ARM Compiler toolchain Using the Linker - ARM Information Center

8.25 Placement of sections with overlays
Using scatter files
You can use the OVERLAY attribute in a scatter file to place multiple execution regions at the same
address. An overlay manager is required to make sure that only one execution region is
instantiated at a time. The ARM Compiler toolchain does not provide an overlay manager.
The following example shows the definition of a static section in RAM followed by a series of
overlays. Here, only one of these sections is instantiated at a time.
Example 8-16 Specifying a root region
EMB_APP 0x8000
{
.
.
STATIC_RAM 0x0 ; contains most of the RW and ZI code/data
{
* (+RW,+ZI)
}
OVERLAY_A_RAM 0x1000 OVERLAY ; start address of overlay...
{
module1.o (+RW,+ZI)
}
OVERLAY_B_RAM 0x1000 OVERLAY
{
module2.o (+RW,+ZI)
}
... ; rest of scatter-loading description...
}
A region marked as OVERLAY is not initialized by the C library at startup. The contents of the
memory used by the overlay region are the responsibility of an overlay manager. If the region
contains initialized data, use the NOCOMPRESS attribute to prevent RW data compression.
The linker defined symbols can be used to obtain the addresses required to copy the code and
data.
The OVERLAY attribute can be used on a single region that is not the same address as a different
region. Therefore, an overlay region can be used as a method to prevent the initialization of
particular regions by the C library startup code. As with any overlay region these must be
manually initialized in your code.
An overlay region can have a relative base. The behavior of an overlay region with a +offset
base address depends on the regions that precede it and the value of +offset. The linker places
consecutive +offset regions at the same base address if they have the same +offset value.
When a +offset execution region ER follows a contiguous overlapping block of overlay
execution regions the base address of ER is:
limit address of the overlapping block of overlay execution regions + offset
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