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<title>Volatile - 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" class="active"><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"><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="#volatile" id="volatile"><h1>Volatile</h1></a>
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<p>Before we focus on what the numbers mean, first let's ask ourselves: Why are we
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doing <em>volatile</em> writes? You've probably never used that keywords before at all.
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What <em>is</em> volatile anyway?</p>
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<p>Well, the optimizer is pretty aggressive, and so it'll skip reads and writes
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when it thinks can. Like if you write to a pointer once, and then again a moment
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later, and it didn't see any other reads in between, it'll think that it can
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just skip doing that first write since it'll get overwritten anyway. Sometimes
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that's correct, but sometimes it's not.</p>
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<p>Marking a read or write as <em>volatile</em> tells the compiler that it really must do
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that action, and in the exact order that we wrote it out. It says that there
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might even be special hardware side effects going on that the compiler isn't
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aware of. In this case, the write to the display control register sets a video
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mode, and the writes to the Video RAM set pixels that will show up on the
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screen.</p>
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<p>Similar to "atomic" operations you might have heard about, all volatile
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operations are enforced to happen in the exact order that you specify them, but
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only relative to other volatile operations. So something like</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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c.write_volatile(5);
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a += b;
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d.write_volatile(7);
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#}</code></pre></pre>
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<p>might end up changing <code>a</code> either before or after the change to <code>c</code> (since the
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value of <code>a</code> doesn't affect the write to <code>c</code>), but the write to <code>d</code> will
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<em>always</em> happen after the write to <code>c</code>, even though the compiler doesn't see any
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direct data dependency there.</p>
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<p>If you ever go on to use volatile stuff on other platforms it's important to
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note that volatile doesn't make things thread-safe, you still need atomic for
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that. However, the GBA doesn't have threads, so we don't have to worry about
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those sorts of thread safety concerns (there's interrupts, but that's another
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matter).</p>
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<a class="header" href="#volatile-by-default" id="volatile-by-default"><h2>Volatile by default</h2></a>
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<p>Of course, writing out <code>volatile_write</code> every time is more than we wanna do.
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There's clarity and then there's excessive. This is a chance to write our first
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<a href="https://doc.rust-lang.org/1.0.0/style/features/types/newtype.html">newtype</a>.
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Basically a type that's got the exact same binary representation as some other
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type, but new methods and trait implementations.</p>
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<p>We want a <code>*mut T</code> that's volatile by default, and also when we offset it...
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well the verdict is slightly unclear on how <code>offset</code> vs <code>wrapping_offset</code> work
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when you're using pointers that you made up out of nowhere. I've asked the
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experts and they genuinely weren't sure, so we'll make an <code>offset</code> method that
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does a <code>wrapping_offset</code> just to be careful.</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(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
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#[repr(transparent)]
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pub struct VolatilePtr<T>(pub *mut T);
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impl<T> VolatilePtr<T> {
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pub unsafe fn read(&self) -> T {
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core::ptr::read_volatile(self.0)
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}
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pub unsafe fn write(&self, data: T) {
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core::ptr::write_volatile(self.0, data);
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}
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pub unsafe fn offset(self, count: isize) -> Self {
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VolatilePtr(self.0.wrapping_offset(count))
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}
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}
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#}</code></pre></pre>
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