How Overlays Work

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How Overlays Work

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.
   One solution is to identify modules of your program which are
relatively independent, and need not call each other directly; call
these modules "overlays".  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.
   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

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 code overlay

   The diagram (*note A code overlay::) 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.
   An overlay loaded into instruction memory and ready for use is
called a "mapped" overlay; its "mapped address" is its address in the
instruction memory.  An overlay not present (or only partially present)
in instruction memory is called "unmapped"; its "load address" is its
address in the larger memory.  The mapped address is also called the
"virtual memory address", or "VMA"; the load address is also called the
"load memory address", or "LMA".
   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
   * 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
   * 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.
   * 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 *Note Overlay Description: (
   * 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.
   The overlay system described above is rather simple, and could be
improved in many ways:
   * 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.
   * 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.
   * 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.