Lab #12: Arithmetic-

Logic Unit

Zack Mattis

Lab: 3/27/17 Report: 4/8/17 Partner: Brendan Schuster

Purpose

In this lab, a fully functioning 4-bit Arithmetic Logic Unit (ALU) was designed

and fully implemented onto a protoboard that is capable of performing a number of

operations. ALUs can perform both arithmetic operations such as addition and

subtraction as well as logical operations including ANDs and ORs. ALUs are utilized

by CPUs to provide the data calculations of the specified functions. This

implementation utilizes a 4 x 4 register previously created and a 4-bit latch to provide

the inputs to the ALU. Additionally, the output of the ALU is fed back into the 4 x 4

register through the use of a 4 x 2:1 MUX to store the calculated hexadecimal values in

the registers. Two 7-segment displays are included to observe the output of the ALU

as well as the output value of the 4 x 4 register. The design can be seen in Figure 1

below. The operation of the design was verified using the LogicPort Logic Analyzer to

observe the signals of the circuit. The design was implemented using one 4x4 register

(74LS670), one ALU (74LS181), two counters (74LS193), one 4 x 2:1 MUX (74LS157),

two D Flip-Flops (74LS74), two 7-segment display drivers/decoders (74LS247), two 7-

segment displays, three 8-bit DIP SPST switches, one SPST switch, 18 resistors, and 4

LEDs.

Figure 1 – 4 bit ALU

Procedure

The design for the 4-bit ALU was created using the Altera Quartus 9.1 software

and can be seen in Figure 2 (see attached). The design uses a 4x4 register combined

with 2 counters for the read/write, and 4-bit latch, a 4-bit ALU, and two decoders for

the seven segment displays that convert the hexadecimal input/output into a visual 7

segment output display. Additionally, a 4 x 2:1 MUX supplies the input of the 4 x 4

register from either user-selected input data or the output of the ALU to store the

calculated data in the registers. This design was then tested using the Quartus’ simulation tool that allows for user selected data for each of the inputs. By manipulating

the data in the 4 x 4 register, and sending it to the latch, the user can select the two data

operands of the ALU. By changing the select lines of the ALU (S[3] – S[0]), the mode

(M), and the carry in (Cin), the user can select which ALU operation he/she wishes to

perform. Four different operations (addition, subtraction, AND, OR) were tested using

Quartus and their respective waveforms can be seen in Figures 3-6 (see attached).

Finally, 4 hexadecimal digits were added together using the 4 registers of the 4x4

register. The values 1, 2, 2, and 3 were added to get the final output of 8, which can be

seen in Figure 7 (see attached).

The verified design was then implemented onto the protoboard using the

previously mentioned IC chips. DIP switches were used to provide the control signals

as well as the 4-bit data signals. A SPST switch was used to provide the clocking signal

for the latch to store the 4-bit hex value from the register. The ALU inputs were from

the 4-bit latch and from the 4x4 register. Through the use of the 4 x 2:1 MUX, the

output of the ALU was wired back into the register so that it could be stored. Both the

register value and the output from the ALU were wired to seven-segment display

decoders and then seven segment displays. These displays allowed for easy viewing of

the hexadecimal value that was being produced. Additionally, LEDs were added to the

output of each counter to view which register was active.

This protoboard implementation was then analyzed using the LogicPort Logic

Analyzer. This device converts the analog signal of the circuit into a digital form of 0s

and 1s. This tool enables effective observation of the circuit’s inputs and outputs. In our testing, we connected the device to 19 data inputs. Each of the four operations

simulated in Quartus were then tested on the Logic Analyzer. The output waveforms

of addition, subtraction, AND, and OR can be seen Figures 8-11 (see attached),

respectively.

Results

The use of a 4x4 register wired in combination with a 4-bit latch allows for

effective input signals for the operands of the ALU. Utilization of a 4 x 2:1 MUX was

a necessary implementation for the ALU in order to store the calculated data output in

the register. Additionally, the use of seven segment displays allowed the user to easily

see the hexadecimal value stored within the register as well as the hexadecimal output

of the ALU. The use of LEDs for the output of the counter also allow the user to see

which register has been selected.

Our completed circuit design can be seen in Figure 12 below. Using this

completed ALU protoboard implementation, we were able to implement a 4-bit

multiplication operation using the pseudocode shown in Figure 13 (see attached). This

pseudocode is used to find the first partial product (a3b0 a2b0 a1b0 a0b0) of the

multiplication of operands A (a3a2a1a0) & B (b3b2b1b0). This multiplication was then

analyzed using the Logic Analyzer to verify its operation. The multiplication waveform

can be seen in Figure 14 (see attached).

Figure 12 – ALU Protoboard Implementation

Conclusion

The design implementation of the 4-bit ALU from Figure 2 proved to be a

successful design. This protoboard circuit works as a fully-functional ALU capable of

performing all of the operations required from a CPU. The LogicPort Logic Analyzer

output waveforms from Figures 8-11 shows the user correctly utilizing the ALU to

perform both arithmetic and logical operations (addition, subtraction, AND, OR).

Without a multiplication operation integrated within the ALU, utilization of the

pseudocode shown in Figure 13 using AND, OR, shift, & addition operations can be

used to calculate partial products to store within registers to calculate a final 4-bit

hexadecimal product. The ALU design and implementation was a practical application

to understanding how the CPUs of computers utilize ALUs to provide the operations

necessary to function properly.

References

1. ECE 0501 Digital Systems Laboratory Custom Course Materials. Laboratory

Notebook. University of Pittsburgh.