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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" class="active"><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"><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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<main>
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<a class="header" href="#io-registers" id="io-registers"><h1>IO Registers</h1></a>
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<p>The GBA has a large number of <strong>IO Registers</strong> (not to be confused with CPU
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registers). These are special memory locations from <code>0x04000000</code> to
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<code>0x040003FE</code>. GBATEK has a <a href="http://problemkaputt.de/gbatek.htm#gbaiomap">full
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list</a>, but we only need to learn
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about a few of them at a time as we go, so don't be worried.</p>
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<p>The important facts to know about IO Registers are these:</p>
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<ul>
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<li>Each has their own specific size. Most are <code>u16</code>, but some are <code>u32</code>.</li>
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<li>All of them must be accessed in a <code>volatile</code> style.</li>
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<li>Each register is specifically readable or writable or both. Actually, with
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some registers there are even individual bits that are read-only or
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write-only.
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<ul>
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<li>If you write to a read-only position, those writes are simply ignored. This
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mostly matters if a writable register contains a read-only bit (such as the
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Display Control, next section).</li>
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<li>If you read from a write-only position, you get back values that are
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<a href="http://problemkaputt.de/gbatek.htm#gbaunpredictablethings">basically
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nonsense</a>. There
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aren't really any registers that mix writable bits with read only bits, so
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you're basically safe here. The only (mild) concern is that when you write a
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value into a write-only register you need to keep track of what you wrote
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somewhere else if you want to know what you wrote (such to adjust an offset
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value by +1, or whatever).</li>
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<li>You can always check GBATEK to be sure, but if I don't mention it then a bit
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is probably both read and write.</li>
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</ul>
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</li>
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<li>Some registers have invalid bit patterns. For example, the lowest three bits
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of the Display Control register can't legally be set to the values 6 or 7.</li>
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</ul>
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<p>When talking about bit positions, the numbers are <em>zero indexed</em> just like an
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array index is.</p>
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