Sega Master System / Mark III / Game Gear
Systems with FM hardware include:
|Read or write when FM hardware not present||Undefined|
|Read||Undefined||Always 0||Last value written, 0 by default|
|Write||No effect||Enable/disable SN76489 audio (1 = enable). Only has effect when bit 0 is set.||Enable/disable YM2413 audio (1 = enable)|
The SN76489 enable bit only works on a Japanese Master System. On the Mark III with FM unit, it has no effect: it is always enabled.
The ability to read from bit 0 allows detection of the FM chip: see FM Chip Detection for example code.
Homebrew YM2413 add-ons for original hardware doesn't support the SN76489 enable bit.