AVR Freaks

In the specific case of the ATmega8515, it is possible to tack on up to 64K of additional external SRAM via '8515's integrated EMI (External Memory Interface). When the EMI is enabled and configured, it will allow that additional external SRAM to be accessed transparently, and the CPU will use the same instructions as though it was accessing the built-in SRAM.

The compiler and linker don't actually have any way of knowing whether or not you've added an external SRAM chip, or whether or not the EMI has been enabled. So it sort of makes sense that it wouldn't say anything about possibly legitimate uses of external memory.

On the other hand, there are lots of other AVRs which don't have anything equivalent to an EMI, and it would make sense for the compiler/linker to complain if your program's data usage exceeded those chips' internal SRAM size.

There may also be a case to be made that the linker should be modified to allow you to configure your project options and inform the linker, on a project-by-project basis, whether or not the EMI is actually being used, and the size of the external SRAM if it is being used. That way, the linker would be able to make sensible decisions about data memory warnings.

However useful that may be, nobody has seen fit to submit patches to give the linker such a capability (it is open source, after all, so if enough users REALLY wanted it, they could band together to create it), so it is left up to the operator to inspect the compiler's output more carefully to catch such potential errors.