2018-11-08 15:20:40 +11:00
<!DOCTYPE HTML>
< html lang = "en" class = "sidebar-visible no-js" >
< head >
<!-- Book generated using mdBook -->
< meta charset = "UTF-8" >
2018-11-18 11:14:42 +11:00
< title > IO Registers - Rust GBA Guide< / title >
2018-11-08 15:20:40 +11:00
< 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" / >
2018-11-10 20:03:37 +11:00
< 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" >
2018-11-08 15:20:40 +11:00
<!-- Fonts -->
2018-11-10 20:03:37 +11:00
< link rel = "stylesheet" href = "../FontAwesome/css/font-awesome.css" >
2018-11-08 15:20:40 +11:00
< 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 -->
2018-11-10 20:03:37 +11:00
< link rel = "stylesheet" href = "../highlight.css" >
< link rel = "stylesheet" href = "../tomorrow-night.css" >
< link rel = "stylesheet" href = "../ayu-highlight.css" >
2018-11-08 15:20:40 +11:00
<!-- Custom theme stylesheets -->
< / head >
< body class = "light" >
<!-- Provide site root to javascript -->
2018-11-10 20:03:37 +11:00
< script type = "text/javascript" > var path _to _root = "../" ; < / script >
2018-11-08 15:20:40 +11:00
<!-- Work around some values being stored in localStorage wrapped in quotes -->
< script type = "text/javascript" >
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') & & theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') & & sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
< / script >
<!-- Set the theme before any content is loaded, prevents flash -->
< script type = "text/javascript" >
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = 'light'; }
document.body.className = theme;
document.querySelector('html').className = theme + ' js';
< / script >
<!-- Hide / unhide sidebar before it is displayed -->
< script type = "text/javascript" >
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
< / script >
< nav id = "sidebar" class = "sidebar" aria-label = "Table of contents" >
2018-11-22 17:00:59 +11:00
< 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 >
2018-11-08 15:20:40 +11:00
< / 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 >
2018-11-18 11:14:42 +11:00
< h1 class = "menu-title" > Rust GBA Guide< / h1 >
2018-11-08 15:20:40 +11:00
< div class = "right-buttons" >
2018-11-10 20:03:37 +11:00
< a href = "../print.html" title = "Print this book" aria-label = "Print this book" >
2018-11-08 15:20:40 +11:00
< 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 >
2018-11-10 20:03:37 +11:00
< 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 >
2018-11-08 15:20:40 +11:00
< / main >
< nav class = "nav-wrapper" aria-label = "Page navigation" >
<!-- Mobile navigation buttons -->
2018-11-18 11:14:42 +11:00
< a rel = "prev" href = "../ch01/volatile.html" class = "mobile-nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
2018-11-10 20:03:37 +11:00
< i class = "fa fa-angle-left" > < / i >
< / a >
2018-11-08 15:20:40 +11:00
2018-11-15 13:53:21 +11:00
< 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" >
2018-11-10 20:03:37 +11:00
< i class = "fa fa-angle-right" > < / i >
< / a >
2018-11-08 15:20:40 +11:00
< div style = "clear: both" > < / div >
< / nav >
< / div >
< / div >
< nav class = "nav-wide-wrapper" aria-label = "Page navigation" >
2018-11-18 11:14:42 +11:00
< a href = "../ch01/volatile.html" class = "nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
2018-11-10 20:03:37 +11:00
< i class = "fa fa-angle-left" > < / i >
< / a >
2018-11-08 15:20:40 +11:00
2018-11-15 13:53:21 +11:00
< a href = "../ch01/the_display_control_register.html" class = "nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
2018-11-10 20:03:37 +11:00
< i class = "fa fa-angle-right" > < / i >
< / a >
2018-11-08 15:20:40 +11:00
< / nav >
< / div >
2018-11-10 20:03:37 +11:00
< 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 >
2018-11-08 15:20:40 +11:00
2018-11-10 20:03:37 +11:00
< 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 >
2018-11-08 15:20:40 +11:00
<!-- Custom JS scripts -->
< / body >
< / html >
