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| <h3 class="section">14.1 How Overlays Work</h3> |
| |
| <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> |
| 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>One solution is to identify modules of your program which are relatively |
| independent, and need not call each other directly; call these modules |
| <dfn>overlays</dfn>. 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>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. |
| |
| <!-- NB: In the below the mapped area's size is greater or equal to the --> |
| <!-- size of all overlays. This is intentional to remind the developer --> |
| <!-- that overlays don't necessarily need to be the same size. --> |
| <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> |
| <p>The diagram (see <a href="A-code-overlay.html#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>An overlay loaded into instruction memory and ready for use is called a |
| <dfn>mapped</dfn> overlay; its <dfn>mapped address</dfn> is its address in the |
| instruction memory. An overlay not present (or only partially present) |
| in instruction memory is called <dfn>unmapped</dfn>; its <dfn>load address</dfn> |
| is its address in the larger memory. The mapped address is also called |
| the <dfn>virtual memory address</dfn>, or <dfn>VMA</dfn>; the load address is also |
| called the <dfn>load memory address</dfn>, or <dfn>LMA</dfn>. |
| |
| <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: |
| |
| <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>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>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="../ld/Overlay-Description.html#Overlay-Description">Overlay Description</a>. |
| |
| <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. |
| |
| </ul> |
| |
| <p>The overlay system described above is rather simple, and could be |
| improved in many ways: |
| |
| <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>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>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. |
| |
| </ul> |
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