A four function ALU A 00 ADD B 01 10 MUX SUB 11 Result AND OR 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 Result ADD A B MUX Select 2 bits SUB AND OR 00 01 10 11

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? MUX Result ADD/SUB A B Select1 AND OR 1 Select0 Add/sub

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