Effects/12xx

12xx is an effect of many uses, including:

Nintendo NES, Game Boy:
Changing the duty cycle if used on square 1 or 2, the possibilities being 00 - 03 If XX = 00 the duty cycle will be 12.5% If XX = 01 the duty cycle will be 25% If XX = 02 the duty cycle will be 50% If XX = 03 the duty cycle will be 75%

Nintendo NES
Changing the noise mode, 0 for white noise and 1 for periodic noise (only works for NES, the Game Boy uses 11xx to change noise modes)

Commodore 64
Changing the pulse width of a C64 square wave in a channel. XX can be 0 to 64 Examples: 1232 = 50% Duty Cycle (default) 124C = 75% Duty Cycle