RtlFindLastBackwardRunClear - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H

NTSYSAPI
ULONG
NTAPI
RtlFindLastBackwardRunClear(
    _In_ PRTL_BITMAP BitMapHeader,
    _In_ ULONG FromIndex,
    _Out_ PULONG StartingRunIndex
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit.