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<h1 class="post-title">MMC3的IRQ中断</h1>
<span class="post-date">周三 11 一月 2023</span>
<p><a href="http://bobrost.com/nes/files/mmc3irqs.txt">原文</a>作者：Allan Blomquist</p>
<p>iNES Mapper #4 (MMC3)提供了一种方便的方法，来对PPU进行帧间改变，以实现NES游戏中的<strong>分屏卷动</strong>等“特殊效果”。与其使用<code>Sprite-0 Hits</code>去择机写入PPU硬件，不如使用IRQ。这可以使CPU在一帧内就做完工作，而不是浪费时间去轮询<code>Sprite-0 Hits</code>标志。 MMC3有4个寄存器，是MMC3为IRQ触发而设的：</p>
<p><code>$C000</code> - <strong>IRQ计数器</strong> - Mapper用它来倒计时以触发IRQ。每一条扫描线，该值减一；当它到0时，IRQ就触发。</p>
<blockquote>
<p><strong>注意：</strong> 背景和精灵必须启用，在<code>$2001</code>(<code>D3</code>、<code>D4</code>)，这样倒数计数才会进行。</p>
</blockquote>
<p><code>$C001</code> - <strong>IRQ计数锁存器</strong> - 写入你想要的值，也就是用来倒数的值；而不是直接往<code>IRQ计数寄存器</code>里写值。</p>
<p><code>$E000</code> - <strong>IRQ禁用/确认器</strong> - 往这里写任意值，都能禁用IRQ的触发，但也能将<code>IRQ计数器锁存器</code>中的值复制到实际的<code>IRQ计数器</code>。一旦确认了一次IRQ，就会保证发生中断。</p>
<p><code>$E001</code> - <strong>IRQ启动器</strong> - 往这里写入任意值，都能启用IRQ的触发。</p>
<p>因此，大多数的商业游戏都是这样使用这些寄存器：</p>
<p>当你想设置的IRQ时：（放在你的NMI/vblank程序中）</p>
<ol>
<li>写<code>$E000</code>以确认任何当前待定的中断</li>
<li>向<code>$C000</code>和<code>$C001</code>写入你要等待的扫描线数</li>
<li>再次写<code>$E000</code>以锁定倒数值</li>
<li>写<code>$E001</code>以启用<code>IRQ计数器</code></li>
</ol>
<p>一旦你的IRQ发生了：（在你的IRQ程序中）</p>
<ol>
<li>写<code>$E000</code>以确认IRQ并禁用<code>IRQ计数器</code></li>
</ol>
<p>你在游戏使用MMC3的IRQ之前，有几件事情你必须设置好。<strong>首先</strong>，你要设置好ROM结尾中断向量表里的IRQ向量，指向IRQ发生时你想被调用的代码。IRQ向量的位置在$FFFE，在NMI向量和RESET向量的右边。当一个IRQ中断触发时，CPU会跳到的16位地址$FFFE上，并且CPU标志被自动存下来，在IRQ程序返回时自动恢复。但是CPU的寄存器不会被保存，所以你必须确保在IRQ程序结束时，它们会被恢复原样（开始时应该入栈AXY寄存器，结束时出栈YXA，注意顺序）。</p>
<p><strong>第二</strong>件你必须做的事情，使用MMC3的IRQ，就要阻止其他的在NES上产生的IRQ中断。NES有一个“帧计数器”，默认会在每帧结束时产生IRQ中断，非常像NMI。如果不禁用这个，你的IRQ代码次数，会比你要的更多，从而搞砸你的游戏。禁用帧计数器的IRQ中断，只需要在你的程序开始时，把<code>$40</code>写入到<code>$4017</code>。</p>
<p><strong>最后</strong>，为了能够使用IRQ，你一定要告诉NES的CPU，你有实际确认它们，而不是忽略了它们，因为它在默认情况下（这也是为什么你不必担心在简单的游戏里，帧计数器的IRQ的发生 - CPU实际上忽略了它们）会启用的IRQ。你必须用“CLI”指令清除CPU中的中断禁用标志。在某些时刻，如果你想重新禁用它们，要用“SEI”。</p>
<p>下面是一个在nbasic中使用MMC3 IRQ中断的最小例子。 它使用一个IRQ中断来改变屏幕上一半的背景颜色，而不需要使用<code>Sprite-0-Hits</code>来确定你的位置。</p>
<blockquote>
<p><strong>注意：</strong> MMC3 IRQ的时机安排是大多数模拟器没有真正做到的事情（例如，一个倒数值X，可能会在扫描线X-1或X+1上，实际触发一个IRQ中断，这取决于你使用的模拟器）。如果你使用IRQ，请在各种模拟器多测试几遍你的游戏。</p>
</blockquote>
<div class="highlight"><pre><span></span><code><span class="c1">;//nesasm文件头</span>
<span class="nf">asm</span>
<span class="w"> </span><span class="na">.inesprg</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="c1">;//1个PRG段</span>
<span class="w"> </span><span class="na">.ineschr</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="c1">;//0个CHR段</span>
<span class="w"> </span><span class="na">.inesmir</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="c1">;//镜像类型0</span>
<span class="w"> </span><span class="na">.inesmap</span><span class="w"> </span><span class="mi">4</span><span class="w"> </span><span class="c1">;//内存使用mapper 4 (MMC3)</span>
<span class="w"> </span><span class="na">.org</span><span class="w"> </span><span class="no">$C000</span>
<span class="w"> </span><span class="na">.bank</span><span class="w"> </span><span class="mi">0</span>
<span class="nf">endasm</span>
<span class="nl">start:</span>
<span class="w"> </span><span class="nf">gosub</span><span class="w"> </span><span class="no">vwait</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2000</span><span class="w"> </span><span class="no">$80</span><span class="w"> </span><span class="c1">;// 打开NMI</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2001</span><span class="w"> </span><span class="no">$08</span><span class="w"> </span><span class="c1">;// 打开BG使MMC3能够计数</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$4017</span><span class="w"> </span><span class="no">$40</span><span class="w"> </span><span class="c1">;// 禁用帧计数器IRQ</span>
<span class="nf">asm</span>
<span class="w"> </span><span class="nf">cli</span><span class="w"> </span><span class="c1">;// 启用IRQ处理过程</span>
<span class="nf">endasm</span>
<span class="nl">gameloop:</span>
<span class="w"> </span><span class="nf">goto</span><span class="w"> </span><span class="no">gameloop</span>
<span class="c1">;// NMI程序</span>
