mirror of https://github.com/fadden/6502bench.git
208 lines
8.3 KiB
HTML
208 lines
8.3 KiB
HTML
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<title>About Disassembly - 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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<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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// Sidenav toggle function.
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//
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<div id="sidenav">
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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" class="active"><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"><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>About Disassembly</h2>
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<div class="grid-container">
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<div class="grid-item-text">
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<p>Well-written assembly-language source code has meaningful
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comments and labels, so that humans can read and understand it.
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For example:</p>
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<pre>
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.org $2000
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sec ;set carry
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ror A ;shift into high bit
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bmi CopyData ;branch always
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.asciiz "first string"
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.asciiz "another string"
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.asciiz "string the third"
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.asciiz "last string"
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CopyData lda #<addrs ;get pointer into
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sta ptr ; address table
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lda #>addrs
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sta ptr+1
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</pre>
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<p>Computers operate at a much lower level, so a piece of software
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called an <i>assembler</i> is used to convert the source code to
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object code that the CPU can execute.
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Object code looks more like this:</p>
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<pre>
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38 6a 30 39 66 69 72 73 74 20 73 74 72 69 6e 67
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00 61 6e 6f 74 68 65 72 20 73 74 72 69 6e 67 00
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73 74 72 69 6e 67 20 74 68 65 20 74 68 69 72 64
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00 6c 61 73 74 20 73 74 72 69 6e 67 00 a9 63 85
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02 a9 20 85 03
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</pre>
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<p>This arrangement works perfectly well until somebody needs to
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modify the software and nobody can find the original sources.
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<i>Disassembly</i> is the act of taking a raw hex
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dump and converting it to source 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/t0-bad-disasm.png" alt="t0-bad-disasm"/>
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</div>
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<div class="grid-item-text">
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<p>Disassembling a blob of data can be tricky. A simple
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disassembler can format instructions, but can't generally tell
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the difference between instructions and data. Many 6502 programs
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intermix code and data freely, so simply dumping everything as
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an instruction stream can result in sections with nonsensical output.</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>One way to separate code from data is to try to execute all
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possible data paths. There are a number of reasons why it's difficult
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or impossible to do this perfectly, but you can get pretty good
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results by identifying execution entry points and just walking through
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the code. When a conditional branch is encountered, both paths are
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traversed. When all code has been traced, every byte that hasn't
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been visited is either
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data used by the program, or dead space not used by anything.</p>
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<p>The process can be improved by keeping track of the flags in the
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6502 status register. For example, in the code fragment shown
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earlier, <code>BMI</code> conditional branch instruction is used.
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A simple tracing algorithm would both follow the branch and fall
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through to the following instruction. However, the code that precedes
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the <code>BMI</code> ensures that the branch is always taken, so a
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clever disassembler would only trace that path.</p>
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<p>(The situation is worse on the 65816, because the length of
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certain instructions is determined by the values of the processor
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status flags.)</p>
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<p>Once the instructions and data are separated and formatted
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nicely, it's still up to a human to figure out what it all means.
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Comments and meaningful labels are needed to make sense of it.
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These should be added to the disassembly listing.</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/t0-sourcegen.png" alt="t0-sourcegen"/>
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</div>
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<div class="grid-item-text">
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<p>SourceGen performs the instruction tracing, and makes it easy
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to format operands and add labels and comments.
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When the disassembled code is ready, SourceGen can generate source code
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for a variety of modern cross-assemblers, and produce HTML listings
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with embedded graphic visualizations.</p>
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</div>
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</div>
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<p>Copyright 2021 faddenSoft</p>
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