mirror of
https://github.com/fadden/6502bench.git
synced 2024-11-19 21:31:30 +00:00
909bdb1fa4
Renamed from "*-incl.html" to "incl-*.html" so they sort together. Moved <div> for all but incl-head inside include file. Overall this shouldn't do anything but move the magic comments around inside the HTML files.
207 lines
8.2 KiB
HTML
207 lines
8.2 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="en">
|
|
<head>
|
|
<!-- START: /incl-head.html -->
|
|
<!-- common head elements -->
|
|
<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"
|
|
integrity="sha384-vtXRMe3mGCbOeY7l30aIg8H9p3GdeSe4IFlP6G8JMa7o7lXvnz3GFKzPxzJdPfGK" crossorigin="anonymous"></script>
|
|
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css"/>
|
|
<link rel="stylesheet" href="../main.css"/>
|
|
<!-- END: /incl-head.html -->
|
|
|
|
<title>About Disassembly - SourceGen Tutorial</title>
|
|
</head>
|
|
|
|
<body>
|
|
|
|
<!-- START: /incl-masthead.html -->
|
|
<div id="masthead">
|
|
<!--<div class="masthead-title" style="background-image: url('images/screenshot-mainwin.png');">-->
|
|
<div class="masthead-title">
|
|
6502bench
|
|
</div>
|
|
</div>
|
|
<!-- END: /incl-masthead.html -->
|
|
|
|
<!-- START: /incl-topnav.html active:#topnav-sgtutorial -->
|
|
<div id="topnav">
|
|
<!-- 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>
|
|
</div>
|
|
<!-- END: /incl-topnav.html -->
|
|
|
|
<!-- START: incl-sidenav.html active:#sidenav-about-disasm -->
|
|
<div id="sidenav">
|
|
<!-- side navigation bar contents -->
|
|
<ul>
|
|
<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 & 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 & 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>
|
|
</div>
|
|
<!-- END: incl-sidenav.html -->
|
|
|
|
<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 #<addrs ;get pointer into
|
|
sta ptr ; address table
|
|
lda #>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 »</a>
|
|
</div>
|
|
|
|
<!-- START: /incl-footer.html -->
|
|
<div id="footer">
|
|
<hr/>
|
|
<p>Copyright 2021 faddenSoft</p>
|
|
<!-- <p id="screen-size"></p>
|
|
<script>
|
|
var w = window.innerWidth;
|
|
var h = window.innerHeight;
|
|
var x = document.getElementById("screen-size");
|
|
x.innerHTML = "DEBUG: initial window size " + w + "x" + h;
|
|
</script> -->
|
|
</div>
|
|
<!-- END: /incl-footer.html -->
|
|
|
|
</body>
|
|
</html>
|