An arithmetic logic unit computes arithmetic or logic function

1
A four function ALU

• An arithmetic logic unit computes arithmetic or
  logic function
• We designed a two function arithmetic unit to
  perform ADD/SUB
• We can add two logical function to it, AND and OR
• A four function ALU will have two bits to specify
  function
• 00 (Add), 01 (Sub), 10 (AND), 11 (OR)
• Then the following set up will work

2
An alternate design of four function ALU

• The previous design is not very efficient as it
  uses an adder and subtract circuit
• We can design an add/subtract unit as discussed
  earlier
• Then we can design a logical unit (AND and OR)
  separately
• Then select appropriate output as result
• What are the control signals, Add/Sub, Select0,
  and Select1?

3
Operation codes

• In micro-controllers, ALU units have many
  function
• 16 arithmetic, 16 logic functions are common
• A 5 bit code decides what function is required
• This is part of an operation code in an
  instruction
• Selection of operands is also specified as part
  of instruction
• In actual instruction execution, operands A and B
  are first selected from different registers
• Then different functions are computed
• Finally, a result based on operation code is
  selected
• This sequence is repeated for every instruction