begin tiled mode docs.

src/video/tiled_backgrounds.rs

@@ -0,0 +1,22 @@
+//! Tiled background documentation (no code).
+//!
+//! When in tiled mode a background shows 2D tiles. This is sometimes called
+//! "text" mode because it's the same style of graphics that was once used for
+//! text terminal displays. The first 64k of VRAM is treated as a collection of
+//! **tiles** and **screenblocks**.
+//!
+//! * A tile is 8x8 pixels. Tiles can be either 4bpp (32 bytes) or 8bpp (64
+//!   bytes). Each background layer can use a different bit depth.
+//! * A screenblock is 32x32 [`TileEntry`] values (2,048 bytes).
+//! * Each background has a `size` value set by its [BackgroundControl]. This
+//!   determines how many tilemaps in a row are actually used to draw the
+//!   background:
+//!   * 0: one tilemap
+//!   * 1: two tilemaps horizontally: left then right.
+//!   * 2: two tilemaps vertically: top then bottom.
+//!   * 3: four tilemaps in a square: upper left, upper right, lower left, lower
+//!     right.
+
+// this makes the doclinks work right.
+#[allow(unused_imports)]
+use crate::prelude::*;