Normally, specifying a fill value for a MEMORY range creates initialized data sections to cover any previously uninitialized areas of memory. To generate ECC data for an entire memory range, the linker either needs to have initialized data in the entire range, or needs to know what value uninitialized memory areas will have at run time.

In cases where you want to generate ECC for an entire memory range, but do not want to initialize the entire range by specifying a fill value, you can use the "vfill" specifier instead of a "fill" specifier to virtually fill the range:

MEMORY {
   FLASH : origin=0x0000  length=0x4000  vfill=0xffffffff
}

The vfill specifier is functionally equivalent to omitting a fill specifier, except that it allows ECC data to be generated for areas of the input memory range that remain uninitialized. This has the benefit of reducing the size of the resulting object file.

The vfill specifier has no effect other than in ECC data generation. It cannot be specified along with a fill specifier, since that would introduce ambiguity.

If fill is specified in the ECC specifier, but vfill is not specified, vfill defaults to 0xff.