mirror of
https://github.com/fadden/6502bench.git
synced 2025-02-28 10:29:02 +00:00
eef1710d5d
Two main changes: - "Seek nearby targets" is no longer enabled by default. - The application asks the user to save new projects immediately. Various minor edits were also made. A couple of the images are slightly stale, showing ".org" rather than ".addrs", but they're in the advanced section and the difference isn't notable.
249 lines
10 KiB
HTML
249 lines
10 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<!-- START: /incl-head.html -->
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<!-- common head elements -->
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<meta charset="utf-8"/>
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<meta name="viewport" content="width=device-width, initial-scale=1"/>
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<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"
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integrity="sha384-vtXRMe3mGCbOeY7l30aIg8H9p3GdeSe4IFlP6G8JMa7o7lXvnz3GFKzPxzJdPfGK" crossorigin="anonymous"></script>
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<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css"/>
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<link rel="stylesheet" href="../main.css"/>
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<!-- END: /incl-head.html -->
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<title>Odds & Ends - SourceGen Tutorial</title>
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</head>
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<body>
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<!-- START: /incl-masthead.html -->
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<div id="masthead">
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<!--<div class="masthead-title" style="background-image: url('images/screenshot-mainwin.png');">-->
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<div class="masthead-title">
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6502bench
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</div>
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</div>
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<!-- END: /incl-masthead.html -->
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<!-- START: /incl-topnav.html active:#topnav-sgtutorial -->
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<div id="topnav">
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<!-- top navigation bar contents -->
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<nav>
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<a id="topnav-home" href=".././">HOME</a>
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<a id="topnav-sgtutorial" class="active" href="../sgtutorial">SourceGen Tutorial</a>
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<a id="topnav-menuicon" href="javascript:void(0);" class="icon" onclick="toggleSidenav()">
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<i class="fa fa-bars"></i>
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</a>
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</nav>
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<script>
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// Sidenav toggle function.
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//
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// Use a jQuery function to toggle the sidenav bar. The initial state
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// is undefined / inherited, so it will pop in and out as the screen
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// resizes around the "large" breakpoint.
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function toggleSidenav() {
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$("#sidenav").toggle("fast");
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}
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</script>
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</div>
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<!-- END: /incl-topnav.html -->
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<!-- START: incl-sidenav.html active:#sidenav-odds-ends -->
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<div id="sidenav">
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<!-- side navigation bar contents -->
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<ul>
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<li id="sidenav-index"><a href="./">Introduction</a></li>
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<li id="sidenav-about-disasm"><a href="about-disasm.html">About Disassembly</a></li>
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<li id="sidenav-using-sourcegen"><a href="using-sourcegen.html">Using SourceGen</a>
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<ul>
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<li id="sidenav-moving-around"><a href="moving-around.html">Moving Around</a></li>
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<li id="sidenav-making-edits"><a href="making-edits.html">Making Edits</a></li>
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<li id="sidenav-labels-symbols"><a href="labels-symbols.html">Labels & Symbols</a></li>
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<li id="sidenav-editing-data"><a href="editing-data.html">Editing Data Operands</a></li>
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<li id="sidenav-generating-code"><a href="generating-code.html">Generating Code</a></li>
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</ul></li>
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<li id="sidenav-digging-deeper"><a href="digging-deeper.html">Digging Deeper</a>
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<ul>
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<li id="sidenav-string-formatting"><a href="string-formatting.html">String Formatting</a></li>
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<li id="sidenav-local-variables"><a href="local-variables.html">Local Variables</a></li>
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<li id="sidenav-inline-data"><a href="inline-data.html">Inline Data</a></li>
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<li id="sidenav-odds-ends" class="active"><a href="odds-ends.html">Odds & Ends</a></li>
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</ul></li>
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<li id="sidenav-advanced-topics"><a href="advanced-topics.html">Advanced Topics</a>
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<ul>
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<li id="sidenav-address-tables"><a href="address-tables.html">Address Tables</a></li>
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<li id="sidenav-extension-scripts"><a href="extension-scripts.html">Extension Scripts</a></li>
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<li id="sidenav-visualizations"><a href="visualizations.html">Visualizations</a></li>
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</ul></li>
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<li id="sidenav-suggestions"><a href="suggestions.html">Suggestions</a></li>
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</ul>
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</div>
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<!-- END: incl-sidenav.html -->
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<div id="main">
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<h2>Odds & Ends</h2>
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<div class="grid-container">
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<div class="grid-item-text">
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<p>The rest of the code isn't really intended to do anything useful. It
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just exists to illustrate some odd situations.</p>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-2078.png" alt="t2-2078"/>
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</div>
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<div class="grid-item-text">
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<p>Look at the code starting at $2078. It ends with a <code>BRK</code>
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at $2081, which as noted earlier is a bad sign. If you look two lines
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above the <code>BRK</code>, you'll see that it's loading the accumulator
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with zero, then doing a <code>BNE</code>, which should never be
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taken (note the cycle count for the <code>BNE</code> is 2).
