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<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control.html" class="active"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li></ol>
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<h1 class="menu-title">Rust GBA Tutorials</h1>
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<a class="header" href="#the-display-control" id="the-display-control"><h1>The Display Control</h1></a>
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<p>The Display Control is our first actual IO Register. GBATEK gives it the
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shorthand <a href="http://problemkaputt.de/gbatek.htm#lcdiodisplaycontrol">DISPCNT</a>, so
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you might see it under that name if you read other guides.</p>
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<p>Among IO Registers, it's one of the simpler ones, but it's got enough complexity
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that we can get a hint of what's to come.</p>
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<p>Also it's the one that you basically always need to set at least once in every
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GBA game, so it's a good starting one to go over for that reason too.</p>
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<p>The Display Control is a <code>u16</code> value located at <code>0x0400_0000</code>.</p>
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<a class="header" href="#video-modes" id="video-modes"><h2>Video Modes</h2></a>
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<p>The lowest three bits (0-2) let you select from among the GBA's six video modes.
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You'll notice that 3 bits allows for eight modes, but the values 6 and 7 are
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prohibited.</p>
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<p>Modes 0, 1, and 2 are "Tiled" modes. These are actually the modes that you
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should eventually learn to use as much as possible. It lets the GBA's limited
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video hardware do as much of the work as possible, leaving more of your CPU time
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for gameplay computations. However, they're also complex enough to deserve their
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own demos and chapters later on, so that's all we'll say about them for now.</p>
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<p>Modes 3, 4, and 5 are "Bitmap" modes. These let you write individual pixels to
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locations on the screen.</p>
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<ul>
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<li><strong>Mode 3</strong> is full resolution (240w x 160h) RBG15 color. You might not be used
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to RGB15, since modern computers have 24 or 32 bit colors. In RGB15, there's 5
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bits for each color channel stored within a <code>u16</code> value, and the highest bit is
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simply ignored.</li>
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<li><strong>Mode 4</strong> is full resolution paletted color. Instead of being a <code>u16</code> color, each
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pixel value is a <code>u8</code> palette index entry, and then the display uses the
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palette memory (which we'll talk about later) to store the actual color data.
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Since each pixel is half sized, we can fit twice as many. This lets us have
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two "pages". At any given moment only one page is active, and you can draw to
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the other page without the user noticing. You set which page to show with
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another bit we'll get to in a moment.</li>
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<li><strong>Mode 5</strong> is full color, but also with pages. This means that we must have a
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reduced resolution to compensate (video memory is only so big!). The screen is
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effectively only 160w x 128h in this mode.</li>
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</ul>
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<a class="header" href="#cgb-mode" id="cgb-mode"><h2>CGB Mode</h2></a>
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<p>Bit 3 is effectively read only. Technically it can be flipped using a BIOS call,
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but when you write to the display control normally it won't write to this bit,
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so we'll call it effectively read only.</p>
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<p>This bit is on if the CPU is in CGB mode.</p>
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<a class="header" href="#page-flipping" id="page-flipping"><h2>Page Flipping</h2></a>
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<p>Bit 4 lets you pick which page to use. This is only relevent in video modes 4 or
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5, and is just ignored otherwise. It's very easy to remember: when the bit is 0
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the 0th page is used, and when the bit is 1 the 1st page is used.</p>
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<p>The second page always starts at <code>0x0600_A000</code>.</p>
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<a class="header" href="#oam-vram-and-blanking" id="oam-vram-and-blanking"><h2>OAM, VRAM, and Blanking</h2></a>
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<p>Bit 5 lets you access OAM during HBlank if enabled. This is cool, but it reduces
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the maximum sprites per scanline, so it's not default.</p>
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<p>Bit 6 lets you adjust if the GBA should treat Object Character VRAM as being 2d
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(off) or 1d (on).</p>
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<p>Bit 7 forces the screen to stay in vblank as long as it's set. This allows the
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fastest use of the VRAM, Palette, and Object Attribute Memory. Obviously if you
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leave this on for too long the player will notice a blank screen, but it might
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be okay to use for a moment or two every once in a while.</p>
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<a class="header" href="#screen-layers" id="screen-layers"><h2>Screen Layers</h2></a>
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<p>Bits 8 through 11 control if Background layers 0 through 3 should be active.</p>
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<p>Bit 12 affects the Object layer.</p>
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<p>Note that not all background layers are available in all video modes:</p>
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<ul>
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<li>Mode 0: all</li>
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<li>Mode 1: 0/1/2</li>
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<li>Mode 2: 2/3</li>
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<li>Mode 3/4/5: 2</li>
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</ul>
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<p>Bit 13 and 14 enable the display of Windows 0 and 1, and Bit 15 enables the
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object display window. We'll get into how windows work later on, they let you do
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some nifty graphical effects.</p>
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<a class="header" href="#in-conclusion" id="in-conclusion"><h2>In Conclusion...</h2></a>
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<p>So what did we do to the display control in <code>hello1</code>?</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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(0x04000000 as *mut u16).write_volatile(0x0403);
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#}</code></pre></pre>
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<p>First let's <a href="https://www.wolframalpha.com/input/?i=0x0403">convert that to
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binary</a>, and we get
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<code>0b100_0000_0011</code>. So, that's setting Mode 3 with background 2 enabled and
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nothing else special.</p>
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<p>However, I think we can do better than that. This is a prime target for more
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newtyping as we attempt a <code>hello2</code> program.</p>
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