| ELWIDTH_SRC | `17:18` | Element Width for Source |
| MODE | `19:23` | see [[discussion]] |
-Note that for 1S2P the EXTRA2 dest and src names are switched.
+Note that for 1S2P the EXTRA2 dest and src names are switched (Rsrc_EXTRA2 is in bits 8:9, Rdest1_EXTRA2 in 10:11)
+
+Note also that LD with update indexed, which takes 2 src and 2 dest (e.g. `lhaux RT,RA,RB`), does not have room for 4 registers and also Twin Prefication. therefore these are treated as RM-2P-2S1D and the src spec for RA is also used for the same RA as a dest.
## R\*_EXTRA2 and R\*_EXTRA3 Encoding