Sega Mega Drive/Mode 4
From Sega Retro
Mode 4 is a VDP setting which allows for Master System backward compatibility on the Sega Mega Drive. It can be enabled with VDP register 01, or with hardware via the M3 pin on the cartridge connector. Once the relevant bit is set, most other VDP registers no longer do anything, with the following exceptions:
- 00 - leftmost 8 pixels can be blanked.
- 08 - used for horizontal scrolling.
- 09 - used for vertical scrolling.
- 0C - screen width setting works, but only 256 pixel wide mode is valid. 320 pixel wide mode results in garbage nametable data on the leftmost 8 columns (64 pixels).
The Master System's native resolution is 256x192 pixels. In mode 4, the Mega Drive positions the display in the centre of a 256x224 screen (256x240 in PAL), which is then stretched to a width of 320 pixels. The left border is considered part of the screen, so in some games the display may appear off centre, such as Alex Kidd in Miracle World: