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        page_size - Global Variable: Is the virtual page size
        extern vm_size_t page_size;
        The  page_size  global  variable  is the size of a virtual
       page on a CPU. A device driver can use this  global  variable
 to partition I/O transfers so that they never cross a
       virtual page boundary. You should use this global variable
       only when: A subsystem cannot use the DMA mapping routines
       (such as, dma_map_alloc, dma_map_load, and so forth) There
       is  a  one-to-one correspondence between the system memory
       map and the I/O's view of the system memory space
                                                    page_size(9v)
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