on whether A is signed or unsigned, sign-extends or zero-extends
*on a per-partition basis*.
+# Scalar source
+
When the source A is scalar and is equal or larger than
the destination it requires copying across multiple
partitions:
| 000 | A7A6 | A5A4 | A3A2 | A1A0 |
| 001 | A5A4 | A3A2 | A1A0 | A1A0 |
| 010 | A3A2 | A1A0 | A3A2 | A1A0 |
+| 011 | A3A2 | A1A0 | A1A0 | A1A0 |
| 100 | A1A0 | A5A4 | A3A2 | A1A0 |
+| 101 | A1A0 | A3A2 | A1A0 | A1A0 |
+| 110 | A1A0 | A1A0 | A3A2 | A1A0 |
| 111 | A1A0 | A1A0 | A1A0 | A1A0 |
+
+When the partitions are all open (1x) only the bits that will fit across
+the whole of the target are copied. In this example, B is 8 bits so only
+8 bits of A are copied.
+
+When the partitions are all closed (4x SIMD) each partition of B is
+2 bits wide, therefore only the *first two* bits of A are copied into
+*each* of the four 2-bit partitions in B.