Sega Master System / Mark III / Game Gear
When a byte is written to port 0x06 on the Game Gear, the PSG output is affected as follows:
If a bit is set, the corresponding channel is output to the
corresponding side. So,
0xff outputs all channels to all sides,
outputs to the left side only, etc.
Note however that this only affects the headphone output. The built-in speaker is unaffected; this means that