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)

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.