@param of The parent IODeviceMemory of which a subrange is to be used for the new descriptor, which will be retained by the subrange IODeviceMemory.
@param offset A byte offset into the parent's memory.
@param length The length of the subrange.
@param of The parent IODeviceMemory of which a subrange is to be used for the new descriptor, which will be retained by the subrange IODeviceMemory.
@param offset A byte offset into the parent's memory.
@param length The length of the subrange.