DEX                   DEX Decrement index X by one                    DEX

  Operation:  X - 1 -> X                                N V - B D I Z C
                                                        / . . . . . / .

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   DEX                 |   $CA   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+