May 2015
December 2013
PPU scrolling - Nesdev wiki
by sylvainulg
Games scroll to show an area much larger than the 256x240 pixel screen [...] Write the starting page (high order bit of X and Y) to bits 0 and 1 of PPUCTRL ($2000) [...] you write that to VRAM before you set the scroll, as seen in the animation below.
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