mirror of
https://github.com/fadden/6502bench.git
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198 lines
7.5 KiB
HTML
198 lines
7.5 KiB
HTML
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<!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 $01, then also continues to the <code>STA</code>.</p>
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<p>Embedded instructions are unusual but not unheard-of. (This trick is
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used extensively in Microsoft BASICs, such as Applesoft.) When you see the
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extra symbol in the opcode field, you need to look closely at what's going
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on.</p>
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</div>
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</div>
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<hr style="width:80%;"/>
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<div class="grid-container">
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<div class="grid-item-text">
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<p>This is the end of the basic tutorial (congratulations!).
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The next sections explore some advanced topics.</p>
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</div>
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</div>
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</div> <!-- #main -->
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<div id="prevnext">
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<a href="#" class="btn-previous">« Previous</a>
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<a href="#" class="btn-next">Next »</a>
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</div>
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<div id="footer">
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<!-- START: /footer-incl.html -->
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<script>$("#footer").load("/footer-incl.html");</script>
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<!-- END: /footer-incl.html -->
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</div>
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</body>
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</html>
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