Technical Q&AQD50 - PCI Video Card Bus Error (17-November-97)
Q
I'm working on a PCI video card and I'm getting a Bus Error when users
drag translucent objects over the Apple in the upper left corner. Why?
A
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.
-- Tim Carroll
Worldwide Developer Technical Support
Technical Q&A
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