RtlFillMemoryUlonglong - NtDoc

#ifndef _NTRTL_H
#if defined(_M_AMD64)

#define RtlFillMemoryUlonglong(Destination, Length, Pattern) \
    __stosq((PULONG64)(Destination), Pattern, (Length) / 8)

#endif
#endif
View code on GitHub

#ifndef _NTRTL_H
#if defined(_M_AMD64)
// ...
#else

NTSYSAPI
VOID
NTAPI
RtlFillMemoryUlonglong(
    _Out_writes_bytes_all_(Length) PVOID Destination,
    _In_ SIZE_T Length,
    _In_ ULONGLONG Pattern
    );

#endif
#endif
View code on GitHub