Fill in a bit about the Gba struct

book/src/pong/02_the_gba_struct.md

@@ -1,3 +1,42 @@
 # The Gba struct
 
+In this section, we'll cover the importance of the Gba struct and how to create it.
+
+# The importance of the Gba struct
+
 Almost all interaction with the Game Boy Advance's hardware goes through the [Gba singleton struct](https://docs.rs/agb/latest/agb/struct.Gba.html).
+Your games using `agb` will typically create this in the `main` function and then handle the abstractions in there.
+
+The Gba struct is used to take advantage of rust's borrow checker, and lean on it to ensure that access to the Game Boy Advance hardware is done 'sensibly'.
+You won't have to worry about 2 bits of your code modifying data in the wrong way!
+This struct is a 'singleton', so you cannot create 2 instances of it at once.
+Attempting to do so will result in a panic which by default crashes the game.
+
+# How all agb games start
+
+Replace the content of the `main` function with the following:
+
+```rust,ignore
+# #![no_std]
+# #![no_main]
+# extern crate agb;
+# #[agb::entry]
+# fn main() -> ! {
+let mut gba = agb::Gba::new();
+
+loop {} // infinite loop for now
+# }
+```
+
+and ignore warnings for now.
+
+# Running your pong game
+
+Although there isn't much to see at the moment (just a black screen), you can start the game by using `cargo run` or whatever worked for you in the introduction.
+
+# What we did
+
+This was a very simple but incredibly important part of any game using `agb`.
+All interactions with the hardware are gated via the Gba struct, so it must be created at the start of your `main` function and never again.
+
+You are now ready to learn about display modes and how to start getting things onto the screen!