VDP

The VDP, also known as Video Display Processor, handles all video output on the Mega Drive. The VDP is often known as the YM7101, which is derived from the Sega Master System VDP for backwards compatibility. The Master System VDP is in turn derived from the Texas Instruments TMS9918A. In most (if not all) Mega Drive models, the VDP is a dedicated large chip on the main motherboard.

It has 64 KB of VRAM, which can be used to store patterns (also known as art) plane and sprite name tables, as well as horizontal scroll data. The benefit of storing name tables and H-Scroll in VRAM is that you can decide where this data is stored, but leaves less room for art. The VDP has 64 bytes of 9-bit RAM to store palettes. This special area of the VDP is known as CRAM, or Colour RAM.

The VDP has the capability of displaying up to 80 sprites on-screen, as well as the A and B planes, and the Window plane. The window plane can be used as a replacement for the A plane in some circumstances. It can also support 64 on-screen colours at any time, as well as scrolling the entire screen or specific parts of it horizontally and vertically. There is also multiple registers that control how the VDP is configured, which are written to the control word as 16-bit words.

Accessing the VDP

To access the VDP, you must use a special area of I/O memory. Below is a map of the VDP's IO memory, starting at $C00000:

$C00000	Data Port
$C00002	Data Port (Mirror)
$C00004	Control Port
$C00006	Control Port (Mirror)
$C00008	H/V Counter
$C0000A	H/V Counter (Mirror)
$C0000C	H/V Counter (Mirror)
$C0000E	H/V Counter (Mirror)
$C00011	SN76489 PSG
$C00013	SN76489 PSG (Mirror)
$C00015	SN76489 PSG (Mirror)
$C00017	SN76489 PSG (Mirror)