The VDP (Video Display Processor, 315-5113) is the video processor of the MegaDrive.
It's an evolution of the Master System VDP (which is an evolution of the TMS9918a video chip, see  for information about the Master System VDP).
It is backward compatible with the Master System one (MODE IV) but this section will only talk about the MODE V (MegaDrive mode).
The VDP think by CELL (a 8x8 pixel pattern).
It can output a 32x28 CELL image (256x224 pixels) or an 40x28 (320x224 pixels) one.
In PAL mode it can also output an 32x30 (256x240 pixels) or 40x30 (320x240 pixels) image.