typedef struct XMEM_HEAP_STATISTICS {  
    ULONG dwSize;  
    ULONG dwPageGroupSize;
    SIZE_T dwMaximumAllocatedSize;  
    SIZE_T dwMinimumFreeDepth;  
    ULONG dwAllocationCount;  
    ULONG dwFreeCount;  
} XMEM_HEAP_STATISTICS  

XMEM_HEAP_STATISTICS statistics;
statistics.dwSize = sizeof( statistics );
BOOL success = XMemGetAllocationStatistics(XALLOC_MEMTYPE_GRAPHICS_WRITECOMBINE, TRUE, &statistics);