1 2 3 Previous Next 34 Replies Latest reply on May 2, 2016 10:32 AM by cl0p3z

    Simple instructions for freezing a Skylake Processor.


      The people at the community over at http://www.mersenneforum.org have found what appears to be a bug in the new Skylake architecture that can freeze any system that has a Skylake processor.


      The Mersenne community are a bunch of mathematicians and other folks that use Intel processors to find record prime numbers. They have found all the record prime numbers of the last 20 years.


      The main website of the community is http://www.mersenne.org where they keep track of all progress in searching for a new record prime number.


      Owners of Skylake systems have found out that the software package that they use to hunt for prime numbers will freeze their system if they use particular settings. Since this bug is reproducible and has been confirmed on motherboards of many different suppliers and with RAM modules of different suppliers the bug seems to be tied to the processor architecture. The bug is reproducible under Windows and Linux.


      This software works perfectly normal on all other Intel processors of past generations.


      Steps to freeze your Skylake system:


      - Download and install Prime95 for Windows on a Skylake system from the website at http://www.mersenne.org/download/

        (If you want to familiarize yourself with the software use the readme, a background in math will be helpful, but is not needed.)

      - In the menu go to 'Advanced | Test' and fill in the number 14942209 in the box labeled 'Exponent to test'

      - Let the program run for some time and at some point, minutes or hours, the system will freeze.


      The prime95 software does multiplications of extreme high numbers using the Fast Fourier Transformation. The implementation of these FFT's in prime95 is handcoded in assembly by George Woltman, and is the most efficient implementation available. This project runs for more than 20 years now and has always been carefully maintained. Tens of thousands of machines run this software 24 hours a day.

      For optimization, different FFT sizes have been implemented in Prime95, only the FFT with length 768K freezes the Skylake.


      It is my fear that like the infamous FDIV bug this issue will require a new stepping and a product recall, since this has security implications as well.


      If you have a Skylake system I invite you to try out the steps above. Please post your findings here and in the mersenneforum at http://mersenneforum.org/showthread.php?t=20714



        • 1. Re: Simple instructions for freezing a Skylake Processor.

          Hi Henk_NL,


          I would need more information about your system configuration in order to replicate the issue. Please download and run our Intel® System Support Utility and Intel® Processor Diagnostic Tool. Send me both results.

          Send me pictures (screenshots) of the errors that you are getting.


          Intel® System Support Utility

          Download Intel® System Support Utility


          Intel® Processor Diagnostic Tool (64-bit)



          Mike C

          • 2. Re: Simple instructions for freezing a Skylake Processor.

            Hi Mike,


            The PEG/SVE/Security Center of Excellence (SeCoE), Intel Product Security Incident Response Team is also working on this issue, but feel free to join or beat them.


            Fint the requested information below. As is stated in the forum at http://mersenneforum.org/showthread.php?t=20714 the problem is related to hyperthreading and the use of CPUsupportsFMA3. Overclocking, underclocking or just running at stock speed does not influence the outcome of the program



            --- Testing Processor 1 ---


            --- IPDT64 - rev ---

            --- Start Time: 12/16/2015 22:42:52---

            --- Skipping Config ---

            --- Reading CPU Manufacturer ---

            Expected --> GenuineIntel

            Detected --> GenuineIntel

            Found --- Genuine Intel Processor ---

            --- Temperature Test ---

            Temperature Test Passed!!!

            Temperature = 79 degrees C below maximum.

            --- Reading Brand String ---

            Detected Brand String:

            Intel Core i7-6700K 4.00GHz

            Brand String Test Passed!!!

            --- Reading CPU Frequency ---

            Expected CPU Frequency is --> 4.00

            Detected CPU Frequency is --> 7.4008

            CPU Frequency Test Passed!!!

            Expected frequency - The highest frequency at which the tested processor was manufactured to operate

            Detected frequency - The frequency at which the tested processor is currently operating

            Power management modes can create marginally higher or lower detected frequency

            than expected frequency. Small variations in clock frequencies are common.

