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Added a SourceGen tutorial with lots of screen shots. Uses "responsive web design" so it works well on mobile devices. This version is using jQuery load() calls to pull in pieces, but that causes a lot of blink when loading because the loads are asynchronous and may not complete until after the initial page render has finished. Tutorial prev/next links not yet working.
198 lines
7.5 KiB
HTML
198 lines
7.5 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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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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<title>Odds & Ends - SourceGen Tutorial</title>
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</head>
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<body>
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<div id="masthead">
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<!-- START: /masthead-incl.html -->
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<script>$("#masthead").load("/masthead-incl.html");</script>
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<!-- END: /masthead-incl.html -->
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</div>
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<div id="topnav">
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<!-- START: /topnav-incl.html active:#topnav-sgtutorial -->
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<script>
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// Load global topnav content, and mark current page active.
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$("#topnav").load("/topnav-incl.html", function() {
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$("#topnav-sgtutorial").addClass("active");
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});
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</script>
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<!-- END: /topnav-incl.html -->
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</div>
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<div id="sidenav">
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<!-- START: /sidenav-incl.html active:#sidenav-odds-ends -->
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<script>
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// Load local sidenav content, and mark current page active.
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$("#sidenav").load("sidenav-incl.html", function() {
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$("#sidenav-odds-ends").addClass("active");
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});
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</script>
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<!-- END: /sidenav-incl.html -->
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</div>
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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>
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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>Edit the label on line $2081, setting it to <kbd>STUFF</kbd>.
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Notice how the references to $2081 through $2084 have changed from
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auto-generated labels to references to <code>STUFF</code>.</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-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>For some projects this may be undesirable. Use
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<samp>Edit > Project Properties</samp>, then in the
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<samp>Analysis Parameters</samp> box un-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-text">
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<p>You'll notice that the references to $2081 and later have switched
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back to auto labels. 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" logic.</p>
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<p>The nearby-target behavior is generally desirable, because it lets you
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avoid explicitly labeling every part of a multi-byte data item. For now,
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use <samp>Edit > Undo</samp> to switch it back on.
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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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</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>The code at $2092 looks a bit strange. <code>LDX</code>, then a
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<code>BIT</code> with a weird symbol, then another <code>LDX</code>. If
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you look at the "bytes" column, you'll notice that the three-byte
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<code>BIT</code> instruction has only one 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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<code>STA</code> runs. Several lines down there's a <code>BNE</code>
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to $2095, which is in the middle of the <code>BIT</code> instruction.
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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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