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6502bench/docs/sgtutorial/about-disasm.html

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Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
2021-06-08 00:14:16 +00:00
<!DOCTYPE html>
<html lang="en">
<head>
Add ${ROOT} substitution Doesn't really affect the pages on the web, but it's easier to preview them in the filesystem when linked to "../main.css" rather than "/main.css". The common portion of the <head> section is now in a separate "include" file. Added some indentation to masthead/topnav/sidenav so it looks nicer in the merged source listing.
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<!-- START: /head-incl.html -->
<!-- common head elements -->
Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
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<meta charset="utf-8"/>
<meta name="viewport" content="width=device-width, initial-scale=1"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"
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<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css"/>
Add ${ROOT} substitution Doesn't really affect the pages on the web, but it's easier to preview them in the filesystem when linked to "../main.css" rather than "/main.css". The common portion of the <head> section is now in a separate "include" file. Added some indentation to masthead/topnav/sidenav so it looks nicer in the merged source listing.
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<link rel="stylesheet" href="../main.css"/>
<!-- END: /head-incl.html -->
Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
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<title>About Disassembly - SourceGen Tutorial</title>
</head>
<body>
<div id="masthead">
<!-- START: /masthead-incl.html -->
Add ${ROOT} substitution Doesn't really affect the pages on the web, but it's easier to preview them in the filesystem when linked to "../main.css" rather than "/main.css". The common portion of the <head> section is now in a separate "include" file. Added some indentation to masthead/topnav/sidenav so it looks nicer in the merged source listing.
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<!--<div class="masthead-title" style="background-image: url('images/screenshot-mainwin.png');">-->
<div class="masthead-title">
6502bench
</div>
Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
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<!-- END: /masthead-incl.html -->
</div>
<div id="topnav">
<!-- START: /topnav-incl.html active:#topnav-sgtutorial -->
Add ${ROOT} substitution Doesn't really affect the pages on the web, but it's easier to preview them in the filesystem when linked to "../main.css" rather than "/main.css". The common portion of the <head> section is now in a separate "include" file. Added some indentation to masthead/topnav/sidenav so it looks nicer in the merged source listing.
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<!-- top navigation bar contents -->
<nav>
<a id="topnav-home" href=".././">HOME</a>
<a id="topnav-sgtutorial" class="active" href="../sgtutorial">SourceGen Tutorial</a>
<a id="topnav-menuicon" href="javascript:void(0);" class="icon" onclick="toggleSidenav()">
<i class="fa fa-bars"></i>
</a>
</nav>
<script>
// Sidenav toggle function.
//
// Use a jQuery function to toggle the sidenav bar. The initial state
// is undefined / inherited, so it will pop in and out as the screen
// resizes around the "large" breakpoint.
function toggleSidenav() {
$("#sidenav").toggle("fast");
}
</script>
Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
2021-06-08 00:14:16 +00:00
<!-- END: /topnav-incl.html -->
</div>
<div id="sidenav">
<!-- START: sidenav-incl.html active:#sidenav-about-disasm -->
Add ${ROOT} substitution Doesn't really affect the pages on the web, but it's easier to preview them in the filesystem when linked to "../main.css" rather than "/main.css". The common portion of the <head> section is now in a separate "include" file. Added some indentation to masthead/topnav/sidenav so it looks nicer in the merged source listing.
2021-06-28 20:37:26 +00:00
<!-- side navigation bar contents -->
Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
2021-06-08 00:14:16 +00:00
<ul>
Add ${ROOT} substitution Doesn't really affect the pages on the web, but it's easier to preview them in the filesystem when linked to "../main.css" rather than "/main.css". The common portion of the <head> section is now in a separate "include" file. Added some indentation to masthead/topnav/sidenav so it looks nicer in the merged source listing.
