Includeasmx86pgtable64h

#define PGDIR_SHIFT 39 #define PTRS_PER_PGD 512

#define PUD_SHIFT 3 0 #define PTRS_PER_PUD 512

#define PMD_SHIFT 21 #define PTRS_PER_PMD 512

The macros ptrs_per_xxx specify how many pointers (i.e., different values) a given directory entry can represent. Since AMD64 employs 9 bits for each directory, 29 = 512 pointers fit into each.

The kernel also needs a means of extracting the individual components from a given address. The kernel uses the bitmasks defined below to do this.

#define PGDIR_MASK (~(PGDIR_SIZE-1))

The masks are applied on a given address by simple bitwise addition.

Continue reading here: Format of Page Tables

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