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<title>The VCount Register - Rust GBA Tutorials</title>
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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"><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" class="active"><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="#the-vcount-register" id="the-vcount-register"><h1>The VCount Register</h1></a>
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<p>There's an IO register called
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<a href="http://problemkaputt.de/gbatek.htm#lcdiointerruptsandstatus">VCOUNT</a> that shows
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you, what else, the Vertical (row) COUNT(er). It's a <code>u16</code> at address
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<code>0x0400_0006</code>, and it's how we'll be doing our very poor quality vertical sync
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code to start.</p>
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<ul>
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<li><strong>What makes it poor?</strong> Well, we're just going to read from the vcount value as
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often as possible every time we need to wait for a specific value to come up,
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and then proceed once it hits the point we're looking for.</li>
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<li><strong>Why is this bad?</strong> Because we're making the CPU do a lot of useless work,
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which uses a lot more power that necessary. Even if you're not on an actual
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GBA you might be running inside an emulator on a phone or other handheld. You
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wanna try to save battery if all you're doing with that power use is waiting
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instead of making a game actually do something.</li>
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<li><strong>Can we do better?</strong> We can, but not yet. The better way to do things is to
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use a BIOS call to put the CPU into low power mode until a VBlank interrupt
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happens. However, we don't know about interrupts yet, and we don't know about
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BIOS calls yet, so we'll do the basic thing for now and then upgrade later.</li>
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</ul>
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<p>So the way that display hardware actually displays each frame is that it moves a
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tiny pointer left to right across each pixel row one pixel at a time. When it's
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within the actual screen width (240px) it's drawing out those pixels. Then it
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goes <em>past</em> the edge of the screen for 68px during a period known as the
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"horizontal blank" (HBlank). Then it starts on the next row and does that loop
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over again. This happens for the whole screen height (160px) and then once again
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it goes past the last row for another 68px into a "vertical blank" (VBlank)
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period.</p>
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<ul>
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<li>One pixel is 4 CPU cycles</li>
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<li>HDraw is 240 pixels, HBlank is 68 pixels (1,232 cycles per full scanline)</li>
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<li>VDraw is 150 scanlines, VBlank is 68 scanlines (280,896 cycles per full refresh)</li>
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</ul>
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<p>Now you may remember some stuff from the display control register section where
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it was mentioned that some parts of memory are best accessed during VBlank, and
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also during hblank with a setting applied. These blanking periods are what was
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being talked about. At other times if you attempt to access video or object
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memory you (the CPU) might try touching the same memory that the display device
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is trying to use, in which case you get bumped back a cycle so that the display
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can finish what it's doing. Also, if you really insist on doing video memory
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changes while the screen is being drawn then you might get some visual glitches.
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If you can, just prepare all your changes ahead of time and then assign then all
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quickly during the blank period.</p>
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<p>So first we want a way to check the vcount value at all:</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 VCOUNT: *mut u16 = 0x0400_0006 as *mut u16;
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pub fn read_vcount() -> u16 {
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unsafe { VCOUNT.read_volatile() }
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}
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#}</code></pre></pre>
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<p>Then we want two little helper functions to wait until VBlank and vdraw.</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 SCREEN_HEIGHT: isize = 160;
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pub fn wait_until_vblank() {
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while read_vcount() < SCREEN_HEIGHT as u16 {}
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}
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pub fn wait_until_vdraw() {
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while read_vcount() >= SCREEN_HEIGHT as u16 {}
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}
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#}</code></pre></pre>
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<p>And... that's it. No special types to be made this time around, it's just a
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number we read out of memory.</p>
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