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<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch00/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch01/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch01/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch01/volatile.html"><strong aria-hidden="true">3.2.</strong> Volatile</a></li><li><a href="../ch01/io_registers.html"><strong aria-hidden="true">3.3.</strong> IO Registers</a></li><li><a href="../ch01/the_display_control_register.html"><strong aria-hidden="true">3.4.</strong> The Display Control Register</a></li><li><a href="../ch01/video_memory_intro.html"><strong aria-hidden="true">3.5.</strong> Video Memory Intro</a></li><li><a href="../ch01/hello2.html"><strong aria-hidden="true">3.6.</strong> hello2</a></li></ol></li><li><a href="../ch02/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="../ch02/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="../ch02/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="../ch02/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li><li><a href="../ch03/index.html"><strong aria-hidden="true">5.</strong> Ch 3: Memory and Objects</a></li><li><ol class="section"><li><a href="../ch03/gba_memory_mapping.html"><strong aria-hidden="true">5.1.</strong> GBA Memory Mapping</a></li><li><a href="../ch03/tile_data.html"><strong aria-hidden="true">5.2.</strong> Tile Data</a></li><li><a href="../ch03/regular_backgrounds.html" class="active"><strong aria-hidden="true">5.3.</strong> Regular Backgrounds</a></li><li><a href="../ch03/regular_objects.html"><strong aria-hidden="true">5.4.</strong> Regular Objects</a></li><li><a href="../ch03/gba_prng.html"><strong aria-hidden="true">5.5.</strong> GBA PRNG</a></li><li><a href="../ch03/memory_game.html"><strong aria-hidden="true">5.6.</strong> memory_game</a></li></ol></li></ol>
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<main>
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<a class="header" href="#regular-backgrounds" id="regular-backgrounds"><h1>Regular Backgrounds</h1></a>
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<p>So, backgrounds, they're cool. Why do we call the ones here "regular"
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backgrounds? Because there's also "affine" backgrounds. However, affine math
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stuff adds a complication, so for now we'll just work with regular backgrounds.
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The non-affine backgrounds are sometimes called "text mode" backgrounds by other
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guides.</p>
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<p>To get your background image working you generally need to perform all of the
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following steps, though I suppose the exact ordering is up to you.</p>
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<a class="header" href="#tiled-video-modes" id="tiled-video-modes"><h2>Tiled Video Modes</h2></a>
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<p>When you want regular tiled display, you must use video mode 0 or 1.</p>
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<ul>
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<li>Mode 0 allows for using all four BG layers (0 through 3) as regular
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backgrounds.</li>
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<li>Mode 1 allows for using BG0 and BG1 as regular backgrounds, BG2 as an affine
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background, and BG3 not at all.</li>
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<li>Mode 2 allows for BG2 and BG3 to be used as affine backgrounds, while BG0 and
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BG1 cannot be used at all.</li>
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</ul>
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<p>We will not cover affine backgrounds in this chapter, so we will naturally be
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using video mode 0.</p>
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<p>Also, note that you have to enable each background layer that you want to use
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within the display control register.</p>
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<a class="header" href="#get-your-palette-ready" id="get-your-palette-ready"><h2>Get Your Palette Ready</h2></a>
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<p>Background palette starts at <code>0x5000000</code> and is 256 <code>u16</code> values long. It'd
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potentially be possible declare a static array starting at a fixed address and
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use a linker script to make sure that it ends up at the right spot in the final
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program, but since we have to use volatile reads and writes with PALRAM anyway,
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we'll just reuse our <code>VolatilePtr</code> type. Something like this:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub const PALRAM_BG_BASE: VolatilePtr<u16> = VolatilePtr(0x500_0000 as *mut u16);
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pub fn bg_palette(slot: usize) -> u16 {
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assert!(slot < 256);
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unsafe { PALRAM_BG_BASE.offset(slot as isize).read() }
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}
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pub fn set_bg_palette(slot: usize, color: u16) {
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assert!(slot < 256);
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unsafe { PALRAM_BG_BASE.offset(slot as isize).write(color) }
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}
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#}</code></pre></pre>
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<p>As we discussed with the tile color depths, the palette can be utilized as a
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single block of palette values (<code>[u16; 256]</code>) or as 16 palbanks of 16 palette
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values each (<code>[[u16;16]; 16]</code>). This setting is assigned per background layer
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via IO register.</p>
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<a class="header" href="#get-your-tiles-ready" id="get-your-tiles-ready"><h2>Get Your Tiles Ready</h2></a>
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<p>Tile data is placed into charblocks. A charblock is always 16kb, so depending on
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color depth it will have either 256 or 512 tiles within that charblock.