<span class="nl">nmi:</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$3F</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$00</span><span class="w"> </span><span class="c1">;// 设置背景色为红色</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2007</span><span class="w"> </span><span class="no">$06</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$E000</span><span class="w"> </span><span class="no">$01</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$C000</span><span class="w"> </span><span class="no">$78</span><span class="w"> </span><span class="c1">;// 设置MMC3让IRQ中断发生在</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$C001</span><span class="w"> </span><span class="no">$78</span><span class="w"> </span><span class="c1">;// 120条扫描线之后</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$E000</span><span class="w"> </span><span class="no">$01</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$E001</span><span class="w"> </span><span class="no">$01</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$00</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$00</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2001</span><span class="w"> </span><span class="no">$08</span><span class="w"> </span><span class="c1">;// 打开BG使MMC3能够计数</span>
<span class="w"> </span><span class="nf">resume</span>
<span class="c1">;// IRQ程序</span>
<span class="nl">irq:</span>
<span class="nf">asm</span>
<span class="w"> </span><span class="nf">pha</span>
<span class="w"> </span><span class="nf">txa</span>
<span class="w"> </span><span class="nf">pha</span><span class="w"> </span><span class="c1">;// 保存寄存器A,X和Y。你必须这么做，因为</span>
<span class="w"> </span><span class="nf">tya</span><span class="w"> </span><span class="c1">;// IRQ中断了你的主线程代码。改变你的</span>
<span class="w"> </span><span class="nf">pha</span><span class="w"> </span><span class="c1">;// 寄存器值，对主线程来说，这是可是</span>
<span class="w"> </span><span class="c1">;// 不太好的...</span>
<span class="nf">endasm</span><span class="w"> </span>
<span class="w"> </span><span class="no">set</span><span class="w"> </span><span class="no">$2001</span><span class="w"> </span><span class="no">$00</span><span class="w"> </span><span class="c1">;// 强制关闭背景，迫使愚蠢的模拟器意识到我在改变背景色</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$3F</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$00</span><span class="w"> </span><span class="c1">;// 设置背景色为绿色</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2007</span><span class="w"> </span><span class="no">$0A</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$E000</span><span class="w"> </span><span class="no">$01</span><span class="w"> </span><span class="c1">;// 确认IRQ并禁止进一步产生IRQ</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$00</span>
<span class="w"> </span><span class="nf">set</span><span class="w"> </span><span class="no">$2006</span><span class="w"> </span><span class="no">$00</span>
<span class="nf">asm</span>
<span class="w"> </span><span class="nf">pla</span>
<span class="w"> </span><span class="nf">tay</span><span class="w"> </span><span class="c1">;// 取回你的寄存器值</span>
<span class="w"> </span><span class="nf">pla</span>
<span class="w"> </span><span class="nf">tax</span>
<span class="w"> </span><span class="nf">pla</span>
<span class="nf">endasm</span>
<span class="w"> </span><span class="nf">resume</span>
<span class="c1">;//等待屏幕刷新</span>
<span class="nl">vwait:</span>
<span class="w"> </span><span class="nf">asm</span>
<span class="w"> </span><span class="nf">lda</span><span class="w"> </span><span class="no">$2002</span>
<span class="w"> </span><span class="nf">bpl</span><span class="w"> </span><span class="no">vwait</span><span class="w"> </span><span class="c1">;//等待回溯开始</span>
<span class="w"> </span><span class="nl">vwait_1:</span>
<span class="w"> </span><span class="nf">lda</span><span class="w"> </span><span class="no">$2002</span>
<span class="w"> </span><span class="nf">bmi</span><span class="w"> </span><span class="no">vwait_1</span><span class="w"> </span><span class="c1">;//等待回溯结束</span>
<span class="w"> </span><span class="nf">endasm</span>
<span class="w"> </span><span class="nf">return</span>
<span class="c1">;//文件脚部</span>
<span class="nf">asm</span>
<span class="w"> </span><span class="c1">;//跳转表指向NMI，Reset和IRQ开始位置</span>
<span class="w"> </span><span class="na">.bank</span><span class="w"> </span><span class="mi">1</span>
<span class="w"> </span><span class="na">.org</span><span class="w"> </span><span class="no">$fffa</span>
<span class="w"> </span><span class="na">.dw</span><span class="w"> </span><span class="no">nmi</span><span class="p">,</span><span class="w"> </span><span class="no">start</span><span class="p">,</span><span class="w"> </span><span class="no">irq</span>
<span class="nf">endasm</span>
</code></pre></div>
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