AVR Freaks

Can we assume you mean avr-gcc? Also by that large upper bound can we assume a 256KB micro with a 16KB bootloader section? (except 2560 does not have a 16K BOOTZSZ option does it?)

Anyway, if I build for a fairly popular 256 AVR (mega2560) and look at the .map file right near the top I see stuff like:

c:/program files (x86)/atmel/studio/7.0/toolchain/avr8/avr8-gnu-toolchain/bin/../lib/gcc/avr/5.4.0/avr6\libgcc.a(_exit.o)
                              C:/Program Files (x86)/Atmel/Studio/7.0/Packs/atmel/ATmega_DFP/1.6.364/gcc/dev/atmega2560/avr6/crtatmega2560.o (exit)

This reveals that this particular micro is AVR architecture "avr6" in the eyes of the GCC tools.

What's more if I look in:

c:/program files (x86)/atmel/studio/7.0/toolchain/avr8/avr8-gnu-toolchain/avr/lib/ldscripts

(which takes the root of part of the path above then adds "avr/lib/ldscripts" to it) I find:

These are the original "X Files" ;-)

These are actually "linker scripts" and for the different types of AVR each gives a different kind of memory layout. We already know from the .map file inspection that the one of interest in this case is the one for "avr6" so is the file called avr6.x. At the top it contains:

The single "avr6.x" actually caters for a number of AVRs so the bounds in this are set to "worst" case. The stuff near the top:

__TEXT_REGION_LENGTH__ = DEFINED(__TEXT_REGION_LENGTH__) ? __TEXT_REGION_LENGTH__ : 1024K;

says that in the absence of being told otherwise (by __TEXT_REGION_LENGTH__ being defined) it should assume a program area limit of 1MB (more than enough for any AVR in this group).

But this also reveals that if __TEXT_REGION_LENGTH__ is defined for the linker it will over-rule this generous limit.

Now in the exploration of the map file it also revealed that a "device pack" is being used for the build and it is in:

C:/Program Files (x86)/Atmel/Studio/7.0/Packs/atmel/ATmega_DFP/1.6.364/gcc/dev/atmega2560/avr6

within that one can find:

So this value of 256K (0x40000) is what is setting the limit for the text region size in ATmega2560. In fact you could see that in the .map file too:

Memory Configuration

Name             Origin             Length             Attributes
text             0x00000000         0x00040000         xr
data             0x00800200         0x00002000         rw !x
eeprom           0x00810000         0x00001000         rw !x
fuse             0x00820000         0x00000003         rw !x
lock             0x00830000         0x00000400         rw !x
signature        0x00840000         0x00000400         rw !x
user_signatures  0x00850000         0x00000400         rw !x

But note in the NM output that this symbol is shown as "W" which means it is only "weakly defined". So if you define it yourself this will over-ride the weak version being supplied form the CRT file.

So if I do this:

then after building, in the .map file I find:

So if I make that number very small and create a large chunk of flash data (a __flash array is a quick way to test this):

-Wl,--defsym=__TEXT_REGION_LENGTH__=0x100

also

#include <avr/io.h>

const __flash uint8_t data[1024];

int main(void)
{

leads to:

mcs7_test.elf section `.text' will not fit in region `text'collect2.exe(0,0): error: ld returned 1 exit status
		make: *** [mcs7_test.elf] Error 1

If I increase the definition of __TEXT_REGION_LENGTH__

Memory Configuration

Name             Origin             Length             Attributes
text             0x00000000         0x00001000         xr

then it builds OK because 1K+some change fits into 4K.

So ultimately I guess you want:

-Wl,--defsym=__TEXT_REGION_LENGTH__=0x3C000

which sets the text region length to 240K. If you build code larger than this you will get that error about "'.text' will not fit"