Overview

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:

Bits

Bit76543210
Read or write when FM hardware not presentUndefined
ReadCounter (see below)Always 0Last value written, 0 by default
WriteNo effectSN76489 mute controlYM2413 mute control

Mute control

The SN76489 control 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 back the values written allows detection of the FM chip: see FM Chip Detection for example code.

Homebrew YM2413 add-ons for original hardware may not support the SN76489 enable bit.

Counter

In this forum post, Charles MacDonald said:

Bits 7-5 of port $F2 return the high bits of a counter that is clocked by the C-Sync input into the 315-5297. The counter is 12 bits wide and counts up on each C-Sync pulse. The bits return are as follows:

D7 : Counter bit 11 
D6 : Counter bit 7 
D5 : Counter bit 3 



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