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DMA Services (cont)
DMA Break-up:
- On x86 based systems, DMA addresses are physical addresses
- An object may span multiple non-contiguous pages in
physical memory
- Such an object cannot be represented by a single DMA
cookie
- Such an object must be broken into multiple pieces,
each represented by a separate DMA cookie
- A DMA "attribute structure" associated with the handle
is used to determine the capabilities of the DMA device, to determine how
the object must be broken up
- The attribute structure is also for DMA devices that
may not be able to access all of physical memory
- If system or device resources are insufficient to allow
DMA access to the entire object at one time, the system will break the
object up into multiple "DMA windows," each of which is represented by
one or more DMA cookies
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