diff --git a/src/ppu.rs b/src/ppu.rs
index cc3c4ceac6a12d0acfede704c42e14944e3a4335..e8292591b222e7f0b86d5d154fccaf3091bc7dd1 100644
--- a/src/ppu.rs
+++ b/src/ppu.rs
@@ -41,6 +41,8 @@ pub struct Ppu {
     bg_map: bool,
     bg_tile: bool,
     switch_lcd: bool,
+    /// The current execution mode of the PPU, should change
+    /// between states over the drawing of a frame.
     mode: PpuMode,
     /// Internal clock counter used to control the time in ticks
     /// spent in each of the PPU modes.