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html.classList.add("sidebar-" + sidebar); </script> <nav id="sidebar" class="sidebar" aria-label="Table of contents"> <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/object_basics.html"><strong aria-hidden="true">5.4.</strong> Object Basics</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> </nav> <div id="page-wrapper" class="page-wrapper"> <div class="page"> <div id="menu-bar" class="menu-bar"> <div id="menu-bar-sticky-container"> <div class="left-buttons"> <button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar"> <i class="fa fa-bars"></i> </button> <button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list"> <i class="fa fa-paint-brush"></i> </button> <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu"> <li role="none"><button role="menuitem" class="theme" id="light">Light <span class="default">(default)</span></button></li> <li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li> <li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li> <li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li> <li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li> </ul> <button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> <i class="fa fa-search"></i> </button> </div> <h1 class="menu-title">Rust GBA Guide</h1> <div class="right-buttons"> <a href="../print.html" title="Print this book" aria-label="Print this book"> <i id="print-button" class="fa fa-print"></i> </a> </div> </div> </div> <div id="search-wrapper" class="hidden"> <form id="searchbar-outer" class="searchbar-outer"> <input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> </form> <div id="searchresults-outer" class="searchresults-outer hidden"> <div id="searchresults-header" class="searchresults-header"></div> <ul id="searchresults"> </ul> </div> </div> <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> <script type="text/javascript"> document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); }); </script> <div id="content" class="content"> <main> <a class="header" href="#volatile" id="volatile"><h1>Volatile</h1></a> <p>Before we focus on what the numbers mean, first let's ask ourselves: Why are we doing <em>volatile</em> writes? You've probably never used that keywords before at all. What <em>is</em> volatile anyway?</p> <p>Well, the optimizer is pretty aggressive, and so it'll skip reads and writes when it thinks can. Like if you write to a pointer once, and then again a moment later, and it didn't see any other reads in between, it'll think that it can just skip doing that first write since it'll get overwritten anyway. Sometimes that's correct, but sometimes it's not.</p> <p>Marking a read or write as <em>volatile</em> tells the compiler that it really must do that action, and in the exact order that we wrote it out. It says that there might even be special hardware side effects going on that the compiler isn't aware of. In this case, the write to the display control register sets a video mode, and the writes to the Video RAM set pixels that will show up on the screen.</p> <p>Similar to "atomic" operations you might have heard about, all volatile operations are enforced to happen in the exact order that you specify them, but only relative to other volatile operations. So something like</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { c.write_volatile(5); a += b; d.write_volatile(7); #}</code></pre></pre> <p>might end up changing <code>a</code> either before or after the change to <code>c</code> (since the value of <code>a</code> doesn't affect the write to <code>c</code>), but the write to <code>d</code> will <em>always</em> happen after the write to <code>c</code>, even though the compiler doesn't see any direct data dependency there.</p> <p>If you ever go on to use volatile stuff on other platforms it's important to note that volatile doesn't make things thread-safe, you still need atomic for that. However, the GBA doesn't have threads, so we don't have to worry about those sorts of thread safety concerns (there's interrupts, but that's another matter).</p> <a class="header" href="#volatile-by-default" id="volatile-by-default"><h2>Volatile by default</h2></a> <p>Of course, writing out <code>volatile_write</code> every time is more than we wanna do. There's clarity and then there's excessive. This is a chance to write our first <a href="https://doc.rust-lang.org/1.0.0/style/features/types/newtype.html">newtype</a>. Basically a type that's got the exact same binary representation as some other type, but new methods and trait implementations.</p> <p>We want a <code>*mut T</code> that's volatile by default, and also when we offset it... well the verdict is slightly unclear on how <code>offset</code> vs <code>wrapping_offset</code> work when you're using pointers that you made up out of nowhere. I've asked the experts and they genuinely weren't sure, so we'll make an <code>offset</code> method that does a <code>wrapping_offset</code> just to be careful.</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { #[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)] #[repr(transparent)] pub struct VolatilePtr<T>(pub *mut T); impl<T> VolatilePtr<T> { pub unsafe fn read(&self) -> T { core::ptr::read_volatile(self.0) } pub unsafe fn write(&self, data: T) { core::ptr::write_volatile(self.0, data); } pub unsafe fn offset(self, count: isize) -> Self { VolatilePtr(self.0.wrapping_offset(count)) } } #}</code></pre></pre> </main> <nav class="nav-wrapper" aria-label="Page navigation"> <!-- Mobile navigation buttons --> <a rel="prev" href="../ch01/hello1.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="../ch01/io_registers.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> <div style="clear: both"></div> </nav> </div> </div> <nav class="nav-wide-wrapper" aria-label="Page navigation"> <a href="../ch01/hello1.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a href="../ch01/io_registers.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> </nav> </div> <script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> <script src="../mark.min.js" type="text/javascript" charset="utf-8"></script> <script src="../searcher.js" type="text/javascript" charset="utf-8"></script> <script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script> <script src="../highlight.js" type="text/javascript" charset="utf-8"></script> <script src="../book.js" type="text/javascript" charset="utf-8"></script> <!-- Custom JS scripts --> </body> </html>