icon - Building a Processor

Building the Adder

First Step to a Computer

One the most basic things we want a computer to do is add and subtract. In fact, being able to add would be nice. There is an integrated circuit (IC) that will aid in the process, 74LS283, the 4-bit binary adder. By feeding it numbers to add for each binary place from the 1's place to the 8's place, one can add two 4-bit binary numbers together and get a result.

The first circuit using the newly purchased breadboard was an 4-bit adder where the two numbers to add were entered via two hexadecimal rotary switches. The two switches were hooked up to LEDs to show the current values. The two switches were connected to the adder, which in turn is connected to the 5 LEDs used to indicate the resulting 4-bit value along with the carry bit. This fairly simple circuit is the picture shown at the top of this page.

In the picture above, the circuit is showing that it is adding the values "3" and "5" to give the result "8" on the result LED indicator.

A Fancier Adder

The next circuit used a octal D flip-flop as a 8-bit register to hold the two numbers to add before the value was sent to the adder. The rotary switchs were not directly connected to the adder in this circuit. Now, rather than having the result of the addition appear immediately after turning the switch, a button at the top right-hand corner is needed to create a clock pulse to cause the register to store the values from the two switches. The register then will provide the two values to add to the adder.

Addition and Subtration

This circuit has an addition of a quad XOR gate connected to the 4-bit adder to perform a 2's compliment subtration. The switch at the top right-hand corner switches the addition to subtraction mode. The addition of the row of four red LED indicators at the bottom right-hand of the breadboard shows the state of the quad XOR gate output.

In addition, a 555 timer has been added to produce a train of clock pulses to perform the addition/subtraction operation. Also, a 4-bit counter has been added along with a row of four green LEDs at the bottom right corner. The 4-bit counter doesn't have any functional significance in the circuit other than to demonstrate that it works.

After this circuit was completed, the breadboard was taken apart to make the accumulator circuit.