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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/gdb/How-Overlays-Work.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
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diff --git a/share/doc/gdb/How-Overlays-Work.html b/share/doc/gdb/How-Overlays-Work.html new file mode 100644 index 0000000..e311ec8 --- /dev/null +++ b/share/doc/gdb/How-Overlays-Work.html @@ -0,0 +1,202 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 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 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Debugging with GDB: How Overlays Work</title> + +<meta name="description" content="Debugging with GDB: How Overlays Work"> +<meta name="keywords" content="Debugging with GDB: How Overlays Work"> +<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="Overlays.html#Overlays" rel="up" title="Overlays"> +<link href="Overlay-Commands.html#Overlay-Commands" rel="next" title="Overlay Commands"> +<link href="Overlays.html#Overlays" rel="previous" title="Overlays"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +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.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="How-Overlays-Work"></a> +<div class="header"> +<p> +Next: <a href="Overlay-Commands.html#Overlay-Commands" accesskey="n" rel="next">Overlay Commands</a>, Up: <a href="Overlays.html#Overlays" accesskey="u" rel="up">Overlays</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="How-Overlays-Work-1"></a> +<h3 class="section">14.1 How Overlays Work</h3> +<a name="index-mapped-overlays"></a> +<a name="index-unmapped-overlays"></a> +<a name="index-load-address_002c-overlay_0027s"></a> +<a name="index-mapped-address"></a> +<a name="index-overlay-area"></a> + +<p>Suppose you have a computer whose instruction address space is only 64 +kilobytes long, but which has much more memory which can be accessed by +other means: special instructions, segment registers, or memory +management hardware, for example. Suppose further that you want to +adapt a program which is larger than 64 kilobytes to run on this system. +</p> +<p>One solution is to identify modules of your program which are relatively +independent, and need not call each other directly; call these modules +<em>overlays</em>. Separate the overlays from the main program, and place +their machine code in the larger memory. Place your main program in +instruction memory, but leave at least enough space there to hold the +largest overlay as well. +</p> +<p>Now, to call a function located in an overlay, you must first copy that +overlay’s machine code from the large memory into the space set aside +for it in the instruction memory, and then jump to its entry point +there. +</p> + +<div class="smallexample"> +<pre class="smallexample"> Data Instruction Larger +Address Space Address Space Address Space ++-----------+ +-----------+ +-----------+ +| | | | | | ++-----------+ +-----------+ +-----------+<-- overlay 1 +| program | | main | .----| overlay 1 | load address +| variables | | program | | +-----------+ +| and heap | | | | | | ++-----------+ | | | +-----------+<-- overlay 2 +| | +-----------+ | | | load address ++-----------+ | | | .-| overlay 2 | + | | | | | | + mapped --->+-----------+ | | +-----------+ + address | | | | | | + | overlay | <-' | | | + | area | <---' +-----------+<-- overlay 3 + | | <---. | | load address + +-----------+ `--| overlay 3 | + | | | | + +-----------+ | | + +-----------+ + | | + +-----------+ + + <a name="A-code-overlay"></a>A code overlay +</pre></div> + +<p>The diagram (see <a href="#A-code-overlay">A code overlay</a>) shows a system with separate data +and instruction address spaces. To map an overlay, the program copies +its code from the larger address space to the instruction address space. +Since the overlays shown here all use the same mapped address, only one +may be mapped at a time. For a system with a single address space for +data and instructions, the diagram would be similar, except that the +program variables and heap would share an address space with the main +program and the overlay area. +</p> +<p>An overlay loaded into instruction memory and ready for use is called a +<em>mapped</em> overlay; its <em>mapped address</em> is its address in the +instruction memory. An overlay not present (or only partially present) +in instruction memory is called <em>unmapped</em>; its <em>load address</em> +is its address in the larger memory. The mapped address is also called +the <em>virtual memory address</em>, or <em>VMA</em>; the load address is also +called the <em>load memory address</em>, or <em>LMA</em>. +</p> +<p>Unfortunately, overlays are not a completely transparent way to adapt a +program to limited instruction memory. They introduce a new set of +global constraints you must keep in mind as you design your program: +</p> +<ul> +<li> Before calling or returning to a function in an overlay, your program +must make sure that overlay is actually mapped. Otherwise, the call or +return will transfer control to the right address, but in the wrong +overlay, and your program will probably crash. + +</li><li> If the process of mapping an overlay is expensive on your system, you +will need to choose your overlays carefully to minimize their effect on +your program’s performance. + +</li><li> The executable file you load onto your system must contain each +overlay’s instructions, appearing at the overlay’s load address, not its +mapped address. However, each overlay’s instructions must be relocated +and its symbols defined as if the overlay were at its mapped address. +You can use GNU linker scripts to specify different load and relocation +addresses for pieces of your program; see <a href="http://sourceware.org/binutils/docs/ld/Overlay-Description.html#Overlay-Description">Overlay Description</a> in <cite>Using ld: the GNU linker</cite>. + +</li><li> The procedure for loading executable files onto your system must be able +to load their contents into the larger address space as well as the +instruction and data spaces. + +</li></ul> + +<p>The overlay system described above is rather simple, and could be +improved in many ways: +</p> +<ul> +<li> If your system has suitable bank switch registers or memory management +hardware, you could use those facilities to make an overlay’s load area +contents simply appear at their mapped address in instruction space. +This would probably be faster than copying the overlay to its mapped +area in the usual way. + +</li><li> If your overlays are small enough, you could set aside more than one +overlay area, and have more than one overlay mapped at a time. + +</li><li> You can use overlays to manage data, as well as instructions. In +general, data overlays are even less transparent to your design than +code overlays: whereas code overlays only require care when you call or +return to functions, data overlays require care every time you access +the data. Also, if you change the contents of a data overlay, you +must copy its contents back out to its load address before you can copy a +different data overlay into the same mapped area. + +</li></ul> + + +<hr> +<div class="header"> +<p> +Next: <a href="Overlay-Commands.html#Overlay-Commands" accesskey="n" rel="next">Overlay Commands</a>, Up: <a href="Overlays.html#Overlays" accesskey="u" rel="up">Overlays</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> |