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Charblocks 0, 1, 2, and 3 are all for background tiles. That's a maximum of 2048
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tiles for backgrounds, but as you'll see in a moment a particular tilemap entry
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can't even index that high. Instead, each background layer is assigned a
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"character base block", and then tilemap entries index relative to the character
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base block of that background layer.</p>
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<p>Now, if you want to move in a lot of tile data you'll probably want to use a DMA
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routine, or at least write a function like memcopy32 for fast <code>u32</code> copying from
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ROM into VRAM. However, for now, and because we're being very explicit since
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this is our first time doing it, we'll write it as functions for individual tile
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reads and writes.</p>
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<p>The math works like indexing a pointer, except that we have two sizes we need to
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go by. First you take the base address for VRAM (<code>0x600_0000</code>), then add the
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size of a charblock (16kb) times the charblock you want to place the tile
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within, and then you add the index of the tile slot you're placing it into times
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the size of that type of tile. Like this:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub fn bg_tile_4bpp(base_block: usize, tile_index: usize) -> Tile4bpp {
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assert!(base_block < 4);
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assert!(tile_index < 512);
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let address = VRAM + size_of::<Charblock4bpp>() * base_block + size_of::<Tile4bpp>() * tile_index;
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unsafe { VolatilePtr(address as *mut Tile4bpp).read() }
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}
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pub fn set_bg_tile_4bpp(base_block: usize, tile_index: usize, tile: Tile4bpp) {
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assert!(base_block < 4);
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assert!(tile_index < 512);
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let address = VRAM + size_of::<Charblock4bpp>() * base_block + size_of::<Tile4bpp>() * tile_index;
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unsafe { VolatilePtr(address as *mut Tile4bpp).write(tile) }
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}
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pub fn bg_tile_8bpp(base_block: usize, tile_index: usize) -> Tile8bpp {
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assert!(base_block < 4);
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assert!(tile_index < 256);
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let address = VRAM + size_of::<Charblock8bpp>() * base_block + size_of::<Tile8bpp>() * tile_index;
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unsafe { VolatilePtr(address as *mut Tile8bpp).read() }
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}
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pub fn set_bg_tile_8bpp(base_block: usize, tile_index: usize, tile: Tile8bpp) {
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assert!(base_block < 4);
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assert!(tile_index < 256);
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let address = VRAM + size_of::<Charblock8bpp>() * base_block + size_of::<Tile8bpp>() * tile_index;
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unsafe { VolatilePtr(address as *mut Tile8bpp).write(tile) }
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}
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#}</code></pre></pre>
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<p>For bulk operations, you'd do the exact same math to get your base destination
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pointer, and then you'd get the base source pointer for the tile you're copying
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out of ROM, and then you'd do the bulk copy for the correct number of <code>u32</code>
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values that you're trying to move (8 per tile moved for 4bpp, or 16 per tile
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moved for 8bpp).</p>
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<p><strong>GBA Limitation Note:</strong> on a modern PC (eg: <code>x86</code> or <code>x86_64</code>) you're probably
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used to index based loops and iterator based loops being the same speed. The CPU
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has the ability to do a "fused multiply add", so the base address of the array
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plus desired index * size per element is a single CPU operation to compute. It's
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slightly more complicated if there's arrays within arrays like there are here,
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but with normal arrays it's basically the same speed to index per loop cycle as
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it is to take a base address and then add +1 offset per loop cycle. However, the
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GBA's CPU <em>can't do any of that</em>. On the GBA, there's a genuine speed difference
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between looping over indexes and then indexing each loop (slow) compared to
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using an iterator that just stores an internal pointer and does +1 offset per
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loop until it reaches the end (fast). The repeated indexing itself can by itself
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be an expensive step. If it's like a 3 element array it's no big deal, but if
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you've got a big slice of data to process, be sure to go over it with <code>.iter()</code>
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and <code>.iter_mut()</code> if you can, instead of looping by index. This is Rust and all,
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so probably you were gonna do that anyway, but just a heads up.</p>
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<a class="header" href="#get-your-tilemap-ready" id="get-your-tilemap-ready"><h2>Get your Tilemap ready</h2></a>
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<p>I believe that at one point I alluded to a tilemap existing. Well, just as the
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tiles are arranged into charblocks, the data describing what tile to show in
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what location is arranged into a thing called a <strong>screenblock</strong>.</p>
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<p>A screenblock is placed into VRAM the same as the tile data charblocks. Starting
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at the base of VRAM (<code>0x600_0000</code>) there are 32 slots for the screenblock array.
