Alignment

#define ALIGN(x, a)     (((x) + (a) - 1) & ~((a) - 1))
#define ALIGN_UP(x,a)   (((x) + (a)) & ~((a) - 1))
#define ALIGN_DOWN(x,a) ((x) & ~((a) - 1))

   x  |  ALIGN(x,4096)  ALIGN_UP(x,4096)  ALIGN_DOWN(x,4096)
------------------------------------------------------------
   0  |              0              4096                   0
   1  |           4096              4096                   0
4095  |           4096              4096                   0
4096  |           4096              8192                4096
4097  |           8192              8192                4096