ARM Compiler toolchain Using the Linker - ARM Information Center

8.21 Automatic placement of __at sections
Using scatter files
The automatic placement of __at sections is enabled by default. This feature is controlled by the
linker command-line option, --autoat.
Note
You cannot use __at section placement with position independent execution regions.
When linking with the --autoat option, the __at sections are not placed by the scatter-loading
selectors. Instead, the linker places the __at section in a compatible region. If no compatible
region is found, the linker creates a load and execution region for the __at section.
All linker --autoat created execution regions have the UNINIT scatter-loading attribute. If you
require a ZI __at section to be zero-initialized then it must be placed within a compatible region.
A linker --autoat created execution region must have a base address that is at least 4
byte-aligned. The linker produces an error message if any region is incorrectly aligned.
A compatible region is one where:
• The __at address lies within the execution region base and limit, where limit is the base
address + maximum size of execution region. If no maximum size is set, the linker sets
the limit for placing __at sections as the current size of the execution region without __at
sections plus a constant, 10240 bytes.
• The execution region meets at least one of the following conditions:
— it has a selector that matches the __at section by the standard scatter-loading rules
— it has at least one section of the same type (RO, RW or ZI) as the __at section
— it does not have the EMPTY attribute.
Note
The linker considers an __at section with type RW compatible with RO.
The following example shows the sections .ARM.__at_0x0 type RO, .ARM.__at_0x2000 type RW,
.ARM.__at_0x4000 type ZI, and .ARM.__at_0x8000 type ZI:
Example 8-11 Automatic placement of __at sections
LR1 0x0
{
ER_RO 0x0 0x2000
{
*(+RO) ; .ARM.__at_0x0 lies within the bounds of ER_RO
}
ER_RW 0x2000 0x2000
{
*(+RW) ; .ARM.__at_0x2000 lies within the bounds of ER_RW
}
ER_ZI 0x4000 0x2000
{
*(+ZI) ; .ARM.__at_0x4000 lies within the bounds of ER_ZI
}
}
; the linker creates a load and execution region for the __at section
; .ARM.__at_0x8000 because it lies outside all candidate regions.
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