ARM Compiler toolchain Using the Linker - ARM Information Center

8.11 Placing functions and data at specific addresses
Using scatter files
Normally, the compiler produces RO, RW and ZI sections from a single source file. These
regions contain all the code and data from the source file. To place a single function or data item
at a fixed address, you must enable the linker to process the function or data separately from the
rest of the input files.
The linker has two methods that enable you to place a section at a specific address:
• You can create a scatter file that defines an execution region at the required address with
a section description that selects only one section.
• For a specially-named section the linker can get the placement address from the section
name. These specially-named sections are called __at sections.
To place a function or variable at a specific address it must be placed in its own section. There
are several ways to do this:
• Place the function or data item in its own source file.
• Use __attribute__((at(address))) to place variables in a separate section at a specific
address.
• Use __attribute__((section("name"))) to place functions and variables in a named
section.
• Use the AREA directive from assembly language. In assembly code, the smallest locatable
unit is an AREA.
• Use the --split_sections compiler option to generate one ELF section for each function
in the source file.
This option results in a small increase in code size for some functions because it reduces
the potential for sharing addresses, data, and string literals between functions. However,
this can help to reduce the final image size overall by enabling the linker to remove unused
functions when you specify armlink --remove.
8.11.1 Example of placing a variable at a specific address without scatter-loading
This example shows how to modify your source code to place code and data at specific
addresses, and does not require a scatter file:
1. Create the source file main.c containing the following code:
#include
extern int sqr(int n1);
int gSquared __attribute__((at(0x5000))); // Place at 0x5000
int main()
{
gSquared=sqr(3);
printf("Value squared is: %d\n", gSquared);
}
2. Create the source file function.c containing the following code:
int sqr(int n1)
{
return n1*n1;
}
3. Compile and link the sources:
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