<!DOCTYPE HTML> <html lang="en" class="sidebar-visible no-js"> <head> <!-- Book generated using mdBook --> <meta charset="UTF-8"> <title>IO Registers - Rust GBA Tutorials</title> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> <meta name="description" content=""> <meta name="viewport" content="width=device-width, initial-scale=1"> <meta name="theme-color" content="#ffffff" /> <link rel="shortcut icon" href="../favicon.png"> <link rel="stylesheet" href="../css/variables.css"> <link rel="stylesheet" href="../css/general.css"> <link rel="stylesheet" href="../css/chrome.css"> <link rel="stylesheet" href="../css/print.css" media="print"> <!-- Fonts --> <link rel="stylesheet" href="../FontAwesome/css/font-awesome.css"> <link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css"> <link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css"> <!-- Highlight.js Stylesheets --> <link rel="stylesheet" href="../highlight.css"> <link rel="stylesheet" href="../tomorrow-night.css"> <link rel="stylesheet" href="../ayu-highlight.css"> <!-- Custom theme stylesheets --> </head> <body class="light"> <!-- Provide site root to javascript --> <script type="text/javascript">var path_to_root = "../";</script> <!-- Work around some values being stored in localStorage wrapped in quotes --> <script type="text/javascript"> try { var theme = localStorage.getItem('mdbook-theme'); 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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/io_registers.html" class="active"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch01/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="../ch01/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch01/hello2.html"><strong aria-hidden="true">3.5.</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></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 Tutorials</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="#io-registers" id="io-registers"><h1>IO Registers</h1></a> <p>The GBA has a large number of <strong>IO Registers</strong> (not to be confused with CPU registers). These are special memory locations from <code>0x04000000</code> to <code>0x040003FE</code>. GBATEK has a <a href="http://problemkaputt.de/gbatek.htm#gbaiomap">full list</a>, but we only need to learn about a few of them at a time as we go, so don't be worried.</p> <p>The important facts to know about IO Registers are these:</p> <ul> <li>Each has their own specific size. Most are <code>u16</code>, but some are <code>u32</code>.</li> <li>All of them must be accessed in a <code>volatile</code> style.</li> <li>Each register is specifically readable or writable or both. Actually, with some registers there are even individual bits that are read-only or write-only. <ul> <li>If you write to a read-only position, those writes are simply ignored. This mostly matters if a writable register contains a read-only bit (such as the Display Control, next section).</li> <li>If you read from a write-only position, you get back values that are <a href="http://problemkaputt.de/gbatek.htm#gbaunpredictablethings">basically nonsense</a>. There aren't really any registers that mix writable bits with read only bits, so you're basically safe here. The only (mild) concern is that when you write a value into a write-only register you need to keep track of what you wrote somewhere else if you want to know what you wrote (such to adjust an offset value by +1, or whatever).</li> <li>You can always check GBATEK to be sure, but if I don't mention it then a bit is probably both read and write.</li> </ul> </li> <li>Some registers have invalid bit patterns. For example, the lowest three bits of the Display Control register can't legally be set to the values 6 or 7.</li> </ul> <p>When talking about bit positions, the numbers are <em>zero indexed</em> just like an array index is.</p> </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/the_display_control_register.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/the_display_control_register.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>