20xy is the SEGA Master System's noise mode effect.
X = 0 or 1, 0 meaning that you can only use 3 frequencies in the noise channel and 1 meaning that you can use any frequency that the SN76489 normally support supports but the third square channel (PSG2) is muted.
Y = 0 or 1, 0 meaning periodic noise and 1 meaning white noise.

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