0xf seem to be unimplemented
0x30 bits can be changed only by writing the 0x40 bit at the same time
0x80 bit is ignored
0x100 and 0x200 clear one another; setting both uses 0x100
0x400 and 0x800 clear one another; setting both uses 0x400
0x1000 and 0x2000 clear one another; setting both uses 0x1000
0x4000 bit is ignored
0x8000 and 0x10000 clear one another; setting both uses 0x8000
0x20000 and 0x40000 clear one another; both can be set at once
0x80000 is read-only ("in vertical blanking interval"?)
0x100000 and 0x200000 clear one another; 0x100000 takes priority
0x400000 can be set, but setting 0x100000 or 0x200000 clears it
0x800000 cannot be set and prevents 0x700000 writes from happening
0xff000000 seem to be unimplemented