SPRUI03E June 2015 – January 2023
It is sometimes useful to load code in one location in memory and run it in another. The linker provides the capability to specify separate load and run allocations for a section. The burden of actually copying the code from the load space to the run space is left to you.
A copy function must be executed before the real function can be executed in its run space. To facilitate this copy function, the assembler provides the .label directive, which allows you to define a load-time address. These load-time addresses can then be used to determine the start address and size of the code to be copied. However, this mechanism will not work if the code contains a call that requires a trampoline to reach its called function. This is because the trampoline code is generated at link time, after the load-time addresses associated with the .label directive have been defined. If the linker detects the definition of a .label symbol in an input section that contains a trampoline call, then a warning is generated.
To solve this problem, you can use the START(), END(), and SIZE() operators (see Section 9.6.10.7). These operators allow you to define symbols to represent the load-time start address and size inside the linker command file. These symbols can be referenced by the copy code, and their values are not resolved until link time, after the trampoline sections have been allocated.
Here is an example of how you could use the START() and SIZE() operators in association with an output section to copy the trampoline code section along with the code containing the calls that need trampolines:
SECTIONS
{ .foo : load = ROM, run = RAM, start(foo_start), size(foo_size)
{ x.obj(.text) }
.text: {} > ROM
.far : { --library=rts.lib(.text) } > FAR_MEM
}A function in x.c.obj contains an run-time-support call. The run-time-support library is placed in far memory and so the call is out-of-range. A trampoline section will be added to the .foo output section by the linker. The copy code can refer to the symbols foo_start and foo_size as parameters for the load start address and size of the entire .foo output section. This allows the copy code to copy the trampoline section along with the original x.c.obj code in .text from its load space to its run space.
See Section 9.7.1 for information about referring to linker symbols in C/C++ code.