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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/io_registers.html"><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" class="active"><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>
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<h1 class="menu-title">Rust GBA Tutorials</h1>
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<main>
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<a class="header" href="#hello2" id="hello2"><h1>hello2</h1></a>
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<p>Okay so let's have a look again:</p>
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<p><code>hello1</code></p>
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<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
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#![no_std]
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#[cfg(not(test))]
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#[panic_handler]
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fn panic(_info: &core::panic::PanicInfo) -> ! {
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loop {}
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}
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#[start]
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fn main(_argc: isize, _argv: *const *const u8) -> isize {
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unsafe {
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(0x04000000 as *mut u16).write_volatile(0x0403);
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(0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
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(0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
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(0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
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loop {}
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}
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}
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</code></pre></pre>
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<p>Now let's clean this up so that it's clearer what's going on.</p>
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<p>First we'll label that display control stuff:</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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pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
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pub const MODE3: u16 = 3;
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pub const BG2: u16 = 0b100_0000_0000;
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#}</code></pre></pre>
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<p>Next we make some const values for the actual pixel drawing</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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pub const VRAM: usize = 0x06000000;
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pub const SCREEN_WIDTH: isize = 240;
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#}</code></pre></pre>
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<p>And then we want a small helper function for putting together a color value.</p>
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<p>Happily, this one can even be declared as a const function. At the time of
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writing, we've got the "minimal const fn" support in nightly. It really is quite
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limited, but I'm happy to let rustc and LLVM pre-compute as much as they can
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when it comes to the GBA's tiny CPU.</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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pub const fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
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blue << 10 | green << 5 | red
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}
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#}</code></pre></pre>
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<p>Finally, we'll make a function for drawing a pixel in Mode 3. Even though it's
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just a one-liner, having the "important parts" be labeled as function arguments
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usually helps you think about it a lot better.</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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pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
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(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
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}
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#}</code></pre></pre>
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<p>So now we've got this:</p>
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<p><code>hello2</code></p>
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<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
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#![no_std]
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#[cfg(not(test))]
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#[panic_handler]
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fn panic(_info: &core::panic::PanicInfo) -> ! {
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loop {}
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}
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#[start]
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fn main(_argc: isize, _argv: *const *const u8) -> isize {
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unsafe {
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DISPCNT.write_volatile(MODE3 | BG2);
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mode3_pixel(120, 80, rgb16(31, 0, 0));
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mode3_pixel(136, 80, rgb16(0, 31, 0));
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mode3_pixel(120, 96, rgb16(0, 0, 31));
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loop {}
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}
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}
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pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
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pub const MODE3: u16 = 3;
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pub const BG2: u16 = 0b100_0000_0000;
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pub const VRAM: usize = 0x06000000;
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pub const SCREEN_WIDTH: isize = 240;
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pub const fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
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blue << 10 | green << 5 | red
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}
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pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
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(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
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}
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</code></pre></pre>
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<p>Exact same program that we started with, but much easier to read.</p>
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<p>Of course, in the full <code>gba</code> crate that this book is a part of we have these and
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other elements all labeled and sorted out for you (not identically, but
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similarly). Still, for educational purposes it's often best to do it yourself at
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least once.</p>
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