            --- FSB NOT Supported on this Processor ---

            --- Running Base Clock test ---

            Detected Base Clock --> 134

            Base Clock test Pass ---

            ..QPI rate Test not supported..

            ..Skipping QPI rate Test..

            Skipping QPI rate Test

            --- Running Floating Point test ---

            Million Floating Points per Second, MFLOPS --> 561.6

            Floating Point Test Pass ---

            --- Running Prime Number Generation Test ---

            Operation Per Second--> 8.63953e+006

            Prime Number Generation Test Pass ---

            --- Reading Cache Size --- 

            - Detected L1 Data Cache Size --> 4 x 32

            - Detected L1 Inst Cache Size --> 4 x 32

            - Detected L2 Cache Size --> 1024

            - Detected L3 Cache Size --> 8192

            Cache Size Test Passed!!!

            --- Determining MMX - SSE capabilities ---

            --- CPU FEATURES DETECTION FOR ---

              --- MMX SSE ---

            MMX - MMX Supported --> Yes

            SSE - SSE Supported --> Yes

            SSE2 - SSE2 Supported --> Yes

            SSE3 - SSE3 Supported --> Yes

            SSSE3 - SSSE3 Supported --> Yes

            SSE4.1 - SSE4.1 Supported --> Yes

            SSE4.2 - SSE4.2 Supported --> Yes

              --- MMX SSE - capabilities check complete ---

            MMX Test Result --- PASS

            SSE Test Result --- PASS

            SSE2 Test Result --- PASS

            SSE3 Test Result --- PASS

            SSSE3 Test Result --- PASS

            SSE4.1 Test Result --- PASS

            SSE4.2 Test Result --- PASS

            MMX SSE Testing Passed !!

            --- Determining AVX AES PCLMULQDQ capabilities ---

              --- CPU FEATURES DETECTION FOR ---

              --- AVX/AES/PCLMULQDQ ---

            AVX - Advanced Vector Extensions Supported --> Yes

            AVX OS Support - AVX Operating System Supported --> Yes

            AES - Advanced Encryption Standard Supported --> Yes

            PCLMULQDQ - Polys Carry-Less Multiply Supported --> Yes

              --- AVX AES PCLMULQDQ capabilities check complete ---

            AVX Compare Test Result --- PASS

            AES Test Result --- PASS

            PCLMULQDQ Test Result --- PASS

            AVX AES PCLMULQDQ Testing Passed !!

            --- Reading Memory Size ---

            Detected Memory Size is --> 32.00GB

            --- Integrated Memory Controller Stress Test ---

            --- Integrated Memory Controller Stress Test Pass!!! ---

            Integrated Memory Controller Test Pass!!!

            ..Platform Controller Hub Test not supported curent chipset..

            ..Skipping Platform Controller Hub Test..

            Skipping Platform Controller Hub Test

            --- Querying for Intel(R) Integrated Graphics Device (IGD) ---

            ..Detected 8086 as Vendor ID on Device 2 on Intel(R) processor..

            ..Intel(R) Integrated Graphics Device Presence Detection Passed..

            ..2D Graphics Visual Display Passed..

            ..Graphics Visual Display Passed..

            ..Rotating Display Passed..

            --- CPU Load ---

            --- Load Level = 8

            CPU Load Passed!!!

            --- Temperature Test ---

            Temperature Test Passed!!!

            Temperature = 71 degrees C below maximum.

            --- Test End Time: 12/16/2015 22:46:51---

            System Information


            Processor Name: Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz

            Processor Information: Intel64 Family 6 Model 94 Stepping 3

            Number of Physical Cores: 4

            Number of Logical Cores: 8

            Installed System Memory: 32 GB

            Operating System: Microsoft Windows 7 Professional 64-Bit

            Graphics Information: Intel(R) HD Graphics 530

            System Product: Z170 OC FORMULA

            System BIOS: P1.70

            • 4. Re: Simple instructions for freezing a Skylake Processor.

