For the SMS1 VDP only:

If bit 0 of VDP register 2 is 0, then bit 10 of the VRAM address for name table lookups will be forced to 0. This will cause the lower 12 rows of the virtual screen to reference the same name table entries as the first 12 rows; if the Y scroll is zero, in a normal 192-line mode, this will cause the first 8 rows to fill both the upper and lower third of the screen.

Only one game relies on this behaviour: the Japanese version of Ys. It is a common bug in homebrew software, however, which may assume the presence of an SMS2 VDP.

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