You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
222 lines
9.0 KiB
HTML
222 lines
9.0 KiB
HTML
4 years ago
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with the
|
||
|
|
Invariant Sections being "Free Software" and "Free Software Needs
|
||
|
|
Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
|
||
|
|
and with the Back-Cover Texts as in (a) below.
|
||
|
|
|
||
|
|
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
|
||
|
|
this GNU Manual. Buying copies from GNU Press supports the FSF in
|
||
|
|
developing GNU and promoting software freedom." -->
|
||
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>General Bytecode Design (Debugging with GDB)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="General Bytecode Design (Debugging with GDB)">
|
||
|
|
<meta name="keywords" content="General Bytecode Design (Debugging with GDB)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<link href="index.html#Top" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Agent-Expressions.html#Agent-Expressions" rel="up" title="Agent Expressions">
|
||
|
|
<link href="Bytecode-Descriptions.html#Bytecode-Descriptions" rel="next" title="Bytecode Descriptions">
|
||
|
|
<link href="Agent-Expressions.html#Agent-Expressions" rel="prev" title="Agent Expressions">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
div.smalllisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style: oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smallexample {font-size: smaller}
|
||
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smalllisp {font-size: smaller}
|
||
|
|
span.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<a name="General-Bytecode-Design"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="n" rel="next">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<a name="General-Bytecode-Design-1"></a>
|
||
|
|
<h3 class="section">F.1 General Bytecode Design</h3>
|
||
|
|
|
||
|
|
<p>The agent represents bytecode expressions as an array of bytes. Each
|
||
|
|
instruction is one byte long (thus the term <em>bytecode</em>). Some
|
||
|
|
instructions are followed by operand bytes; for example, the <code>goto</code>
|
||
|
|
instruction is followed by a destination for the jump.
|
||
|
|
</p>
|
||
|
|
<p>The bytecode interpreter is a stack-based machine; most instructions pop
|
||
|
|
their operands off the stack, perform some operation, and push the
|
||
|
|
result back on the stack for the next instruction to consume. Each
|
||
|
|
element of the stack may contain either a integer or a floating point
|
||
|
|
value; these values are as many bits wide as the largest integer that
|
||
|
|
can be directly manipulated in the source language. Stack elements
|
||
|
|
carry no record of their type; bytecode could push a value as an
|
||
|
|
integer, then pop it as a floating point value. However, GDB will not
|
||
|
|
generate code which does this. In C, one might define the type of a
|
||
|
|
stack element as follows:
|
||
|
|
</p><div class="example">
|
||
|
|
<pre class="example">union agent_val {
|
||
|
|
LONGEST l;
|
||
|
|
DOUBLEST d;
|
||
|
|
};
|
||
|
|
</pre></div>
|
||
|
|
<p>where <code>LONGEST</code> and <code>DOUBLEST</code> are <code>typedef</code> names for
|
||
|
|
the largest integer and floating point types on the machine.
|
||
|
|
</p>
|
||
|
|
<p>By the time the bytecode interpreter reaches the end of the expression,
|
||
|
|
the value of the expression should be the only value left on the stack.
|
||
|
|
For tracing applications, <code>trace</code> bytecodes in the expression will
|
||
|
|
have recorded the necessary data, and the value on the stack may be
|
||
|
|
discarded. For other applications, like conditional breakpoints, the
|
||
|
|
value may be useful.
|
||
|
|
</p>
|
||
|
|
<p>Separate from the stack, the interpreter has two registers:
|
||
|
|
</p><dl compact="compact">
|
||
|
|
<dt><code>pc</code></dt>
|
||
|
|
<dd><p>The address of the next bytecode to execute.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>start</code></dt>
|
||
|
|
<dd><p>The address of the start of the bytecode expression, necessary for
|
||
|
|
interpreting the <code>goto</code> and <code>if_goto</code> instructions.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
<p>Neither of these registers is directly visible to the bytecode language
|
||
|
|
itself, but they are useful for defining the meanings of the bytecode
|
||
|
|
operations.
|
||
|
|
</p>
|
||
|
|
<p>There are no instructions to perform side effects on the running
|
||
|
|
program, or call the program’s functions; we assume that these
|
||
|
|
expressions are only used for unobtrusive debugging, not for patching
|
||
|
|
the running code.
|
||
|
|
</p>
|
||
|
|
<p>Most bytecode instructions do not distinguish between the various sizes
|
||
|
|
of values, and operate on full-width values; the upper bits of the
|
||
|
|
values are simply ignored, since they do not usually make a difference
|
||
|
|
to the value computed. The exceptions to this rule are:
|
||
|
|
</p><dl compact="compact">
|
||
|
|
<dt>memory reference instructions (<code>ref</code><var>n</var>)</dt>
|
||
|
|
<dd><p>There are distinct instructions to fetch different word sizes from
|
||
|
|
memory. Once on the stack, however, the values are treated as full-size
|
||
|
|
integers. They may need to be sign-extended; the <code>ext</code> instruction
|
||
|
|
exists for this purpose.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt>the sign-extension instruction (<code>ext</code> <var>n</var>)</dt>
|
||
|
|
<dd><p>These clearly need to know which portion of their operand is to be
|
||
|
|
extended to occupy the full length of the word.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<p>If the interpreter is unable to evaluate an expression completely for
|
||
|
|
some reason (a memory location is inaccessible, or a divisor is zero,
|
||
|
|
for example), we say that interpretation “terminates with an error”.
|
||
|
|
This means that the problem is reported back to the interpreter’s caller
|
||
|
|
in some helpful way. In general, code using agent expressions should
|
||
|
|
assume that they may attempt to divide by zero, fetch arbitrary memory
|
||
|
|
locations, and misbehave in other ways.
|
||
|
|
</p>
|
||
|
|
<p>Even complicated C expressions compile to a few bytecode instructions;
|
||
|
|
for example, the expression <code>x + y * z</code> would typically produce
|
||
|
|
code like the following, assuming that <code>x</code> and <code>y</code> live in
|
||
|
|
registers, and <code>z</code> is a global variable holding a 32-bit
|
||
|
|
<code>int</code>:
|
||
|
|
</p><div class="example">
|
||
|
|
<pre class="example">reg 1
|
||
|
|
reg 2
|
||
|
|
const32 <i>address of z</i>
|
||
|
|
ref32
|
||
|
|
ext 32
|
||
|
|
mul
|
||
|
|
add
|
||
|
|
end
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>In detail, these mean:
|
||
|
|
</p><dl compact="compact">
|
||
|
|
<dt><code>reg 1</code></dt>
|
||
|
|
<dd><p>Push the value of register 1 (presumably holding <code>x</code>) onto the
|
||
|
|
stack.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>reg 2</code></dt>
|
||
|
|
<dd><p>Push the value of register 2 (holding <code>y</code>).
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>const32 <i>address of z</i></code></dt>
|
||
|
|
<dd><p>Push the address of <code>z</code> onto the stack.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>ref32</code></dt>
|
||
|
|
<dd><p>Fetch a 32-bit word from the address at the top of the stack; replace
|
||
|
|
the address on the stack with the value. Thus, we replace the address
|
||
|
|
of <code>z</code> with <code>z</code>’s value.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>ext 32</code></dt>
|
||
|
|
<dd><p>Sign-extend the value on the top of the stack from 32 bits to full
|
||
|
|
length. This is necessary because <code>z</code> is a signed integer.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>mul</code></dt>
|
||
|
|
<dd><p>Pop the top two numbers on the stack, multiply them, and push their
|
||
|
|
product. Now the top of the stack contains the value of the expression
|
||
|
|
<code>y * z</code>.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>add</code></dt>
|
||
|
|
<dd><p>Pop the top two numbers, add them, and push the sum. Now the top of the
|
||
|
|
stack contains the value of <code>x + y * z</code>.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>end</code></dt>
|
||
|
|
<dd><p>Stop executing; the value left on the stack top is the value to be
|
||
|
|
recorded.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Bytecode-Descriptions.html#Bytecode-Descriptions" accesskey="n" rel="next">Bytecode Descriptions</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|