We have the building blocks of a game here, but we're still lacking player input. A game that plays itself isn't very much fun, so let's fix that.
Paste this code below your Main loop.
Like MemCopy, this is a function that can be reused from different places, using the call instruction.
{{#include ../../unbricked/input/main.asm:input-routine}}
Unfortunately, reading input on the Game Boy is fairly involved (as you can see!), and it would be quite difficult to explain what this function does right now. So, I ask that you make an exception, and trust me that this function does read input. Alright? Good!
Now that we know how to use functions, let's call the UpdateKeys function in our main loop to read user input.
UpdateKeys writes the held buttons to a location in memory that we called wCurKeys, which we can read from after the function returns.
Because of this, we only need to call UpdateKeys once per frame.
This is important, because not only is it faster to reload the inputs that we've already processed, but it also means that we will always act on the same inputs, even if the player presses or releases a button mid-frame.
First, let's set aside some room for the two variables that UpdateKeys will use; paste this at the end of the main.asm:
{{#include ../../unbricked/input/main.asm:vars}}
Each variable must reside in RAM, and not ROM, because ROM is "Read-Only" (so you can't modify it).
Additionally, each variable only needs to be one byte large, so we use db ("Define Byte") to reserve one byte of RAM for each.
Before we read these variables we will also want to initialize them.
We can do that below our initialization of wFrameCounter.
{{#include ../../unbricked/input/main.asm:initialize-vars}}
We're going to use the and opcode, which we can use to set the zero flag (z) to the value of the bit.
We can use this along with the PAD constants in hardware.inc to read a particular key.
{{#include ../../unbricked/input/main.asm:main}}
Now, if you compile the project, you should be able to move the paddle left and right using the d-pad!! Hooray, we have the beginnings of a game!