Just a Two-Bit Adder - The Motorola MC353G Half Adder

This artwork is based on a micro-image of one of the very first computer chips. The actual chip is in the lower left corner of the artwork, and it's over fifty years old. This was a chip that could add two bits of information. The picture below shows an x-ray view of the chip. The "chip" that you see is actually the package for the true chip. The silicon computer chip is attached to a Header and covered by the Can, so you can't see it. It is protected in the package and is connected to the outside world with Interconnect Wires that attach the chip's pads to the Leads that allow the package to be plugged into a circuit board. This MC353 Half Adder chip was made by Motorola.

About the Half Adder

The 353 chip contains a simple circuit called a Half Adder (HA). The simple circuit uses XOR and AND logic gates. The circuit takes bits, A & B, adds them together for a sum, S, and possibly a carry, C. HAs can be attached to each other to add muli-bit numbers. For example let's assume we have two binary numbers we want to add. A is 01 and B is 11. The first HA would handle the one's position and add A=1 and B=1. The outputs would be the sum, S = 0, and a carry C = 1. Now we need another HA to the add the carry to the two's position. First, add the carry, C = 1, to A's two position, A = 0, the output is S = 0, and the new carry is, C = 0. With a third HA we add the B's two position, B = 1 and the carry, C = 0, the output is S = 1, and the new carry is, C = 0. So, taking all the HA bits, the result of adding 01 + 11 is 0100. Luckily there are better circuits for adding big numbers.