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Each screenblock is 2048 bytes (<code>0x800</code>). Naturally, if our tiles are using up
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charblock space within VRAM and our tilemaps are using up screenblock space
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within the same VRAM... well it would just be a <em>disaster</em> if they ran in to
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each other. Once again, it's up to you as the programmer to determine how much
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space you want to devote to each thing. Each complete charblock uses up 8
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screenblocks worth of space, but you don't have to fill a complete charblock
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with tiles, so you can be very fiddly with how you split the memory.</p>
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<p>Each screenblock is composed of a series of <em>screenblock entry</em> values, which
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describe what tile index to use and if the tile should be flipped and what
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palbank it should use (if any). Because both regular backgrounds and affine
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backgrounds are composed of screenblocks with entries, and because the affine
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background has a smaller format for screenblock entries, we'll name
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appropriately.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[derive(Clone, Copy)]
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#[repr(transparent)]
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pub struct RegularScreenblock {
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pub data: [RegularScreenblockEntry; 32 * 32],
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}
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#[derive(Debug, Clone, Copy, Default)]
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#[repr(transparent)]
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pub struct RegularScreenblockEntry(u16);
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#}</code></pre></pre>
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<p>So, with one entry per tile, a single screenblock allows for 32x32 tiles worth of
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background.</p>
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<p>The format of a regular screenblock entry is quite simple compared to some of
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the IO register stuff:</p>
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<ul>
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<li>10 bits for tile index (base off of the character base block of the background)</li>
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<li>1 bit for horizontal flip</li>
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<li>1 bit for vertical flip</li>
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<li>4 bits for picking which palbank to use (if 4bpp, otherwise it's ignored)</li>
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</ul>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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impl RegularScreenblockEntry {
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pub fn tile_id(self) -> u16 {
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self.0 & 0b11_1111_1111
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}
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pub fn set_tile_id(&mut self, id: u16) {
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self.0 &= !0b11_1111_1111;
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self.0 |= id;
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}
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pub fn horizontal_flip(self) -> bool {
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(self.0 & (1 << 0xA)) > 0
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}
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pub fn set_horizontal_flip(&mut self, bit: bool) {
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if bit {
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self.0 |= 1 << 0xA;
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} else {
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self.0 &= !(1 << 0xA);
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}
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}
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pub fn vertical_flip(self) -> bool {
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(self.0 & (1 << 0xB)) > 0
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}
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pub fn set_vertical_flip(&mut self, bit: bool) {
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if bit {
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self.0 |= 1 << 0xB;
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} else {
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self.0 &= !(1 << 0xB);
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}
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}
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pub fn palbank_index(self) -> u16 {
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self.0 >> 12
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}
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pub fn set_palbank_index(&mut self, palbank_index: u16) {
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self.0 &= 0b1111_1111_1111;
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self.0 |= palbank_index << 12;
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}
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}
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#}</code></pre></pre>
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<p>Now, at either 256 or 512 tiles per charblock, you might be thinking that with a
