1. ECEN 248 Lab 4: Multiplexer Based Arithmetic Logic Unit
names
Dept. of Electrical and Computer Engineering

2. Lab 4 in Brief
Design a 4-bit arithmetic logic unit which does elementary computation like:
Addition
Subtraction
Bit-wise AND
Use a multiplexer based control to choose between the operations

3. Unsigned Number Representation
This is used for unsigned numbers (i.e., positive and negative are not defined) 3

4. Two’s Complement
Used to represent both positive and negative (signed) integers
Positive numbers are represented by their ordinary binary representation
Negative numbers are represented by the two’s complement

5. Converting to Two’s Complement
Step 1: Start with the ordinary binary representation of the number
Ex: If we want to convert -3 to Two’s Complement, we start with representing 3 as 011
Step 2: Invert all the bits of the number
Ex: Inverting all the bits of 011 results in 100
Step 3: Add 1 to the resulting value
Ex: = 101
If the addition results in an overflow, ignore it.
Taking the Two’s Complement of the result will give you the positive number back! 5

6. Multiplexers (Mux)
A device which selects a single input from a series of inputs and passes its value to the output
The selection is done based on data fed to the control lines (S) of the mux
2:1 Multiplexer 4:1 Multiplexer

7. Arithmetic Logic Unit (ALU)
Performs arithmetic and logic operations
Fundamental building block of a CPU
Inputs to the ALU:
Two numbers (operands)
Control bits to control what operation the ALU is performing
Outputs of the ALU:
The computed result
An indication if the operation has resulted in an overflow

8. Arithmetic Logic Unit (ALU)

9. Specifics and Design Hints
Components to design the ALU:
One 2:1 Multiplexer block (Four 2:1 Multiplexers with same select line (C0))
One XOR Chip (Four XOR Gates)
One 4-Bit Adder Block
5 LEDs to show Outputs
Hints for the design of the ALU:
Multiplexer Select Line Input is C0
C1 decides whether addition or Subtraction
For subtraction when C1=1, use XOR gates to invert the inputs and Cin of the full adder to add ‘1’(i.e. C0) for the 2’s complement of the 2nd operand.
Four bit adder block does addition and subtraction.

10. Deadlines Today Next Week Work on Lab 4 Lab 4 Post-Lab due
Lab 5 Pre-Lab: no deliverables