An overlay description provides an easy way to describe sections which are to be loaded as part of a single memory image but are to be run at the same memory address. At run time, some sort of overlay manager will copy the overlaid sections in and out of the runtime memory address as required, perhaps by simply manipulating addressing bits. This approach can be useful, for example, when a certain region of memory is faster than another.
Overlays are described using the OVERLAY command. The
OVERLAY command is used within a SECTIONS command, like an
output section description. The full syntax of the OVERLAY
command is as follows:
OVERLAY [start] : [NOCROSSREFS] [AT ( ldaddr )]
{
secname1
{
output-section-command
output-section-command
...
} [:phdr...] [=fill]
secname2
{
output-section-command
output-section-command
...
} [:phdr...] [=fill]
...
} [>region] [:phdr...] [=fill]
Everything is optional except OVERLAY (a keyword), and each
section must have a name (secname1 and secname2 above). The
section definitions within the OVERLAY construct are identical to
those within the general SECTIONS contruct (see SECTIONS),
except that no addresses and no memory regions may be defined for
sections within an OVERLAY.
The sections are all defined with the same starting address. The load
addresses of the sections are arranged such that they are consecutive in
memory starting at the load address used for the OVERLAY as a
whole (as with normal section definitions, the load address is optional,
and defaults to the start address; the start address is also optional,
and defaults to the current value of the location counter).
If the NOCROSSREFS keyword is used, and there any references
among the sections, the linker will report an error. Since the sections
all run at the same address, it normally does not make sense for one
section to refer directly to another. See NOCROSSREFS.
For each section within the OVERLAY, the linker automatically
provides two symbols. The symbol __load_start_secname is
defined as the starting load address of the section. The symbol
__load_stop_secname is defined as the final load address of
the section. Any characters within secname which are not legal
within C identifiers are removed. C (or assembler) code may use these
symbols to move the overlaid sections around as necessary.
At the end of the overlay, the value of the location counter is set to the start address of the overlay plus the size of the largest section.
Here is an example. Remember that this would appear inside a
SECTIONS construct.
OVERLAY 0x1000 : AT (0x4000)
{
.text0 { o1/*.o(.text) }
.text1 { o2/*.o(.text) }
}
This will define both .text0 and .text1 to start at
address 0x1000. .text0 will be loaded at address 0x4000, and
.text1 will be loaded immediately after .text0. The
following symbols will be defined if referenced: __load_start_text0,
__load_stop_text0, __load_start_text1,
__load_stop_text1.
C code to copy overlay .text1 into the overlay area might look
like the following.
extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
&__load_stop_text1 - &__load_start_text1);
Note that the OVERLAY command is just syntactic sugar, since
everything it does can be done using the more basic commands. The above
example could have been written identically as follows.
.text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
PROVIDE (__load_start_text0 = LOADADDR (.text0));
PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
PROVIDE (__load_start_text1 = LOADADDR (.text1));
PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));