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VRAM Data Bus
From SnesLab
The VRAM Data Bus is the data bus that connects S-PPU1 and S-PPU2 to the VRAM chips. It consists of two 8-bit data buses: VDA and VDB. VDA0-VDA7 connects to the VRAM chip which has VAA going to it, and VDB0-VDB7 connects to the other VRAM chip, which has VDB going to it.
It is drawn in blue in the colorized jwdonal schematic.