PSG Registers R4 - R6
From Intellivision Wiki
The upper 4 bits of period for channel A, B, or C. Channel A uses R4, channel B uses R5, and channel C uses R6. See PSG Channels for more information.
0000:0000:0000:pppp where: pppp indicates the upper 4 bits of 12-bit period for channel A, B, or C