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318 lines
13 KiB
HTML
318 lines
13 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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<title>Local Variables - SourceGen Tutorial</title>
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<body>
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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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<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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<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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<!-- START: incl-sidenav.html active:#sidenav-local-variables -->
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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-moving-around"><a href="moving-around.html">Moving Around</a></li>
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<li id="sidenav-simple-edits"><a href="simple-edits.html">Simple 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-digging-deeper"><a href="digging-deeper.html">Digging Deeper</a>
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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" class="active"><a href="local-variables.html">Local Variables</a></li>
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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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<li id="sidenav-suggestions"><a href="suggestions.html">Suggestions</a></li>
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<div id="main">
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<h2>Local Variables</h2>
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<div class="grid-container">
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<div class="grid-item-image">
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<img src="images/t2-203d-start.png" alt="t2-203d-start"/>
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</div>
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<div class="grid-item-text">
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<p>Let's move on to the code at $203D. It starts by storing a couple of
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values into direct page address $02/03. This appears to be setting up a
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pointer to $2063, which is a data area inside the file. So let's make it
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official.</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/t2-203d-edit1.png" alt="t2-203d-edit1"/>
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</div>
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<div class="grid-item-text">
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<p>Select the line at address $2063, and use
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<samp>Actions > Edit Label</samp> to
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give it the label "<kbd>XDATA?</kbd>". The question mark on the end is there to
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remind us that we're not entirely sure what this is. Now edit the
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operand on line $203D, and set it to the symbol "<kbd>XDATA</kbd>",
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with the part "low". The question mark isn't really part of the label,
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so you don't need to type it here.</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/t2-203d-after-edit2.png" alt="t2-203d-after-edit2"/>
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</div>
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<div class="grid-item-text">
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<p>Edit the operand on line $2041,
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and set it to "<kbd>XDATA</kbd>" with the part "high". (The symbol text box
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gets focus immediately, so you can start typing the symbol name as soon
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as the dialog opens; you don't need to click around first.) If all
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went well, the operands should now read <code>LDA #<XDATA?</code>
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and <code>LDA #>XDATA?</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-create-ptr-entry.png" alt="t2-create-ptr-entry"/>
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</div>
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<div class="grid-item-text">
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<p>Let's give the pointer a name. Select line $203D, and use
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<samp>Actions > Create Local Variable Table</samp>
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to create an empty table. Click <samp>New Symbol</samp> on the right side.
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Leave the Address button selected. Set the Label field to "<kbd>PTR1</kbd>",
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the Value field to "$02", and the width to "2" (it's a 2-byte pointer).
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Click "OK" to create the entry, and then
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"OK" to update the table.</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-after-ptr-entry.png" alt="t2-after-ptr-entry"/>
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</div>
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<div class="grid-item-text">
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<p>There's now a <code>.VAR</code> statement
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(similar to a <code>.EQU</code>) above line $203D,
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and the stores to $02/$03 have changed to
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"<samp>PTR1</samp>" and "<samp>PTR1+1</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/t2-20a7.png" alt="t2-20a7"/>
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</div>
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<div class="grid-item-text">
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<p>Double-click on the <code>JSR</code> opcode on line $2045 to jump to
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<samp>L20A7</samp>.
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The code here just loads a value from $3000 into the accumulator
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and returns, so not much to see here. Hit the back-arrow in the
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toolbar to jump back to the <code>JSR</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/t2-2048.png" alt="t2-2048"/>
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</div>
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<div class="grid-item-text">
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<p>The next bit of code masks the accumulator so it holds a value between
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0 and 3, then doubles it and uses it as an index into <code>PTR1</code>.
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We know <code>PTR1</code> points to <code>XDATA</code>,
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which looks like it has some 16-bit addresses. The
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values loaded are stored in two more zero-page locations, $04-05.
