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.
Since indexing was frequently carried out by altering the address part of an instruction (for instance, by adding one to access the following data item in a list), such instructions were always located in the second half-word, and the first half-word was filled with a NOP instruction.