A little though gives a useful workaround: two modes,
controlled by a single bit in `RM.EXTRA`, determine whether the 5th
register is set to RC or whether to RT+VL. This then leaves only
-4 registers to qualify as scalar/vector, and this can use four
-EXTRA2 designators which fits into the available space.
+4 registers to qualify as scalar/vector, which can use four
+EXTRA2 designators and fits into the available 9-bit space.
RS=RT+VL Mode: