It's all well and good to draw three pixels, but they don't do anything yet. We want them to to something, and for that we need to get some input from the user.

The GBA, as I'm sure you know, has an arrow pad, A and B, L and R, Start and Select. That's a little more than the NES/GB/CGB had, and a little less than the SNES had. As you can guess, we get key state info from an IO register.

Also, we will need a way to keep the program from running "too fast". On a modern computer or console you do this with vsync info from the GPU and Monitor, and on the GBA we'll be using vsync info from an IO register that tracks what the display hardware is doing.

For this chapter we'll make a copy of hello2.rs named snake.rs and then fill it in as we go. Normally you might not place the entire program into a single source file, but since these are examples it's slightly better to have them be completely self contained than it is to have them be "properly organized" for the long term.