typedef struct _HEAP_FREE_ENTRY
{
     union
     {
          struct
          {
               WORD Size;
               UCHAR Flags;
               UCHAR SmallTagIndex;
          };
          struct
          {
               PVOID SubSegmentCode;
               WORD PreviousSize;
               union
               {
                    UCHAR SegmentOffset;
                    struct
                    {
                         UCHAR LFHFlags;
                         UCHAR UnusedBytes;
                    };
                    struct
                    {
                         WORD FunctionIndex;
                         WORD ContextValue;
                    };
                    struct
                    {
                         ULONG InterceptorValue;
                         WORD UnusedBytesLength;
                    };
                    struct
                    {
                         UCHAR EntryOffset;
                         UCHAR ExtendedBlockSignature;
                    };
                    struct
                    {
                         ULONG Code1;
                         WORD Code2;
                    };
                    UCHAR Code3;
               };
               UCHAR Code4;
          };
          UINT64 AgregateCode;
     };
     LIST_ENTRY FreeList;
} HEAP_FREE_ENTRY, *PHEAP_FREE_ENTRY;

Windows Vista Kernel Structures