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10 bit index you can index past the end of one charblock and into the next.
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You'd be right, mostly.</p>
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<p>As long as you stay within the background memory region for charblocks (that is,
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0 through 3), then it all works out. However, if you try to get the background
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rendering to reach outside of the background charblocks you'll get an
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implementation defined result. It's not the dreaded "undefined behavior" we're
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often worried about in programming, but the results <em>are</em> determined by what
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you're running the game on. With GBA hardware you get a bizarre result
|
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(basically another way to put garbage on the screen). With a DS it acts as if
|
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the tiles were all 0s. If you use an emulator it might or might not allow for
|
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you to do this, it's up to the emulator writers.</p>
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<a class="header" href="#set-your-io-registers" id="set-your-io-registers"><h2>Set Your IO Registers</h2></a>
|
|
<p>Instead of being just a single IO register to learn about this time, there's two
|
|
separate groups of related registers.</p>
|
|
<a class="header" href="#background-control" id="background-control"><h3>Background Control</h3></a>
|
|
<ul>
|
|
<li>BG0CNT (<code>0x400_0008</code>): BG0 Control</li>
|
|
<li>BG1CNT (<code>0x400_000A</code>): BG1 Control</li>
|
|
<li>BG2CNT (<code>0x400_000C</code>): BG2 Control</li>
|
|
<li>BG3CNT (<code>0x400_000E</code>): BG3 Control</li>
|
|
</ul>
|
|
<p>Each of these are a read/write <code>u16</code> location. This is where we get to all of
|
|
the important details that we've been putting off.</p>
|
|
<ul>
|
|
<li>2 bits for the priority.</li>
|
|
<li>2 bits for "character base block", the charblock that all of the tile indexes
|
|
for this background are offset from.</li>
|
|
<li>1 bit for mosaic effect being enabled (we'll get to that below).</li>
|
|
<li>1 bit to enable 8bpp, otherwise 4bpp is used.</li>
|
|
<li>5 bits to pick the "screen base block", the screen block that serves as the
|
|
<em>base</em> value for this background.</li>
|
|
<li>1 bit that is <em>not</em> used in regular mode, but in affine mode it can be enabled
|
|
to cause the affine background to wrap around at the edges.</li>
|
|
<li>2 bits for the background size.</li>
|
|
</ul>
|
|
<p>The size works a little funny. When size is 0 only the base screen block is
|
|
used. If size is 1 or 2 then the base screenblock and the following screenblock
|
|
are placed next to each other (horizontally for 1, vertically for 2). If the
|
|
size is 3 then the base screenblock and the following three screenblocks are
|
|
arranged into a 2x2 grid of screenblocks.</p>
|
|
<a class="header" href="#background-offset" id="background-offset"><h3>Background Offset</h3></a>
|
|
<ul>
|
|
<li>BG0HOFS (<code>0x400_0010</code>): BG0 X-Offset</li>
|
|
<li>BG0VOFS (<code>0x400_0012</code>): BG0 Y-Offset</li>
|
|
<li>BG1HOFS (<code>0x400_0014</code>): BG1 X-Offset</li>
|
|
<li>BG1VOFS (<code>0x400_0016</code>): BG1 Y-Offset</li>
|
|
<li>BG2HOFS (<code>0x400_0018</code>): BG2 X-Offset</li>
|
|
<li>BG2VOFS (<code>0x400_001A</code>): BG2 Y-Offset</li>
|
|
<li>BG3HOFS (<code>0x400_001C</code>): BG3 X-Offset</li>
|
|
<li>BG3VOFS (<code>0x400_001E</code>): BG3 Y-Offset</li>
|
|
</ul>
|
|
<p>Each of these are a <em>write only</em> <code>u16</code> location. Bits 0 through 8 are used, so
|
|
the offsets can be 0 through 511. They also only apply in regular backgrounds.
|
|
If a background is in an affine state then you'll use different IO registers to
|
|
control it (discussed in a later chapter).</p>
|
|
<p>The offset that you assign determines the pixel offset of the display area
|
|
relative to the start of the background scene, as if the screen was a camera
|
|
looking at the scene. In other words, as a BG X offset value increases, you can
|
|
think of it as the camera moving to the right, or as that background moving to
|
|
the left. Like when mario walks toward the goal. Similarly, when a BG Y offset
|
|
increases the camera is moving down, or the background is moving up, like when
|
|
mario falls down from a high platform.</p>
|
|
<p>Depending on how much the background is scrolled and the size of the background,
|
|
it will loop.</p>
|
|
<a class="header" href="#mosaic" id="mosaic"><h2>Mosaic</h2></a>
|
|
<p>As a special effect, you can apply mosaic to backgrounds and objects. It's just
|
|
a single flag for each background, so all backgrounds will use the same mosaic
|
|
settings when they have it enabled. What it actually does is split the normal
|
|
image into "blocks" and then each block gets the color of the top left pixel of
|
|
that block. This is the effect you see when link hits an electric foe with his
|
|
sword and the whole screen "buzzes" at you.</p>
|
|
<p>The mosaic control is a <em>write only</em> <code>u16</code> IO register at <code>0x400_004C</code>.</p>
|
|
<p>There's 4 bits each for:</p>
|
|
<ul>
|
|
<li>Horizontal BG stretch</li>
|
|
<li>Vertical BG stretch</li>
|
|
<li>Horizontal object stretch</li>
|
|
<li>Vertical object stretch</li>
|
|
</ul>
|
|
<p>The inputs should be 1 <em>less</em> than the desired block size. So if you set a
|
|
stretch value of 5 then pixels 0-5 would be part of the first block (6 pixels),
|
|
then 6-11 is the next block (another 6 pixels) and so on.</p>
|
|
<p>If you need to make a pixel other than the top left part of each block the one
|
|
that determines the mosaic color you can carefully offset the background or
|
|
image by a tiny bit, but of course that makes every mosaic block change its
|
|
target pixel. You can't change the target pixel on a block by block basis.</p>
|
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