rdtsc()

[Christina_C_Intel]

Does anyone have any examples or insight on how to print the rdtsc() value on IA-32? I'm using intrinsics, but asmlanguage will be helpful too.

Thanks

[Intel_C_Intel]

How about this?

unsigned __int64 tick = 0;

void getTick(void)
{
__asm{
_emit 0x0f
_emit 0x31
lea ecx,tick
mov dword ptr [ecx], eax
mov dword ptr [ecx+4],edx
}
}

main() {
while (1) {
getTick();
printf("%I64u ", tick);
}
}

Aart Bik

http://www.aartbik.com/

[Christina_C_Intel]

Yes, that works.

Thanks

[simbalee]

Does it work on a computer with multi-core?
It seems not always work on Core Duo.

[TimP]

If you want the same code to work in both 32- and 64-bit compilation, you should use the _rdtsc() intrinsic, supported by Intel and MS C++.

rdtsc works fine on Core 2 Duo, where it is based on the Front Side Bus clock. I do find that I must repeat a code over a longer time intervalto get repeatable timing on Core 2 than on earlier Intel architectures. My results are sufficiently repeatable for elapsed time intervals of 0.0001 seconds or more.

[simbalee]

I am using a 32bit WinXP Prof. on a Core Duo E6400, instead of 64bit OS. The code I am using to get clocktick is something like this:

unsigned __int64 GetCycleCount()
{
__asm_emit 0x0F
__asm_emit0x31
}

It does work on the PC, but not always work. And it seems has nothing to do with elapsed time. Say, it can measure a thread that run for less than 0.1ms, on the other hand, it gives 0 to a thread that may last more than 100ms. And the result is repeatable, which means the function always gives zero for some conditions. That is very weird.

[TimP]

It looks like you are seeing only the low order 32 bits of the timer registers. Aart's example shows you how to get all 64 bits in 32-bit asm. Compiling in 64-bit mode (should you change OS in the future), you need a different version to get 64 bits. With gcc compatible compilers, you need yet other variations. So, I repeat the suggestion of looking up __rdtsc() http://msdn2.microsoft.com/en-us/library/twchhe95(VS.80).aspx

if you are willing to use a compiler recent enough to support it. I think even the free Microsoft compilers from 2003 on include this intrinsic.

[simbalee]

You are right. Dr. Bik's code works well on Core Duo. I got the code some years before and did not realized that it was for IA-32.