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
+
+ partition: p p p (3 bits)
+ a : AAAA AAAA AAAA AAAA (16 bits)
+ b : B7B6 B5B4 B3B2 B1B0 (8 bits)
+