DEY                   DEY Decrement index Y by one                    DEY

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

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   DEY                 |   $88   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+

What it does: Reduces the Y Register by 1.

Major uses: Like DEX, DEY is often used as a counter for loop structures. But DEY is the more common of the two since the Y Register can simultaneously serve two purposes within a loop by permitting the very popular Indirect Y addressing mode. A common way to print a screen message (the ASCII form of the message is at $5000 in this example, and the message ends with 0): LDY #0:LOOP LDA $5000,Y:BEQ END:STA SCREEN,Y:INY:JMP LOOP:END continue with the program.