this should really be done after other patches are in, to keep things sane, it also depends on bug #871 analysis and integration into sv_analysis.py
changed to match modes (being written) in https://libre-soc.org/openpower/sv/normal/ https://libre-soc.org/openpower/sv/ldst/ https://libre-soc.org/openpower/sv/cr_ops/
pack/unpack is now set as part of a "sub-mode" of svstep.