I need to have a function or opcode that does something similar to GetTickCount() or RDTSC, and to be fast. RDTSC, as it is written in Agner's tute, cannot be used in Windows, and I was really frightened after I used the RDTSC opcode to measure several instructions take thousands of cycles (seems the RDTSC is skipped in Protected mode, and after I modify eax, the stupid results come ):
;..
RDTSC
push eax
fld real4 ptr
fstp real4 ptr
RDTSC
pop ebx
sub eax,ebx
PrintDec eax ; here eax gives some frightening results.
;..



thanks in advance
Posted on 2002-12-12 21:07:57 by Ultrano
RDTSC can be used in Windows programs : it returns the number of cycles for the start of the machine in eax:edx.
Posted on 2002-12-13 01:33:11 by JCP
As ReadIoSys says, it can be used in windows.

However, it is not virtualised (like eax etc. are), so when a process switched RDTSC keeps on counting. So if there is a task switch inside your timed section, then the results are unreliable.

If the code is short, then the chances of a task switch (and thus an unreliable value from it) are much lower. The bigger the code, the more chance of a task switch.

Also run the benchmarking code several times, there may be cache issues you are hitting etc. on the first load.

Mirno
Posted on 2002-12-13 05:56:47 by Mirno
Of course I bear that in mind! But there were several algos which when I run them, they produce result of 2000 or near, and I run the test.exe several (30 - 40) times and get the medium results. The frightening results come when I run a fxxx instruction - maybe the fpu makes some initialisation for these 2000 cycles, will check thoroughly
Posted on 2002-12-13 07:49:38 by Ultrano
Hi Ultrano,
Have you seen this site? Ratch
http://cedar.intel.com/software/idap/media/pdf/rdtscpm1.pdf
Posted on 2002-12-14 00:13:01 by Ratch
Ultrano, I just pick smallest time of several runs for exact time. Or I force minimum value to be returned 10 times to validate it is shortest time (sometimes processor does not return smallest time again!?).
Posted on 2002-12-14 02:28:29 by bitRAKE