typedef struct _HEAP_VIRTUAL_ALLOC_ENTRY
{
     LIST_ENTRY Entry;
     HEAP_ENTRY_EXTRA ExtraStuff;
     ULONG CommitSize;
     ULONG ReserveSize;
     HEAP_ENTRY BusyBlock;
} HEAP_VIRTUAL_ALLOC_ENTRY, *PHEAP_VIRTUAL_ALLOC_ENTRY;

Windows Vista Kernel Structures