where square brackets are zero if A is unsigned, and contains
the specified bits if signed.
-When the source A is larger width than B, truncation
-always occurs: it is a matter of how much truncation
-in each partition
+When the source A is scalar and is equal or larger than
+the destination it requires copying across multiple
+partitions:
partition: p p p (3 bits)
a : AAAA AAAA AAAA AAAA (16 bits)
b : B7B6 B5B4 B3B2 B1B0 (8 bits)
+
| partition | o3 | o2 | o1 | o0 |
| --------- | -- | -- | -- | -- |
| 000 | A7A6 | A5A4 | A3A2 | A1A0 |