The instructions that transmit data between the index registers and the memory work only on the left half (address) portion of memory. These instructions are LDXn (load index n from address of storage word). And STXn (store the contents of index n in address of storage word). The effective address of both of these instructions includes modification by index registers. A corresponding set of instructions for transmitting data to or from the right half of memory would facilitate list structure operations. The present order code makes it impossible to so list-chaining operations (car or cdr) without disturbing the A or Q registers.