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Let's make these a pointer as well.</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/t2-204e-lv.png" alt="t2-204e-lv"/>
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</div>
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<div class="grid-item-text">
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<p>Double-click the operand on line $204E ("<samp>$04</samp>"),
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and click <samp>Create Local Variable</samp>. Set the Label
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to "<kbd>PTR2</kbd>" and the width to 2. Click <samp>OK</samp>
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to create the symbol, then <samp>OK</samp>
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to close the operand editor, which should still be set to Default format --
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we didn't actually edit the operand, we just used the operand edit
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dialog as a convenient way to create a local variable table entry. All
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accesses to $04/$05 now use <code>PTR2</code>, and there's a new entry in the local
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variable table we created earlier.</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/t2-2055.png" alt="t2-2055"/>
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</div>
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<div class="grid-item-text">
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<p>The next section of code, at $2055, copies bytes from <code>PTR2</code>
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to $0400, stopping when it hits a zero byte.
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It looks like this is copying null-terminated strings.</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-fmt-xdata.png" alt="t2-fmt-xdata"/>
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</div>
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<div class="grid-item-text">
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<p>This confirms our idea that <code>XDATA</code> holds 16-bit addresses,
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so let's format it. Select lines $2063 to $2066, and
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<samp>Actions > Edit Operand</samp>.
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The editor window should say "<samp>8 bytes selected</samp>" at the top.
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Click the <samp>16-bit words, little-endian</samp> radio button,
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and then in the <samp>Display As</samp> box, click <samp>Address</samp>.
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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-fmt-xdata-done.png" alt="t2-fmt-xdata-done"/>
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</div>
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<div class="grid-item-text">
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<p>The values at <code>XDATA</code> should now be four
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<code>.DD2</code> 16-bit addresses.
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If you scroll up, you'll see that the <code>.ZSTR</code> strings
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near the top now have labels that match the operands in <code>XDATA</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-text">
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<p>Now that we know what <code>XDATA</code> holds, let's rename it.
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Change the label to <kbd>STRADDR</kbd>. The symbol parts in the
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operands at $203D and $2041 update automatically.</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-image">
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<img src="images/t2-enable-counts.png" alt="t2-enable-counts"/>
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</div>
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<div class="grid-item-text">
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<p>Let's take a quick look at the cycle-count feature. Use
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<samp>Edit > Settings</samp> to open the app settings panel.
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In the <samp>Miscellaneous</samp> group on the right side, click the
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<samp>Show cycle counts for instructions</samp> checkbox, then click
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<samp>OK</samp>. (There's also a toolbar button for this.)</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-show-cycle-counts.png" alt="t2-show-cycle-counts"/>
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</div>
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<div class="grid-item-text">
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<p>Every line with an instruction now has a cycle count on it. The cycle
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counts are adjusted for everything SourceGen can figure out. For example,
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the <code>BEQ</code> on line $205A shows "2+" cycles, meaning that
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it takes at least two cycles but might take more. That's because
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conditional branches take an
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extra cycle if the branch is taken. The <code>BNE</code> on line
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$2061 shows 3 cycles, because we know that the branch is always
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taken and doesn't cross a page boundary.</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>(If you want to see why it's always taken,
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look at the value of the 'Z' flag in the "flags" column, which indicates
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the state of the flags before the instruction on that line is executed.
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Lower-case 'z' means the zero-flag is clear (0), upper-case 'Z' means it's
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set (1). The analyzer determined that the flag was clear for instructions
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following the <code>BEQ</code> because we're on the branch-not-taken path.
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The following instruction, <code>ORA #$80</code>, cleared the 'Z' flag and
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set the 'N' flag, so a <code>BMI</code> would also be an always-taken branch.)</p>
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<p>The cycle-count comments can be added to generated source code as well.</p>
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<p>If you add an end-of-line comment, it appears after the cycle count.
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(Try it.)</p>
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</div>
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</div>
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</div> <!-- #main -->
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