gba/docs/ch1/hello2.html
2018-11-11 14:56:11 -07:00

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<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"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch1/hello2.html" class="active"><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/gba_snake.html"><strong aria-hidden="true">4.3.</strong> gba_snake</a></li></ol></li></ol>
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<h1 class="menu-title">Rust GBA Tutorials</h1>
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<a class="header" href="#hello2" id="hello2"><h1>hello2</h1></a>
<p>Okay so let's have a look again:</p>
<p><code>hello1</code></p>
<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
#![no_std]
#[cfg(not(test))]
#[panic_handler]
fn panic(_info: &amp;core::panic::PanicInfo) -&gt; ! {
loop {}
}
#[start]
fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
unsafe {
(0x04000000 as *mut u16).write_volatile(0x0403);
(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);
loop {}
}
}
</code></pre></pre>
<p>Now let's clean this up so that it's clearer what's going on.</p>
<p>First we'll label that display control stuff:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
pub const MODE3: u16 = 3;
pub const BG2: u16 = 0b100_0000_0000;
#}</code></pre></pre>
<p>Next we make some const values for the actual pixel drawing</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const VRAM: usize = 0x06000000;
pub const SCREEN_WIDTH: isize = 240;
#}</code></pre></pre>
<p>And then we want a small helper function for putting together a color value.</p>
<p>Happily, this one can even be declared as a const function. At the time of
writing, we've got the &quot;minimal const fn&quot; support in nightly. It really is quite
limited, but I'm happy to let rustc and LLVM pre-compute as much as they can
when it comes to the GBA's tiny CPU.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const fn rgb16(red: u16, green: u16, blue: u16) -&gt; u16 {
blue &lt;&lt; 10 | green &lt;&lt; 5 | red
}
#}</code></pre></pre>
<p>Finally, we'll make a function for drawing a pixel in Mode 3. Even though it's
just a one-liner, having the &quot;important parts&quot; be labeled as function arguments
usually helps you think about it a lot better.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
}
#}</code></pre></pre>
<p>So now we've got this:</p>
<p><code>hello2</code></p>
<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
#![no_std]
#[cfg(not(test))]
#[panic_handler]
fn panic(_info: &amp;core::panic::PanicInfo) -&gt; ! {
loop {}
}
#[start]
fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
unsafe {
DISPCNT.write_volatile(MODE3 | BG2);
mode3_pixel(120, 80, rgb16(31, 0, 0));
mode3_pixel(136, 80, rgb16(0, 31, 0));
mode3_pixel(120, 96, rgb16(0, 0, 31));
loop {}
}
}
pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
pub const MODE3: u16 = 3;
pub const BG2: u16 = 0b100_0000_0000;
pub const VRAM: usize = 0x06000000;
pub const SCREEN_WIDTH: isize = 240;
pub const fn rgb16(red: u16, green: u16, blue: u16) -&gt; u16 {
blue &lt;&lt; 10 | green &lt;&lt; 5 | red
}
pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
}
</code></pre></pre>
<p>Exact same program that we started with, but much easier to read.</p>
<p>Of course, in the full <code>gba</code> crate that this book is a part of we have these and
other elements all labeled and sorted out for you. Still, for educational
purposes it's often best to do it yourself at least once.</p>
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