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> Windows > nop |
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LocoDelAssembly 23 Jun 2009, 18:48
The result in the vast majority of processors will be that it does not impose an extra clock latency in the code per iteration. The reason is that the processor is capable of handling both instruction in parallel. If you replace nop with "xor eax, eax" you'll probably still get the same times.
As for if NOP consumes a clock cycle it will depend, but for sure it will waste processor resources so it may make a loop take longer to complete if instructions willing to be executed have to wait for NOP to release processor execution units (some AMD processors can avoid sending NOPs to execution units but them still consume processor resources). |
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23 Jun 2009, 18:48 |
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eskizo 23 Jun 2009, 19:09
Thanks Loco. By the way, whaits the best way to build a "time/clocks spend checker" ? I mean, how to check how many 'nanoseconds' a block of code spends...
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23 Jun 2009, 19:09 |
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arigity 23 Jun 2009, 19:33
the intel optimization manual has a section on nops.
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23 Jun 2009, 19:33 |
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Azu 23 Jun 2009, 19:49
eskizo wrote: Thanks Loco. By the way, whaits the best way to build a "time/clocks spend checker" ? I mean, how to check how many 'nanoseconds' a block of code spends... |
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23 Jun 2009, 19:49 |
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windwakr 23 Jun 2009, 19:59
If you do any sort of timing you should set the priority of your program to high or realtime.
EDIT: fixed Ok, If you need a good article on rdtsc timing read THIS. Last edited by windwakr on 23 Jun 2009, 20:18; edited 3 times in total |
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23 Jun 2009, 19:59 |
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Azu 23 Jun 2009, 20:04
With any kind of timing, no?
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23 Jun 2009, 20:04 |
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Madis731 27 Jun 2009, 12:35
Timing with RDTSC are usually only accurate upto the point where context-switch occurs. That is the primary reason for realtime-priority. Rule of thumb says, measure clocks that usually end in less than one millisecond which is about 2000000 clocks on 2GHz CPU. If you measure with RDTSC you need to be aware that any code that takes less than bus multiplier clocks, you see 0 or the bus multiplier.
Example: 2GHz T7200 CPU has a 166.3MHz bus and a 12x multiplier. Time difference between 2 RDTSCs can be 0, 12, 24, 36, 48 and you cannot get more accurate than that. 1) If you test your code 10 times and you get results 0,12,12,0,0,0,12,12,0,12 you can be pretty sure that it was 6 clocks. 2) If you want to be *really* sure though, you should loop your code say 1024 times and then do SHR [result],10  When you get 6, then it usually is 6±0.5 clocks. |
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27 Jun 2009, 12:35 |
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LocoDelAssembly 27 Jun 2009, 16:29
Quote:
Why?  Have you really seen two consecutive RDTSC giving exactly the same count??? |
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27 Jun 2009, 16:29 |
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windwakr 27 Jun 2009, 16:52
Code:
rdtsc
mov ebx,eax
rdtsc
always either different by 105 or 98, weird. Madis731 wrote:
Ok, 98 goes into my multiplier 7 times, but what about 105? 7.5 times, how do you explain that? BTW: That processor was only $2! It came with some junk motherboard we bought at a flea market.
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27 Jun 2009, 16:52 |
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LocoDelAssembly 27 Jun 2009, 17:17
After tried countless times this code:
Code: format pe gui 4.0 mov edi, 3 align 16 .loop: rdtsc mov ebx, eax rdtsc sub eax, ebx dec edi jnz .loop int3 I always get EAX=5. CPUID reports this: Code: Name: AMD Athlon 64 3200+ Code Name: Venice Core Speed: 2009.4 MHz Multiplier: 10.0x Bus Speed: 200.9 MHz HT Link: 1004.7 MHz |
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27 Jun 2009, 17:17 |
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Azu 27 Jun 2009, 21:10
Madis731 wrote: Timing with RDTSC windwakr wrote:
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27 Jun 2009, 21:10 |
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