how can I get the ZONE_DMA size ??

Hello I learned that the ZONE_DMA size is 16MB from the book KDE, but on my laptop the ZONE_SIZE is maybe 12M. I print the page directory informations, and from the NO.768 to NO.770 pte is not direct mapping until the NO.771 pte. So, how can I get the ZONE_DMA real size ? Is there any kernel functions or configuration files ? I want to get the accurate number. Thanks, HeChuan