DEC                   DEC Decrement memory by one                     DEC

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

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  ZeroPage      |   DEC $FF             |   $C6   |    2    |    5     |
  |  ZeroPage,X    |   DEC $FF,X           |   $D6   |    2    |    6     |
  |  Absolute      |   DEC $FFFF           |   $CE   |    3    |    6     |
  |  Absolute,X    |   DEC $FFFF,X         |   $DE   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+
  For penalty cycles on the 65816, check the desired addressing mode.

 65816 Extensions:

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  | Implied        |   DEC                 |   $3A   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+

What it does: Reduces the value of a byte in memory by 1. The N and Z flags are affected.

Major uses: A useful alternative to SBC when you are reducing the value of a memory address. DEC is simpler and shorter than SBC, and although DEC doesn't affect the C flag, you can still decrement double-byte numbers (see "Decrement Double-Byte Numbers" in Appendix D).

The other main use for DEC is to control a memory index when the X and Y Registers are too busy to provide this service. For example, you could define, say, address $505 as a counter for a loop structure. Then: LOOP STA $8000:DEC $505:BEQ ENDJMP LOOP. This structure would continue storing A into $8000 until address $505 was decremented down to zero. This imitates DEX or DEY and allows you to set up as many nested loop structures (loops within loops) as you wish.