=x+y'+1 where y'+1 is 2' complement of y, so the output is F=X-Y

Note: Subtraction is performed using adder circuit by 2's complement method. So we do not have a subtractor circuit in the Arithmetic unit

Logic Unit

Consider a simple logic unit given below which can perform two logic operations AND and EXCLUSIVE OR. The multiplexer selects either of the operations depending on the selection line S0.

• If S0=0 out put is G=X AND Y
• If S0=1 out put is G= X ^ Y (Exclusive OR)
• Some additional hard ware is also added to perform other Boolean operations
  Example:
  X'=X ^ 1
  X^Y^Y^XY=X OR Y
  

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