Window mask settings, mirroring $2123-$2125. These control what layers each window is active on, as well as whether the window is inverted.
Each of the three addresses has the binary format ABCDabcd:
A/a = Enable window 2
B/b = Invert window 2
C/c = Enable window 1
D/d = Invert window 1
The lowercase letters affect BG1, BG3, and sprites, while the upper case letters affect BG2, BG4, and the fixed color. Which of the layers in each set of three is then determined by the particular address you modify:
