Type A instructions have, in sequence, a 3-bit "prefix" (instruction code), a 15-bit "decrement" field, a 3-bit "tag" field, and a 15-bit "address" field. They are conditional jump operations based on the values in the decrement registers specified in the "tag" field. Some also subtract the "decrement" field from the contents of the index registers. The implementation requires that the second two bits of the instruction code be non-zero, giving a total of six possible type A instructions. One (STR, instruction code binary 101) was not implemented until the IBM 709.