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The trick is in the two lines before that, which use self-modifying code to
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change the <code>LDA</code> immediate operand from $00 to $ff.
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The <code>BNE</code> is actually a branch-always.</p>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-override-status.png" alt="t2-override-status.png"/>
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</div>
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<div class="grid-item-text">
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<p>We can fix this by correcting the status flags. Select line $207F,
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and then <samp>Actions > Override Status Flags</samp>. This lets us specify what
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the flags should be before the instruction is executed. For each flag,
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we can override the default behavior and specify that the flag is
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clear (0), set (1), or indeterminate (could be 0 or 1). In this case,
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we know that the self-modified code will be loading a non-zero value, so
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in the "<samp>Z</samp>" column click on the button in the "<samp>Zero</samp>" row.
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Click "<samp>OK</samp>".</p>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-2078-done.png" alt="t2-2078-done"/>
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</div>
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<div class="grid-item-text">
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<p>The <code>BNE</code> is now an always-taken branch, and the code
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list rearranges itself appropriately (and the cycle count is now 3).</p>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-2086.png" alt="t2-2086"/>
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</div>
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<div class="grid-item-text">
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<p>Continuing on, the code at $2086 touches a few consecutive locations
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that have auto-generated labels.</p>
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<div class="grid-item-text">
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<p>Edit the label on line $2081, setting it to <kbd>STUFF</kbd>.
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The operand label on line $2086 also changed. But what if we want
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to treat these as a single four-byte item?</p>
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</div>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-seek-nearby.png" alt="t2-seek-nearby"/>
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</div>
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<div class="grid-item-text">
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<p>Use <samp>Edit > Project Properties</samp>, then in the
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<samp>Analysis Parameters</samp> box check
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<samp>Seek nearby targets</samp>, and click <samp>OK</samp>.</p>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-2081-stuff.png" alt="t2-2081-stuff"/>
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</div>
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<div class="grid-item-text">
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<p>You'll notice that the references to $2081 and later are now
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also references to <code>STUFF</code>. The nearby-target behavior
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is often useful, because it lets you avoid explicitly labeling every
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part of a multi-byte data item. References to the byte before
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a string or array are automatically resolved to <code>LABEL-1</code>.
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You can use <samp>Edit > Undo</samp> to turn it back off:
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changes to project properties are added to the undo/redo buffer
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just like any other change to the project.</p>
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<p>(This feature used to be enabled by default, but is now
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disabled for new projects because it can be more distracting than
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helpful for some types of code.)
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<p>If you scroll up, you'll see that the references to
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<code>PTR1+1</code> and <code>PTR2+1</code> were
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not affected, because local variables use explicit widths rather
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than the "nearby target" logic.</p>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-2092.png" alt="t2-2092"/>
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</div>
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<div class="grid-item-text">
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<p>While we're here, the code at $2092 looks a bit strange.
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<code>LDX</code>, then a <code>BIT</code> with a weird symbol, then
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another <code>LDX</code>. If you look at the "Bytes" column, you'll
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notice that the three-byte <code>BIT</code> instruction has only one
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byte on its line.</p>
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</div>
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</div>
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<div class="grid-container">
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<div class="grid-item-text">
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<p>The trick here is that the <code>LDX #$01</code> is embedded inside the
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<code>BIT</code> instruction. When the code runs through here, X is set
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to $00, then the <code>BIT</code> instruction sets some flags, then the
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following <code>STA</code> runs. Several lines down at $20A4 there's a
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<code>BNE</code> to $2095, which is in the middle of the <code>BIT</code>
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instruction. It loads X with $01, then also continues to the <code>STA</code>.</p>
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<p>Embedded instructions are unusual but not unheard-of. (This trick is
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used extensively in Microsoft BASICs, such as Applesoft.) When you see the
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extra symbol in the opcode field, you need to look closely at what's going
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on.</p>
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</div>
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</div>
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<hr style="width:80%;"/>
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<div class="grid-container">
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<div class="grid-item-text">
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<p>This is the end of the basic tutorial (congratulations!).
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The next sections explore some advanced topics.</p>
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</div>
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</div>
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</div> <!-- #main -->
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<div id="prevnext">
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<a href="inline-data.html" class="btn-previous">« Previous</a>
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<hr/>
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<p>Copyright 2021 faddenSoft</p>
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