On systems with FM hardware, port $f2 allows detection, and enabling, of the YM2413 FM chip. On some systems, it also allows disabling the SN76489 PSG chip.

Systems with FM hardware include:


Read or write when FM hardware not presentUndefined
ReadUndefinedAlways 0Last value written, 0 by default
WriteNo effectEnable/disable SN76489 audio (1 = enable). Only has effect when bit 0 is set.Enable/disable YM2413 audio (1 = enable)

In summary:

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.

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