How to Disable AVX in Prime95 (prime.txt Settings)

Quick facts: Prime95 30.19 build 20 can skip its AVX, AVX2, and AVX-512 code paths if you add settings to prime.txt, the configuration file in the Prime95 folder. The documented keys are CpuSupportsAVX=0, CpuSupportsAVX2=0, and CpuSupportsAVX512F=0. Since version 30.11, settings live in prime.txt; older guides that tell you to edit local.txt are describing the previous arrangement. Disabling AVX changes what you are testing, so read the trade-offs below before you treat a pass as equivalent.

Why people disable AVX at all

AVX instructions let the processor do more arithmetic per clock cycle, which raises power draw and heat under a continuous load like the Torture Test. A widely cited community source, the Tom’s Hardware Intel Temperature Guide, reported that on older Core processors from the 2nd to 8th generations, versions after 26.6 running AVX code could produce temperatures up to about 20 degrees Celsius higher than version 26.6, and described those temperatures as unrealistic compared with typical workloads. That is a second-hand community guide figure for older platforms, not an official GIMPS conclusion and not a number to apply directly to a modern processor with AVX offset and power-limit management. The underlying point still holds on many systems: an AVX-heavy Prime95 run is a deliberately extreme load, and some testers want a run that resembles a heavy non-AVX workload more closely, or want to isolate whether AVX power draw is what pushes their cooling over the edge.

The exact settings

Close Prime95 first, so it does not overwrite the file when it exits. Open the folder where you extracted Prime95 and find prime.txt. Open it in a plain text editor and add the lines you need, one per line, then save and relaunch the program:

CpuSupportsAVX=0
CpuSupportsAVX2=0
CpuSupportsAVX512F=0

You do not have to add all three. Adding only the AVX-512 line, for example, keeps AVX2 paths active while removing the widest vector path. These keys tell the program to act as if the CPU does not support the feature; they do not change your processor, your BIOS, or any other application. To undo the change, delete the lines or set them back to 1 and restart Prime95. If you followed an old tutorial and created the entries in local.txt instead, move them to prime.txt, because current versions read their settings from prime.txt following the migration recorded in the version 30.11 change notes. Keep a backup copy of the original prime.txt before editing, so you can restore your worker and communication settings exactly.

What a test with AVX disabled actually proves

With AVX disabled, the Torture Test still compares results against known correct answers and still stresses caches, cores, and memory, but it no longer proves stability for AVX workloads. Video encoders, scientific software, emulators, and some games do use AVX instructions, and an overclock that passes only with AVX disabled can still fail in those programs. Treat the AVX-off run as a diagnostic and as a heat reference, not as a replacement for a full test. A clear method is to run both: first with AVX enabled to find the true worst case, then with AVX disabled to see how much of the heat and power is AVX-driven. If the system passes with AVX off and fails quickly with AVX on at the same settings, your limit is likely power, voltage under AVX load, or cooling, and a BIOS AVX offset is often the more honest fix for daily use.

How this relates to version 26.6

Version 26.6 is searched for the same reason: it is the last release before AVX optimisation entered the FFT code in version 27.1, so it naturally produces a lighter, non-AVX load. Using an ancient build is not necessary to get that effect, and 26.6 lacks later features such as PRP proof generation, AVX-512 support, and heterogeneous core support. The modern equivalent is current Prime95 with the prime.txt switches above. The full story is on the Prime95 26.6 page. For BIOS-side control instead, see Prime95 and AVX offset.

Verify the switch took effect

After relaunching, do not assume the edit worked just because the file saved. Start a short Torture Test run and watch power and temperature behaviour compared with your earlier AVX-enabled run at the same preset and settings; a clear change in the heat pattern is practical evidence that a different code path is in use. Check also that Prime95 did not recreate or overwrite prime.txt on exit, by reopening the file after you close the program and confirming your lines are still present exactly as written. If nothing changed, the usual causes are editing a copy of prime.txt in a different extracted folder, leaving a second Prime95 instance running, or a spelling difference in the key name. Fix the file location first, then the spelling, and test again with the same short run so the comparison stays honest.

Common mistakes

The most common mistake is editing the wrong file, local.txt, and concluding the switches do not work. The second is editing prime.txt while Prime95 is still running, so the program rewrites the file on exit. The third is forgetting the edit exists: weeks later, benchmark or GIMPS results look low and the hidden cause is a disabled instruction path. Add a comment to your own notes, not to prime.txt in a way that breaks its format, recording the date you added the switches and why. Finally, do not disable AVX to hide an overheating problem you intend to ignore. If a cooler cannot handle the AVX run, the correct responses are better cooling, lower voltage, an AVX offset, or stock settings, in whichever combination you can verify with a repeat test.

Next steps

Return to the tutorials hub for the main workflow in How to Use Prime95, and compare presets in Small vs Large vs Blend before your next run.