Which timing mechanism did you use?

This was a while ago and I don't remember the exact details...

I had two loops with a large number of operations. One called a trivial Win32 function (GetACP), the other made a System Call (NtClose) as well as the same trivial Win32 function. I used QueryPerformanceCounter to time the loops.

From there, I estimated the number of machine cycles per iteration (using my clock speed of the time), and subtracted the "trivial" version which made no system calls.

Clever approach!

Peeking at "ntoskrnl.exe" in Ghidra, it appears that "NtVdmControl" is a true "do-nothing" system call in Windows 10 x64, and Windows 11 removed the function entirely.