DEX                   DEX Decrement index X by one                    DEX

  Operation:  X - 1 -> X                                N Z C I D V
                                                        / / . . . .
                                 (Ref: 7.6)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   DEX                 |   $CA   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+