through to SVi=4 selects SVSHAPE3.
* When `SVi` is 5, `SVSTATE.srcstep` is returned.
* When `SVi` is 6, `SVSTATE.dststep` is returned.
+* When `SVi` is 7, `SVSTATE.ssubstep` is returned.
+* When `SVi` is 8, `SVSTATE.dsubstep` is returned.
* When `SVi` is 0b1100 pack/unpack in SVSTATE is cleared
* When `SVi` is 0b1101 pack in SVSTATE is set, unpack is cleared
* When `SVi` is 0b1110 unpack in SVSTATE is set, pack is cleared