For this lab you will implement a basic $4-$bit ALU. Our ALU will take in two $4-$bit inputs,

perform a selected operation, then output the result. The four operations our ALU will

implement will be: ADD, AND, NOT, and EQUALITY

Getting Started

Open the provided Lab$\_2\_$template project in Logisim. You'll see several inputs and outputs

already added to the circuit. They are:

Two $4-$bit pins for the ALU inputs

One $2-$bit pin for selecting the ALU operation

One $4-$bit pin for the ALU output

One $1-$bit pin to store the carry bit of an ADD operation

Using only simple logic gates, implement each of the four operations. Each operation should

receive its input data from the same two $4-$bit ALU input pins and output its result to the $4-$

bit ALU output pin. You will use a multiplexer to select which operations output is stored in

the ALU output pin. Implementing the ADD operation

The ADD operation will sum the values of ALU$\_$input$\_$a and ALU$\_$input$\_$b$.$ The result of

the summation will be stored in ALU$\_$output and the final carry bit will be stored in

ADD$\_$carry. The ADD result will only be stored when ALU$\_$select is $00.$ If the ADD

operation is not selected, the ADD portion of the circuit should not affect the value of

ALU$\_$output in anyway. If ADD is not selected, the value of ADD$\_$carry should be $0.$

Hint: As mentioned in the lecture slides, chaining multiple $1-$bit adders together allows for

multibit numbers to be summed.

Implementing the AND operation

The AND operation will perform a logical AND operation on the values of ALU$\_$input$\_$a

and ALU$\_$input$\_$b$.$ The result of the logical AND will be stored in ALU$\_$output. The AND

result will only be stored when ALU$\_$select is $01.$ If the AND operation is not selected, the

AND portion of the circuit should not affect the value of ALU$\_$output in anyway.

Implementing the NOT operation

The NOT operation will perform a logical NOT operation on the value of ALU$\_$input$\_$a$.$

The result of the logical NOT will be stored in ALU$\_$output. The NOT result will only be

stored when ALU$\_$select is $10.$ If the NOT operation is not selected, the NOT portion of the

circuit should not affect the value of ALU$\_$output in anyway.

Implementing the EQUALITY operation

The EQUALITY operation will compare the values of ALU$\_$input$\_$a and ALU$\_$input$\_$b and

determine if they are equal. If the two input values are the same, ALU$\_$output should be all

$0$s$.$ If the values are not equal ALU$\_$output should be a non$-$zero value. Note is does not

matter what the specific output of the EQUALITY operation is when two values are not

equal, just as long as the output is non$-$zero. The EQUALITY result will only be stored

when ALU$\_$select is $11.$ If the EQUALITY operation is not selected, the EQUALITY

portion of the circuit should not affect the value of ALU$\_$output in anyway.

Hint: For the AND, NOT, and EQUALITY operations, do not overcomplicate your

solutions. You can start by developing a truth table for the operation then design the circuit

Build a Multiplexer

Lastly, create a $4-$bit, $4$ to $1$ multiplexer circuit. Using only simple logic gates, build a

multiplexer circuit so that each combination of ALU$\_$select connections a different

operation to ALU$\_$output. Note that while similar to the multiplexer from Lab $1,$ the inputs

and output of this multiplexer are $4$ bits wide rather than $1$ bit wide