• It has hardware circuits that performs arithmetic & logical operation
  • H/w section are
    • Adder
    • Accumulator
    • General purpose register (GPR)
    • Counters
    • Shifters
    • Complementer

Adder ->

Circuit which adds 2 numbers & gives result

Accumulator ->

Register that temporarily holds results of previous operation in ALU

General Purpose Register (GPR) 

  • Operand in CPU – immediately available

  • GPR stores 

    • operand 
    • operand address(all used for multiple purposes)
    • constant 

Scratch Pad Memory or Registers:

  • To store intermediate results temp

  • There are usually one or more scratch pad registers 

  • Purely internal h/w resources

  • Not addressable by program

Shifter & complementer

  • provides left shift & right shift required for various arithmetic operation

  • complementer provides 2’s complement of binary numbers 

Positive & negative no.s 

ALU Size:

  • Performance of computer is dependent on ALU Size

  • Need not be same as memory word length 

  • In original IBM PC< 16 bit ,  ALU whereas memory word length is 8 bit