9.10.7 fillupper

This attribute can be used to specify the upper byte of a variable stored into a space(prog) section.

For example:

int foo[26] __attribute__((space(prog),fillupper(0x23))) = { 0xDEAD };

will fill the upper bytes of array foo with 0x23, instead of 0x00. foo[0] will still be initialized to 0xDEAD.

The command line option -mfillupper=0x23 will perform the same function.