CopyBits will occasionally read a few bytes above the base address of the
PixMap. If the video page starts exactly on the card's memory space,
then these accesses are made to unmapped memory.
This was documented for NuBus cards on page 3-118 of Inside Macintosh:Imaging with QuickDraw. The solution is to make the base address of the PixMap to be at least 20 bytes into the address space. Moving the PixMap in 32 bytes will align the PixMap to a cache line boundary.