* Source:: Examining source files
* Data:: Examining data
* Tracepoints:: Debugging remote targets non-intrusively
+* Overlays:: Debugging programs that use overlays
* Languages:: Using @value{GDBN} with different languages
* Source:: Examining source files
* Data:: Examining data
* Tracepoints:: Debugging remote targets non-intrusively
+* Overlays:: Debugging programs that use overlays
* Languages:: Using @value{GDBN} with different languages
> end
@end smallexample
+@node Overlays
+@chapter Debugging Programs That Use Overlays
+@cindex overlays
+@kindex overlay
+@kindex ovly @r{(@code{ovly})}
+
+If your program is too large to fit completely in your target system's
+memory, you can sometimes use @dfn{overlays} to work around this
+problem. @value{GDBN} provides some support for programs that use
+overlays.
+
+@menu
+* How Overlays Work:: A general explanation of overlays.
+@end menu
+
+@node How Overlays Work
+@section How Overlays Work
+
+Suppose you have a computer whose instruction address space is only 64
+kilobytes long, but which has much more memory that can be accessed by
+other means: special instructions, segment registers, or bank switch
+registers, 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
+@dfn{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 code.
+
+An overlay loaded into instruction memory and ready for use is called a
+@dfn{mapped} overlay; its @dfn{mapped address} is its address in the
+instruction memory. An overlay not present (or only partial present) in
+instruction memory is called @dfn{unmapped}; its @dfn{load address} is
+its address in the larger memory.
+
+Overlays introduce a number of complications:
+@itemize bullet
+
+@item
+Since you cannot have all your overlays loaded at once,
+
+
+This will probably overwrite the
+previous overlay loaded in that space; you will need to re-load
+
+
+
+
+modify your main program such that, before it calls a function located
+in an overlay, it first copies that overlay from the large memory into
+the instruction memory.
+
+
+
+
+copy the overlay into
+place
+
+
+want to run which is larger than 64 kilobytes.
+
+
+
+ a 64kb instruction address
+space.
+
+to run on a system which has a small
+amount of memory available from which it can execute machine
+instructions, and a large amount of separate data memory,
+
+
+
@node Languages
@chapter Using @value{GDBN} with Different Languages
@cindex languages
@cindex dynamic linking
@item add-symbol-file @var{filename} @var{address}
@itemx add-symbol-file @var{filename} @var{address} @r{[} -readnow @r{]} @r{[} -mapped @r{]}
-@itemx add-symbol-file @var{filename} @r{-s}@var{section} @var{address}
+@itemx add-symbol-file @var{filename} @r{-s}@var{section} @var{address} @dots{}
The @code{add-symbol-file} command reads additional symbol table
information from the file @var{filename}. You would use this command
when @var{filename} has been dynamically loaded (by some other means)
thus read keeps adding to the old. To discard all old symbol data
instead, use the @code{symbol-file} command without any arguments.
+@cindex relocatable object files, reading symbols from
+@cindex object files, relocatable, reading symbols from
+@cindex reading symbols from relocatable object files
+@cindex symbols, reading from relocatable object files
+@cindex @file{.o} files, reading symbols from
+Although @var{filename} is typically a shared library file, an
+executable file, or some other object file which has been fully
+relocated for loading into a process, you can also load symbolic
+information from relocatable @file{.o} files, as long as:
+
+@itemize @bullet
+@item
+the file's symbolic information refers only to linker symbols defined in
+that file, not to symbols defined by other object files,
+@item
+every section the file's symbolic information refers to has actually
+been loaded into the inferior, as it appears in the file, and
+@item
+you can determine the address at which every section was loaded, and
+provide these to the @code{add-symbol-file} command.
+@end itemize
+
+@noindent
+Some embedded operating systems, like Sun Chorus and VxWorks, can load
+relocatable files into an already running program; such systems
+typically make the requirements above easy to meet. However, it's
+important to recognize that many native systems use complex link
+procedures (@code{.linkonce} section factoring and C++ constructor table
+assembly, for example) that make the requirements difficult to meet. In
+general, one cannot assume that using @code{add-symbol-file} to read a
+relocatable object file's symbolic information will have the same effect
+as linking the relocatable object file into the program in the normal
+way.
+
@code{add-symbol-file} does not repeat if you press @key{RET} after using it.
You can use the @samp{-mapped} and @samp{-readnow} options just as with
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