Use software Logisim to build a 4-bit ALU capable of doing addition, subtraction, logical AND, and logical OR.

First create a circuit called 1-bitAdder with three inputs a,b, carryIn, two outputs sum and carryout. Remember input is a square and output is a circle. You may use logical equation in slide 24 or combine slide 26 and 27 in LogicalDesign1.pptx. You may also follow what I did in class.

Then create a circuit called 1-bitAlu, to include 1-bitAdder, AND, OR and a multiplexer, and 2-bit Operation. It has three input a, b, carryIn, and two output c and carryout. It has 1-bit signal called BInvert which is used to invert b to feed into the second input of 1-bitAdder. It also has 2-bits control signal called Operation. See slide 28 in LogicalDesign1.pptx

Use four 1-bitAlu to create a 4-bitAlu. with three control signals: Bnegate (1), Operation (2),

Control signals are

000, logical AND

001, logical OR

010, ADD

110, Subtract

Both of your input should be 4-bits. You could use a splitter in Wire to split it into 4 1-bit line to feed into each of the four 1-bitAlu.

Use a splitter to split three control signal into two lines. Left most bit is used a BInvert and initial carryIn (carryIn for the right most 1-bitAdder). The rest two bits can be used to feed into each 1-bitAlu as Operation.