RtlFillMemoryUlong - NtDoc

#ifndef _NTRTL_H
#if defined(_M_AMD64)

FORCEINLINE
VOID
RtlFillMemoryUlong(
    _Out_writes_bytes_all_(Length) PVOID Destination,
    _In_ SIZE_T Length,
    _In_ ULONG Pattern
    )
{
    PULONG Address = (PULONG)Destination;

    //
    // If the number of DWORDs is not zero, then fill the specified buffer
    // with the specified pattern.
    //

    if ((Length /= 4) != 0) {

        //
        // If the destination is not quadword aligned (ignoring low bits),
        // then align the destination by storing one DWORD.
        //

        if (((ULONG64)Address & 4) != 0) {
            *Address = Pattern;
            if ((Length -= 1) == 0) {
                return;
            }

            Address += 1;
        }

        //
        // If the number of QWORDs is not zero, then fill the destination
        // buffer a QWORD at a time.
        //

         __stosq((PULONG64)(Address),
                 Pattern | ((ULONG64)Pattern << 32),
                 Length / 2);

        if ((Length & 1) != 0) {
            Address[Length - 1] = Pattern;
        }
    }

    return;
}

#endif
#endif
View code on GitHub

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

NTSYSAPI
VOID
NTAPI
RtlFillMemoryUlong(
    _Out_writes_bytes_all_(Length) PVOID Destination,
    _In_ SIZE_T Length,
    _In_ ULONG Pattern
    );

#endif
#endif
View code on GitHub