              When I registered here as a member, only the second attempt got me an email to my address that I had to use to confirm the validity of my account. More people from the Mersenneforum have tried to join but failed, because they did not receive a confirmation link in their email box. On behalf of future user 'ATH' I post this clarification on how to obtain the freezing of the Skylake system:




              In order to replicate the error make sure hyperthreading is enabled on a Skylake 6700K.


              Download Prime95 version 27.9, as the error seems to occur more frequently here:



              The error also occurs in the newest version 28.7:


              but if you try this you need to create a file called "local.txt" in the directory with the line:


              because version 28.7 uses AVX2/FMA3 by default and the error seems to occur only with AVX.


              Start Prime95.exe and choose "Options" - "Torture test", and fill out the popup box like this:


              except change the bottom one "Time to run each FFT size (in minutes)" to like 120min or more, however long you want to run the test.



              In summary the error occurs only with HT on and all 8 virtual threads running tests and only with AVX. The error is currently only experienced with 768 FFT (Fast Fourier transform) size in Prime95. 

              • 5. Re: Simple instructions for freezing a Skylake Processor.

                To put on the table, signing up to this forum involves jumping through a few hoops.


                Several people who have valuable information and want to share on this matter have had difficulty joining.


                Please see http://mersenneforum.org/showthread.php?t=20714 for more details.


                Kindest regards.

                • 6. Re: Simple instructions for freezing a Skylake Processor.

                  Hi Henk_NL,


                  I am going to work on this, I will let you the results.



                  Mike C

                  • 7. Re: Simple instructions for freezing a Skylake Processor.

                    I can reproduce this issue as well.  In addition to that, I and others over at this thread: http://www.tomshardware.com/forum/id-2830772/skylake-build-randomly-freezing-crashing.html have been battling an issue for months now, where the system completely hangs when idle.  Anywhere from a few hours being idle, to several days.  We've tried RMA'ing every piece of hardware except the CPU, and tried every setting possible, including running RAM at the default 2133MHz, but so far nothing has resolved the issue.  There is some underlying platform issue here.

                    • 8. Re: Simple instructions for freezing a Skylake Processor.

                      Hi Henk_NL,


                      I noted you are using running the processor at 7.4008MHz. The base frequency of the Intel® Core™ i7-6700K is 4.0MHz. Are you getting the same results using the stock settings of the processor and using memories of 2133MHz at 1.25V.


                      Mike C

                      • 9. Re: Simple instructions for freezing a Skylake Processor.

                        The problem is not related to the base frequency, temperature or vcore: http://www.bilder-hochladen.net/files/big/hb0a-9w-c8c7.jpg



                        • 10. Re: Simple instructions for freezing a Skylake Processor.

                          The results that I posted were not mine but one of the other people who stumbled upon this and posted their results in the mersenne forum.


                          Indeed, the results obtained are the same regardless of systems having been overclocked or not, or even are underclocked. The fatal combination seems to be:


                          - Skylake AND

                          - hyperthreading AND

                          - CpuSupportsFMA3=0. (only needed if you use the latest version of prime95, since previous version stem from before the introduction use FMA3 anyway)


                          There are people contemplating writing a piece of software based on prime95 that will simplify freezing the Skylake based on just a few instructions. Very scary indeed and further analysis of the downloadable code is required

                          • 11. Re: Simple instructions for freezing a Skylake Processor.

                            Hi Henk_NL,


                            Thank you for your feedback, our engineer department is working on your case now.


                            Mike C

                            • 12. Re: Simple instructions for freezing a Skylake Processor.



                              See 768k Skylake Problem/Bug - Page 19 - mersenneforum.org


                              These two things might be related. Anyway it certainly brings back the memories of the FDIV bug.


                              I saw the bug report I filed was categorized as 'performance issue'. It may soon seriously affect the performance of the Intel stock price on the stocks exchange, but I guess such an option does not exist in the drop down menu.

                              • 13. Re: Simple instructions for freezing a Skylake Processor.

                                Then of course it may as well be two separate issues as well.

                                • 14. Re: Simple instructions for freezing a Skylake Processor.

                                  Hi Mike.C. from Intel.


                                  I do hope your engineering department is working this.


                                  You might want to get your public and investor relations departments on this really soon now....

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