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Bit-Breakdown Diagram
From SnesLab
A Bit-Breakdown Diagram shows what the individual bits of a word do. It is very common on the SNES for registers to pack more than one value into 8 or 16 bits. When writing to a register, care must be taken to ensure only the relevant bits are changed.
In the official Super Nintendo development manual, bit 0 is conventionally labeled "D0" (data).