Since indexing was commonly done by modifying the address part of an instruction (say, by adding one to access the next data item in a list), such instructions always had to be in the second half-word, and the first half-word was padded with a NOP instruction.
Indexing operations often involved modifying the address component of an instruction (by adding one, for instance, to access the following data element in a sequence). Consequently, these instructions were invariably situated in the second half-word, while the first half-word was supplemented with a NOP instruction to ensure proper alignment.