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202 lines
9.6 KiB
HTML
202 lines
9.6 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>How Overlays Work (Debugging with GDB)</title>
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<meta name="description" content="How Overlays Work (Debugging with GDB)">
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<meta name="keywords" content="How Overlays Work (Debugging with GDB)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Overlays.html#Overlays" rel="up" title="Overlays">
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<link href="Overlay-Commands.html#Overlay-Commands" rel="next" title="Overlay Commands">
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<link href="Overlays.html#Overlays" rel="prev" title="Overlays">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="How-Overlays-Work"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="How-Overlays-Work-1"></a>
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<h3 class="section">14.1 How Overlays Work</h3>
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<a name="index-mapped-overlays"></a>
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<a name="index-unmapped-overlays"></a>
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<a name="index-load-address_002c-overlay_0027s"></a>
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<a name="index-mapped-address"></a>
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<a name="index-overlay-area"></a>
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<p>Suppose you have a computer whose instruction address space is only 64
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kilobytes long, but which has much more memory which can be accessed by
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other means: special instructions, segment registers, or memory
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management hardware, for example. Suppose further that you want to
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adapt a program which is larger than 64 kilobytes to run on this system.
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</p>
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<p>One solution is to identify modules of your program which are relatively
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independent, and need not call each other directly; call these modules
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<em>overlays</em>. Separate the overlays from the main program, and place
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their machine code in the larger memory. Place your main program in
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instruction memory, but leave at least enough space there to hold the
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largest overlay as well.
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</p>
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<p>Now, to call a function located in an overlay, you must first copy that
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overlay’s machine code from the large memory into the space set aside
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for it in the instruction memory, and then jump to its entry point
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there.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> Data Instruction Larger
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Address Space Address Space Address Space
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+-----------+ +-----------+ +-----------+
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| | | | | |
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+-----------+ +-----------+ +-----------+<-- overlay 1
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| program | | main | .----| overlay 1 | load address
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| variables | | program | | +-----------+
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| and heap | | | | | |
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+-----------+ | | | +-----------+<-- overlay 2
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| | +-----------+ | | | load address
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+-----------+ | | | .-| overlay 2 |
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mapped --->+-----------+ | | +-----------+
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address | | | | | |
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| overlay | <-' | | |
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| area | <---' +-----------+<-- overlay 3
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| | <---. | | load address
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+-----------+ `--| overlay 3 |
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+-----------+ | |
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+-----------+
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+-----------+
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<a name="A-code-overlay"></a>A code overlay
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</pre></div>
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<p>The diagram (see <a href="#A-code-overlay">A code overlay</a>) shows a system with separate data
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and instruction address spaces. To map an overlay, the program copies
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its code from the larger address space to the instruction address space.
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Since the overlays shown here all use the same mapped address, only one
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may be mapped at a time. For a system with a single address space for
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data and instructions, the diagram would be similar, except that the
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program variables and heap would share an address space with the main
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program and the overlay area.
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</p>
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<p>An overlay loaded into instruction memory and ready for use is called a
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<em>mapped</em> overlay; its <em>mapped address</em> is its address in the
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instruction memory. An overlay not present (or only partially present)
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in instruction memory is called <em>unmapped</em>; its <em>load address</em>
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is its address in the larger memory. The mapped address is also called
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the <em>virtual memory address</em>, or <em>VMA</em>; the load address is also
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called the <em>load memory address</em>, or <em>LMA</em>.
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</p>
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<p>Unfortunately, overlays are not a completely transparent way to adapt a
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program to limited instruction memory. They introduce a new set of
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global constraints you must keep in mind as you design your program:
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</p>
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<ul>
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<li> Before calling or returning to a function in an overlay, your program
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must make sure that overlay is actually mapped. Otherwise, the call or
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return will transfer control to the right address, but in the wrong
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overlay, and your program will probably crash.
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</li><li> If the process of mapping an overlay is expensive on your system, you
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will need to choose your overlays carefully to minimize their effect on
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your program’s performance.
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</li><li> The executable file you load onto your system must contain each
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overlay’s instructions, appearing at the overlay’s load address, not its
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mapped address. However, each overlay’s instructions must be relocated
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and its symbols defined as if the overlay were at its mapped address.
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You can use GNU linker scripts to specify different load and relocation
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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>.
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</li><li> The procedure for loading executable files onto your system must be able
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to load their contents into the larger address space as well as the
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instruction and data spaces.
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</li></ul>
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<p>The overlay system described above is rather simple, and could be
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improved in many ways:
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</p>
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<ul>
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<li> If your system has suitable bank switch registers or memory management
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hardware, you could use those facilities to make an overlay’s load area
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contents simply appear at their mapped address in instruction space.
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This would probably be faster than copying the overlay to its mapped
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area in the usual way.
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</li><li> If your overlays are small enough, you could set aside more than one
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overlay area, and have more than one overlay mapped at a time.
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</li><li> You can use overlays to manage data, as well as instructions. In
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general, data overlays are even less transparent to your design than
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code overlays: whereas code overlays only require care when you call or
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return to functions, data overlays require care every time you access
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the data. Also, if you change the contents of a data overlay, you
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must copy its contents back out to its load address before you can copy a
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different data overlay into the same mapped area.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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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>
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</div>
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</body>
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</html>
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