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<title>Ch 3: Memory and Objects - Rust GBA Guide</title>
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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" class="active"><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"><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_rng.html"><strong aria-hidden="true">5.5.</strong> GBA RNG</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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<h1 class="menu-title">Rust GBA Guide</h1>
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<main>
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<a class="header" href="#ch-3-memory-and-objects" id="ch-3-memory-and-objects"><h1>Ch 3: Memory and Objects</h1></a>
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<p>Alright so we can do some basic "movement", but we left a big trail in the video
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memory of everywhere we went. Most of the time that's not what we want at all.
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If we want more hardware support we're going to have to use a new video mode. So
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far we've only used Mode 3, but modes 4 and 5 are basically the same. Instead,
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we'll switch focus to using a tiled graphical mode.</p>
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<p>First we will go over the complete GBA memory mapping. Part of this is the
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memory for tiled graphics, but also things like all those IO registers, where
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our RAM is for scratch space, all that stuff. Even if we can't put all of them
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to use at once, it's helpful to have an idea of what will be available in the
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long run.</p>
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<p>Tiled modes bring us three big new concepts that each have their own complexity:
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tiles, backgrounds, and objects. Backgrounds and objects both use tiles, but the
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background is for creating a very large static space that you can scroll around
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the view within, and the objects are about having a few moving bits that appear
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over the background. Careful use of backgrounds and objects is key to having the
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best looking GBA game, so we won't even be able to cover it all in a single
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chapter.</p>
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<p>And, of course, since most games are pretty boring if they're totally static
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we'll touch on the kinds of RNG implementations you might want to have on a GBA.
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Most general purpose RNGs that you find are rather big compared to the amount of
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memory we want to give them, and they often use a lot of <code>u64</code> operations, so
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they end up much slower on a 32-bit machine like the GBA (you can lower 64-bit
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ops to combinations of 32-bit ops, but that's quite a bit more work). We'll
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cover a few RNG options that size down the RNG to a good size and a good speed
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without trading away too much in terms of quality.</p>
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<p>To top it all off, we'll make a simple "memory game" sort of thing. There's some
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face down cards in a grid, you pick one to check, then you pick the other to
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check, and then if they match the pair disappears.</p>
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<a class="header" href="#drawing-priority" id="drawing-priority"><h2>Drawing Priority</h2></a>
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<p>Both backgrounds and objects can have "priority" values associated with them.
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TONC and GBATEK have <em>opposite</em> ideas of what it means to have the "highest"
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priority. TONC goes by highest numerical value, and GBATEK goes by what's on the
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z-layer closest to the user. Let's list out the rules as clearly as we can:</p>
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<ul>
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<li>Priority is always two bits, so 0 through 3.</li>
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<li>Priority conceptually proceeds in drawing passes that count <em>down</em>, so any
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priority 3 things can get covered up by priority 2 things. In truth there's
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probably depth testing and buffering stuff going on so it's all one single
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pass, but conceptually we will imagine it happening as all of the 3 elements,
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then all of 2, and so on.</li>
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<li>Objects always draw over top of backgrounds of equal priority.</li>
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<li>Within things of the same type and priority, the lower numbered element "wins"
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and gets its pixel drawn (bg0 is favored over bg1, obj0 is favored over obj1,
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etc).</li>
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</ul>
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