What happens with the adress when you access any kind of memory? let's have a look Reading/writing 8 Bytes from a memory address: A0 A1 A2 A3 ... 0 0 0 0 1 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 0 1 0 0 1 1 0 1 1 1 0 As you can see, A0 will alternate for every byte you're accessing. Now, open the datasheet of your display controller and read the description of the RS pin. Putting it to A0 will just fail as you keep switching it, even if you're writing plain image data! Using an address pin for RS a neat little "trick" to map image data as well as control data to your controllers memory space without having to manually switch a GPIO. But it has to be a high address pin (at least, the image data for your whole screen must need less address signals!)