-10 bit mode may be expanded by 16 bit mode later, adding capabilities
-that do not fit in the extreme limited space.
+Note that illeg and nop are all zeros, including in the 16-bit mode.
+Given that C is allocated to OpenPOWER ISA Major opcodes EXT000 and
+EXT001 this ensures that in both 10-bit *and* 16-bit mode, a 16-bit
+run of all zeros is considered "illegal" whilst 0b0000.0000.1000.0000
+is "nop"