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" >
2018-11-10 20:03:37 +11:00
< title > Video Memory Intro - Rust GBA Tutorials< / 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" >
2018-11-13 10:29:17 +11:00
< 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_register.html" > < strong aria-hidden = "true" > 3.3.< / strong > The Display Control Register< / a > < / li > < li > < a href = "../ch1/video_memory_intro.html" class = "active" > < 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 > < li > < a href = "../ch2/index.html" > < strong aria-hidden = "true" > 4.< / strong > Ch 2: User Input< / a > < / li > < li > < ol class = "section" > < li > < a href = "../ch2/the_key_input_register.html" > < strong aria-hidden = "true" > 4.1.< / strong > The Key Input Register< / a > < / li > < li > < a href = "../ch2/the_vcount_register.html" > < strong aria-hidden = "true" > 4.2.< / strong > The VCount Register< / a > < / li > < li > < a href = "../ch2/light_cycle.html" > < strong aria-hidden = "true" > 4.3.< / strong > light_cycle< / 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 >
< h1 class = "menu-title" > Rust GBA Tutorials< / h1 >
< 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 = "#video-memory-intro" id = "video-memory-intro" > < h1 > Video Memory Intro< / h1 > < / a >
< p > The GBA's Video RAM is 96k stretching from < code > 0x0600_0000< / code > to < code > 0x0601_7FFF< / code > .< / p >
< p > The Video RAM can only be accessed totally freely during a Vertical Blank
(aka " vblank" ). At other times, if the CPU tries to touch the same part of video
memory as the display controller is accessing then the CPU gets bumped by a
cycle to avoid a clash.< / p >
< p > Annoyingly, VRAM can only be properly written to in 16 and 32 bit segments (same
with PALRAM and OAM). If you try to write just an 8 bit segment, then both parts
of the 16 bit segment get the same value written to them. In other words, if you
write the byte < code > 5< / code > to < code > 0x0600_0000< / code > , then both < code > 0x0600_0000< / code > and ALSO
< code > 0x0600_0001< / code > will have the byte < code > 5< / code > in them. We have to be extra careful when
trying to set an individual byte, and we also have to be careful if we use
< code > memcopy< / code > or < code > memset< / code > as well, because they're byte oriented by default and
don't know to follow the special rules.< / p >
< a class = "header" href = "#rgb15" id = "rgb15" > < h2 > RGB15< / h2 > < / a >
2018-11-11 17:39:26 +11:00
< p > As I said before, RGB15 stores a color within a < code > u16< / code > value using 5 bits for
each color channel.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
pub const RED: u16 = 0b0_00000_00000_11111;
pub const GREEN: u16 = 0b0_00000_11111_00000;
pub const BLUE: u16 = 0b0_11111_00000_00000;
#}< / code > < / pre > < / pre >
< p > In Mode 3 and Mode 5 we write direct color values into VRAM, and in Mode 4 we
write palette index values, and then the color values go into the PALRAM.< / p >
2018-11-10 20:03:37 +11:00
< a class = "header" href = "#mode-3" id = "mode-3" > < h2 > Mode 3< / h2 > < / a >
2018-11-11 17:39:26 +11:00
< p > Mode 3 is pretty easy. We have a full resolution grid of rgb15 pixels. There's
160 rows of 240 pixels each, with the base address being the top left corner. A
particular pixel uses normal " 2d indexing" math:< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
let row_five_col_seven = 5 + (7 * SCREEN_WIDTH);
#}< / code > < / pre > < / pre >
< p > To draw a pixel, we just write a value at the address for the row and col that
we want to draw to.< / p >
2018-11-10 20:03:37 +11:00
< a class = "header" href = "#mode-4" id = "mode-4" > < h2 > Mode 4< / h2 > < / a >
2018-11-11 17:39:26 +11:00
< p > Mode 4 introduces page flipping. Instead of one giant page at < code > 0x0600_0000< / code > ,
there's Page 0 at < code > 0x0600_0000< / code > and then Page 1 at < code > 0x0600_A000< / code > . The resolution
for each page is the same as above, but instead of writing < code > u16< / code > values, the
memory is treated as < code > u8< / code > indexes into PALRAM. The PALRAM starts at
< code > 0x0500_0000< / code > , and there's enough space for 256 palette entries (each a < code > u16< / code > ).< / p >
< p > To set the color of a palette entry we just do a normal < code > u16< / code > write_volatile.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
(0x0500_0000 as *mut u16).offset(target_index).write_volatile(new_color)
#}< / code > < / pre > < / pre >
< p > To draw a pixel we set the palette entry that we want the pixel to use. However,
we must remember the " minimum size" write limitation that applies to VRAM. So,
if we want to change just a single pixel at a time we must< / p >
< ol >
< li > Read the full < code > u16< / code > it's a part of.< / li >
< li > Clear the half of the < code > u16< / code > we're going to replace< / li >
< li > Write the half of the < code > u16< / code > we're going to replace with the new value< / li >
< li > Write that result back to the address.< / li >
< / ol >
< p > So, the math for finding a byte offset is the same as Mode 3 (since they're both
a 2d grid). If the byte offset is EVEN it'll be the high bits of the < code > u16< / code > at
half the byte offset rounded down. If the offset is ODD it'll be the low bits of
the < code > u16< / code > at half the byte.< / p >
< p > Does that make sense?< / p >
< ul >
< li > If we want to write pixel (0,0) the byte offset is 0, so we change the high
bits of < code > u16< / code > offset 0. Then we want to write to (1,0), so the byte offset is
1, so we change the low bits of < code > u16< / code > offset 0. The pixels are next to each
other, and the target bytes are next to each other, good so far.< / li >
< li > If we want to write to (5,6) that'd be byte < code > 5 + 6 * 240 = 1445< / code > , so we'd
target the low bits of < code > u16< / code > offset < code > floor(1445/2) = 722< / code > .< / li >
< / ul >
< p > As you can see, trying to write individual pixels in Mode 4 is mostly a bad
time. Fret not! We don't < em > have< / em > to write individual bytes. If our data is
arranged correctly ahead of time we can just write < code > u16< / code > or < code > u32< / code > values
directly. The video hardware doesn't care, it'll get along just fine.< / p >
2018-11-10 20:03:37 +11:00
< a class = "header" href = "#mode-5" id = "mode-5" > < h2 > Mode 5< / h2 > < / a >
2018-11-11 17:39:26 +11:00
< p > Mode 5 is also a two page mode, but instead of compressing the size of a pixel's
data to fit in two pages, we compress the resolution.< / p >
< p > Mode 5 is full < code > u16< / code > color, but only 160w x 128h per page.< / p >
2018-11-10 20:03:37 +11:00
< a class = "header" href = "#in-conclusion" id = "in-conclusion" > < h2 > In Conclusion...< / h2 > < / a >
2018-11-11 17:39:26 +11:00
< p > So what got written into VRAM in < code > hello1< / code > ?< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
(0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
(0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
(0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
#}< / code > < / pre > < / pre >
< p > So at pixels < code > (120,80)< / code > , < code > (136,80)< / code > , and < code > (120,96)< / code > we write three values. Once
again we probably need to < a href = "https://www.wolframalpha.com/" > convert them< / a > into
binary to make sense of it.< / p >
< ul >
< li > 0x001F: 0b11111< / li >
< li > 0x03E0: 0b11111_00000< / li >
< li > 0x7C00: 0b11111_00000_00000< / li >
< / ul >
< p > Ah, of course, a red pixel, a green pixel, and a blue pixel.< / p >
2018-11-08 15:20:40 +11:00
< / main >
< nav class = "nav-wrapper" aria-label = "Page navigation" >
<!-- Mobile navigation buttons -->
2018-11-11 18:25:26 +11:00
< a rel = "prev" href = "../ch1/the_display_control_register.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
2018-11-11 17:39:26 +11:00
< a rel = "next" href = "../ch1/hello2.html" class = "mobile-nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< 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" >
2018-11-11 18:25:26 +11:00
< a href = "../ch1/the_display_control_register.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
2018-11-11 17:39:26 +11:00
< a href = "../ch1/hello2.html" class = "nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< 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 >
