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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" class="active"><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"><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>
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<h1 class="menu-title">Rust GBA Tutorials</h1>
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<a class="header" href="#hello1" id="hello1"><h1>hello1</h1></a>
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<p>Ready? Here goes:</p>
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<p><code>hello1.rs</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>Throw that into your project, build the program (as described back in Chapter
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0), and give it a run. You should see a red, green, and blue dot close-ish to
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the middle of the screen. If you don't, something already went wrong. Double
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check things, phone a friend, write your senators, try asking Ketsuban on the
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<a href="https://discordapp.com/invite/aVESxV8">Rust Community Discord</a>, until you're
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able to get your three dots going.</p>
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<a class="header" href="#explaining-hello1" id="explaining-hello1"><h2>Explaining hello1</h2></a>
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<p>So, what just happened? Even if you're used to Rust that might look pretty
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strange. We'll go over each part extra carefully.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#![feature(start)]
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#fn main() {
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#}</code></pre></pre>
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<p>This enables the <a href="https://doc.rust-lang.org/beta/unstable-book/language-features/start.html">start
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feature</a>,
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which you would normally be able to read about in the unstable book, except that
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the book tells you nothing at all except to look at the <a href="https://github.com/rust-lang/rust/issues/29633">tracking
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issue</a>.</p>
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<p>Basically, a GBA game is even more low-level than the <em>normal</em> amount of
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low-level that you get from Rust, so we have to tell the compiler to account for
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that by specifying a <code>#[start]</code>, and we need this feature on to do that.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#![no_std]
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#fn main() {
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#}</code></pre></pre>
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<p>There's no standard library available on the GBA, so we'll have to live a core
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only life.</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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#[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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#}</code></pre></pre>
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<p>This sets our <a href="https://doc.rust-lang.org/nightly/nomicon/panic-handler.html">panic
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handler</a>.
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Basically, if we somehow trigger a panic, this is where the program goes.
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However, right now we don't know how to get any sort of message out to the user
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so... we do nothing at all. We <em>can't even return</em> from here, so we just sit in
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an infinite loop. The player will have to reset the universe from the outside.</p>
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<p>The <code>#[cfg(not(test))]</code> part makes this item only exist in the program when
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we're <em>not</em> in a test build. This is so that <code>cargo test</code> and such work right as
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much as possible.</p>
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<pre><pre class="playpen"><code class="language-rust">#[start]
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fn main(_argc: isize, _argv: *const *const u8) -> isize {
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</code></pre></pre>
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<p>This is our <code>#[start]</code>. We call it <code>main</code>, but it's not like a <code>main</code> that you'd
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see in a Rust program. It's <em>more like</em> the sort of <code>main</code> that you'd see in a C
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program, but it's still <strong>not</strong> that either. If you compile a <code>#[start]</code> program
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for a target with an OS such as <code>arm-none-eabi-nm</code> you can open up the debug
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info and see that your result will have the symbol for the C <code>main</code> along side
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the symbol for the start <code>main</code> that we write here. Our start <code>main</code> is just its
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own unique thing, and the inputs and outputs have to be like that because that's
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how <code>#[start]</code> is specified to work in Rust.</p>
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<p>If you think about it for a moment you'll probably realize that, those inputs
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and outputs are totally useless to us on a GBA. There's no OS on the GBA to call
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our program, and there's no place for our program to "return to" when it's done.</p>
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<p>Side note: if you want to learn more about stuff "before main gets called" you
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can watch a great <a href="https://www.youtube.com/watch?v=dOfucXtyEsU">CppCon talk</a> by
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Matt Godbolt (yes, that Godbolt) where he delves into quite a bit of it. The
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talk doesn't really apply to the GBA, but it's pretty good.</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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unsafe {
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#}</code></pre></pre>
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<p>I hope you're all set for some <code>unsafe</code>, because there's a lot of it to be had.</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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(0x04000000 as *mut u16).write_volatile(0x0403);
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#}</code></pre></pre>
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<p>Sure!</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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(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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#}</code></pre></pre>
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<p>Ah, of course.</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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loop {}
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}
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}
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#}</code></pre></pre>
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<p>And, as mentioned above, there's no place for a GBA program to "return to", so
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we can't ever let <code>main</code> try to return there. Instead, we go into an infinite
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<code>loop</code> that does nothing. The fact that this doesn't ever return an <code>isize</code>
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value doesn't seem to bother Rust, because I guess we're at least not returning
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any other type of thing instead.</p>
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<p>Fun fact: unlike in C++, an infinite loop with no side effects isn't Undefined
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Behavior for us rustaceans... <em>semantically</em>. In truth LLVM has a <a href="https://github.com/rust-lang/rust/issues/28728">known
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bug</a> in this area, so we won't
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actually be relying on empty loops in any future programs.</p>
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<a class="header" href="#all-those-magic-numbers" id="all-those-magic-numbers"><h2>All Those Magic Numbers</h2></a>
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<p>Alright, I cheated quite a bit in the middle there. The program works, but I
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didn't really tell you why because I didn't really tell you what any of those
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magic numbers mean or do.</p>
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<ul>
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<li><code>0x04000000</code> is the address of an IO Register called the Display Control.</li>
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<li><code>0x06000000</code> is the start of Video RAM.</li>
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</ul>
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<p>So we write some magic to the display control register once, then we write some
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other magic to three locations of magic to the Video RAM. We get three dots,
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each in their own location... so that second part makes sense at least.</p>
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<p>We'll get into the magic number details in the other sections of this chapter.</p>
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<a class="header" href="#sidebar-volatile" id="sidebar-volatile"><h2>Sidebar: Volatile</h2></a>
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<p>We'll get into what all that is in a moment, but first let's ask ourselves: Why
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are we doing <em>volatile</em> writes? You've probably never used it before at all.
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What is volatile anyway?</p>
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<p>Well, the optimizer is pretty aggressive some of the time, and so it'll skip
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reads and writes when it thinks can. Like if you write to a pointer once, and
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then again a moment later, and it didn't see any other reads in between, it'll
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think that it can just skip doing that first write since it'll get overwritten
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anyway. Sometimes that's right, but sometimes it's wrong.</p>
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<p>Marking a read or write as <em>volatile</em> tells the compiler that it really must do
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that action, and in the exact order that we wrote it out. It says that there
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might even be special hardware side effects going on that the compiler isn't
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aware of. In this case, the write to the display control register sets a video
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mode, and the writes to the Video RAM set pixels that will show up on the
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screen.</p>
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<p>Similar to "atomic" operations you might have heard about, all volatile
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operations are enforced to happen in the exact order that you specify them, but
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only relative to other volatile operations. So something like</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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c.volatile_write(5);
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a += b;
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d.volatile_write(7);
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#}</code></pre></pre>
|
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<p>might end up changing <code>a</code> either before or after the change to <code>c</code> (since the
|
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value of <code>a</code> doesn't affect the write to <code>c</code>), but the write to <code>d</code> will
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<em>always</em> happen after the write to <code>c</code> even though the compiler doesn't see any
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direct data dependency there.</p>
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<p>If you ever use volatile stuff on other platforms it's important to note that
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volatile doesn't make things thread-safe, you still need atomic for that.
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However, the GBA doesn't have threads, so we don't have to worry about thread
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safety concerns.</p>
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