Prime95 26.6: The Last Pre-AVX Version

Version 26.6 is an old Prime95 release that still gets searched for every day. This page explains why, what it cannot do, and the better modern alternative. We do not offer a download of the 26.6 binary here, because we have not verified a 26.6 package ourselves; the current verified release is on the download page.

Why 26.6 is still searched for: a community heat-testing tradition

According to community sources, notably the Tom’s Hardware Intel Temperature Guide (a second-hand, community guide, not GIMPS documentation), versions after 26.6 run AVX code on supported processors, which can raise temperatures by up to about 20 degrees Celsius compared with 26.6 on 2nd to 8th generation Intel Core processors. The same guide recommends 26.6 Small FFTs as a CPU heat test because its steady load stays close to TDP. That tradition is why overclocking forums keep pointing at 26.6 for heat testing older chips. Treat those numbers as community reporting for older platforms, not as an official specification.

What 26.6 is missing

The official changelog shows how much came after 26.6. AVX-optimised FFTs arrived in 27.1 and 27.3, FMA support in 28.3, and AVX-512 FFTs plus cache-based Torture Test sizing in 29.8. Modern features such as PRP testing with proof files, support for hybrid big and little core CPUs, and years of bug fixes simply do not exist in 26.6. Using it as your main GIMPS client today makes little sense.

The modern alternative: 30.19 with AVX disabled

If your goal is a 26.6-style heat test without AVX, the current version can do it. Prime95 30.19 supports documented settings in prime.txt such as CpuSupportsAVX=0 (with matching switches for AVX2 and AVX-512F), which stop the program from using those instruction paths. You keep the current, verified build and its result checking, without the AVX heat profile. Get the current release from the download page, and see the version history for how the versions differ.