166 lines
8.4 KiB
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166 lines
8.4 KiB
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<title>How Overlays Work - Debugging with GDB</title>
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<link rel="up" href="Overlays.html#Overlays" title="Overlays">
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<link rel="next" href="Overlay-Commands.html#Overlay-Commands" title="Overlay Commands">
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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<div class="node">
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<a name="How-Overlays-Work"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Overlay-Commands.html#Overlay-Commands">Overlay Commands</a>,
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Up: <a rel="up" accesskey="u" href="Overlays.html#Overlays">Overlays</a>
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<hr>
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</div>
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<h3 class="section">14.1 How Overlays Work</h3>
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<p><a name="index-mapped-overlays-950"></a><a name="index-unmapped-overlays-951"></a><a name="index-load-address_002c-overlay_0027s-952"></a><a name="index-mapped-address-953"></a><a name="index-overlay-area-954"></a>
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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>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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<dfn>overlays</dfn>. 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>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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<!-- NB: In the below the mapped area's size is greater or equal to the -->
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<!-- size of all overlays. This is intentional to remind the developer -->
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<!-- that overlays don't necessarily need to be the same size. -->
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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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+-----------+
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<a name="A-code-overlay"></a>A code overlay
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</pre>
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<p>The diagram (see <a href="A-code-overlay.html#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>An overlay loaded into instruction memory and ready for use is called a
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<dfn>mapped</dfn> overlay; its <dfn>mapped address</dfn> 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 <dfn>unmapped</dfn>; its <dfn>load address</dfn>
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is its address in the larger memory. The mapped address is also called
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the <dfn>virtual memory address</dfn>, or <dfn>VMA</dfn>; the load address is also
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called the <dfn>load memory address</dfn>, or <dfn>LMA</dfn>.
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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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<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>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>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="../ld/Overlay-Description.html#Overlay-Description">Overlay Description</a>.
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<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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</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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<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>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>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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</ul>
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</body></html>
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