the encoding embeds predication into the swizzle as well as constants 1/1.0 and 0/0.0
-As a Scalar instruction, mv.swiz and fmv.swiz operate on four 32-bit
+**As a Scalar instruction**
+
+When part of the Scalar Power ISA (not SVP64 Vectorised)
+mv.swiz and fmv.swiz operate on four 32-bit
quantities, reducing this instruction to 2-in, 2-out pairs of 64-bit
registers:
| Z | RA+1 | RT+1 | lo-half |
| W | RA+1 | RT+1 | hi-half |
-When RA=RT (in-place swizzle) any
+When RA=RT (in-place swizzle) any portion of RT not covered by
+the Swizzle is unmodified. For example a Swizzle of "..XY"
+will copy the contents RA+1 into RT but leave RT+1 unmodified.
+
+When RA!=RT any part of RT or RT+1 not set as a destination by
+the Swizzle will be set to zero. A Swizzle of "..XY" would
+copy the contents RA+1 into RT, but set RT+1 to zero.
+
+When (RA,RA+1) overlaps with (RT,RT+1) and RA!=RT, the results
+are `UNDEFINED`.
+
+**SVP64 Vectorised**
+
+When Vectorised, TODO
# Format