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
1232 = 50% Duty Cycle (default)
124C = 75% Duty Cycle

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