typedef struct _POOL_HEADER
{
     union
     {
          ULONG PreviousSize: 9;
          struct
          {
               ULONG PoolIndex: 7;
               ULONG BlockSize: 9;
               ULONG PoolType: 7;
          };
          ULONG Ulong1;
     };
     union
     {
          ULONG PoolTag;
          struct
          {
               WORD AllocatorBackTraceIndex;
               WORD PoolTagHash;
          };
     };
} POOL_HEADER, *PPOOL_HEADER;

Windows Vista Kernel Structures