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. There are conditional jump operations based on the values in the index registers specified in the tag field. Some instructions 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.
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 There are conditional jump operations based on the values in the index registers specified in the tag field Some instructions 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