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<li id="sidenav-index"><a href="./">Introduction</a></li>
<li id="sidenav-about-disasm" class="active"><a href="about-disasm.html">About Disassembly</a></li>
<li id="sidenav-using-sourcegen"><a href="using-sourcegen.html">Using SourceGen</a>
<ul>
<li id="sidenav-moving-around"><a href="moving-around.html">Moving Around</a></li>
<li id="sidenav-simple-edits"><a href="simple-edits.html">Simple Edits</a></li>
<li id="sidenav-labels-symbols"><a href="labels-symbols.html">Labels &amp; Symbols</a></li>
<li id="sidenav-editing-data"><a href="editing-data.html">Editing Data Operands</a></li>
<li id="sidenav-generating-code"><a href="generating-code.html">Generating Code</a></li>
</ul></li>
<li id="sidenav-digging-deeper"><a href="digging-deeper.html">Digging Deeper</a>
<ul>
<li id="sidenav-string-formatting"><a href="string-formatting.html">String Formatting</a></li>
<li id="sidenav-local-variables"><a href="local-variables.html">Local Variables</a></li>
<li id="sidenav-inline-data"><a href="inline-data.html">Inline Data</a></li>
<li id="sidenav-odds-ends"><a href="odds-ends.html">Odds &amp; Ends</a></li>
</ul></li>
<li id="sidenav-advanced-topics"><a href="advanced-topics.html">Advanced Topics</a>
<ul>
<li id="sidenav-address-tables"><a href="address-tables.html">Address Tables</a></li>
<li id="sidenav-extension-scripts"><a href="extension-scripts.html">Extension Scripts</a></li>
<li id="sidenav-visualizations"><a href="visualizations.html">Visualizations</a></li>
</ul></li>
</ul>
Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
2021-06-08 00:14:16 +00:00
<!-- END: sidenav-incl.html -->
</div>
<div id="main">
<h2>About Disassembly</h2>
<div class="grid-container">
<div class="grid-item-text">
<p>Well-written assembly-language source code has meaningful
comments and labels, so that humans can read and understand it.
For example:</p>
<pre>
.org $2000
sec ;set carry
ror A ;shift into high bit
bmi CopyData ;branch always
.asciiz "first string"
.asciiz "another string"
.asciiz "string the third"
.asciiz "last string"
CopyData lda #&lt;addrs ;get pointer into
sta ptr ; address table
lda #&gt;addrs
sta ptr+1
</pre>
<p>Computers operate at a much lower level, so a piece of software
called an <i>assembler</i> is used to convert the source code to
object code that the CPU can execute.
Object code looks more like this:</p>
<pre>
38 6a 30 39 66 69 72 73 74 20 73 74 72 69 6e 67
00 61 6e 6f 74 68 65 72 20 73 74 72 69 6e 67 00
73 74 72 69 6e 67 20 74 68 65 20 74 68 69 72 64
00 6c 61 73 74 20 73 74 72 69 6e 67 00 a9 63 85
02 a9 20 85 03
</pre>
<p>This arrangement works perfectly well until somebody needs to
modify the software and nobody can find the original sources.
<i>Disassembly</i> is the act of taking a raw hex
dump and converting it to source code.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t0-bad-disasm.png" alt="t0-bad-disasm"/>
</div>
<div class="grid-item-text">
<p>Disassembling a blob of data can be tricky. A simple
disassembler can format instructions, but can't generally tell
the difference between instructions and data. Many 6502 programs
intermix code and data freely, so simply dumping everything as
an instruction stream can result in sections with nonsensical output.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-text">
<p>One way to separate code from data is to try to execute all
possible data paths. There are a number of reasons why it's difficult
or impossible to do this perfectly, but you can get pretty good
results by identifying execution entry points and just walking through
the code. When a conditional branch is encountered, both paths are
traversed. When all code has been traced, every byte that hasn't
been visited is either
data used by the program, or dead space not used by anything.</p>
<p>The process can be improved by keeping track of the flags in the
6502 status register. For example, in the code fragment shown
earlier, <code>BMI</code> conditional branch instruction is used.
A simple tracing algorithm would both follow the branch and fall
through to the following instruction. However, the code that precedes
the <code>BMI</code> ensures that the branch is always taken, so a
clever disassembler would only trace that path.</p>
<p>(The situation is worse on the 65816, because the length of
certain instructions is determined by the values of the processor
status flags.)</p>
<p>Once the instructions and data are separated and formatted
nicely, it's still up to a human to figure out what it all means.
Comments and meaningful labels are needed to make sense of it.
These should be added to the disassembly listing.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t0-sourcegen.png" alt="t0-sourcegen"/>
</div>
<div class="grid-item-text">
<p>SourceGen performs the instruction tracing, and makes it easy
to format operands and add labels and comments.
When the disassembled code is ready, SourceGen can generate source code
for a variety of modern cross-assemblers, and produce HTML listings
with embedded graphic visualizations.</p>
</div>
</div>
</div> <!-- grid-container -->
<div id="prevnext">
<a href="using-sourcegen.html" class="btn-next">Next &raquo;</a>
</div>
<div id="footer">
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Add ${ROOT} substitution Doesn't really affect the pages on the web, but it's easier to preview them in the filesystem when linked to "../main.css" rather than "/main.css". The common portion of the <head> section is now in a separate "include" file. Added some indentation to masthead/topnav/sidenav so it looks nicer in the merged source listing.
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<hr/>
<p>Copyright 2021 faddenSoft</p>
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Execute scripts This change applies the substitution scripts on the HTML files, replacing away the jQuery load() calls with the actual file contents, and setting the correct URLs to the prev/next buttons.
2021-06-08 00:14:16 +00:00
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