For this code:
Boot GETA $0,DTrap %set dynamic- and forced-trap handler
the GAS MMIX port complains as follows:
loc-6.s:4: Error: LOC expression stepping backwards is not supported
It'd reasonable to special-case this value (or any value with the low 56 bits
zero) as pointing to a special unnamed segment, and not stepping relative within
the current segment. Compare to special-casing of constants for Data_Segment,
Pool_Segment and Stack_Segment.
It'd also be reasonable to treat absolute LOC arguments as unsigned and only
check for "stepping backwards" when there's previous contents.
Committed patch in <http://sourceware.org/ml/binutils/2008-06/msg00138.html>.