Viewport position (Scrolling)
These registers can be accessed even during Mode 3, but modifications may not take effect immediately (see further below).
FF42–FF43 — SCY, SCX: Background viewport Y position, X position
These two registers specify the top-left coordinates of the visible 160×144 pixel area within the 256×256 pixels BG map. Values in the range 0–255 may be used.
The PPU calculates the bottom-right coordinates of the viewport with those formulas: bottom := (SCY + 143) % 256 and right := (SCX + 159) % 256.
As suggested by the modulo operations, in case the values are larger than 255 they will “wrap around” towards the top-left corner of the tilemap.
Example from the homebrew game Mindy’s Hike:
Mid-frame behavior
The scroll registers are re-read on each tile fetch, except for the low 3 bits of SCX, which are only read at the beginning of the scanline (for the initial shifting of pixels).
All models before the CGB-D read the Y coordinate once for each bitplane (so a very precisely timed SCY write allows “desyncing” them), but CGB-D and later use the same Y coordinate for both no matter what.