VRAM is RAM in the SNES for storing tiles and tilemaps to be processed by the PPU. It is stored separately from the WRAM, and can only be accessed through the hardware registers. It consists of two 8bit 32kb SRAM (64kb) chips one of each is tied to PPU1 and PPU2. VRAM can only be accessed during a blanked screen ($2100 bit 7 set) because the PPU needs to access it during active display. The Vblank / vertical retrace period is the time where VRAM can be accessed although time is limited. The screen is blanked during this time and amongst other tasks, VRAM is modified.
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