[avr-gcc-list] Re: Format of HEX file when program memory addresses exceed 64K

[David Brown]

On 06/05/2010 20:34, Graham Davies wrote:
> I've spent over an hour looking for this information on the Web and in
> the WinAVR documentation but I can't find it so I'm asking for some
> pointers.
>
> I'm using WinAVR-20081205 with AVRStudio 4.15, letting the GCC plug-in
> do all the heavy lifting (I'm not writing my own Makefiles or link
> scripts or anthing like that). The target is an ATmega1280, which is
> significant to my question because it has more than 64Kbytes of program
> memory. The output format is Intel HEX and I totally grok what I'm
> getting now as my program size is less than 64Kbytes. I even know where
> this is coming from. The generated Makefile uses 'avr-objcopy -O ihex'
> to get this.
>
> What I need to know is what the HEX file is going to look like when my
> program inevitably grows is size above 64Kbytes. What mechanism will
> 'objcopy' use to get above sixteen address bits? I need to know this
> exactly because I need to write a Flash loader to read such a file.
>
> I just need to know where to look for this information or what to Google
> for.
>
> Thanks, Graham.

Just a little note about versions - your WinAVR versions is over a year 
out of date, and AVRStudio is now at 4.18 (IIRC).  I'm not suggesting 
that you have to update - it's never a good idea to update tools in the 
middle of a project unless you have good reason.  But if you find you 
have problems, then give the newer versions a try.

In addition to other people's suggestions about where to find the 
relevant documentation, the obvious idea is to try it and see.  Make a 
few large dummy functions so that your code is big enough, and generate 
the hex file.

An example function would be:

volatile long int dummy1, dummy2, dummy3;
void __attribute__((used)) dummyFunc(void) {
        dummy1 = dummy2 + dummy3;
        dummy1 = dummy2 + dummy3;
        // Make lots of copy-and-paste copies
        dummy1 = dummy2 + dummy3;
}

The volatile and "used" attribute are so that the compiler can't be 
smart and remove the code.