Prime95 is a program which searches for very large prime numbers. It has become extremely popular among PC enthusiasts and overclockers as a stability testing utility.

It includes a "Torture Test" mode designed specifically for testing PC subsystems for errors in order to help ensure the correct operation of Prime95 on that system, but can also be used to stress test a PC. People who are overclocking their system may use the torture test as an indication of the stability of their system.

If your computer can run Prime95 for 24 hours without any errors, then you can usually assume that your CPU, and the parts of the motherboard which interface with the CPU and RAM, are stable and working well.

What's New in version 25.9:

• Performance increases for the 64-bit version by taking advantage of the eight extra SSE2 registers. The 32-bit version may also be a bit faster by taking advantage of the Core 2 architecture. The FFT code was originally optimized for the Pentium 4 where instructions like "movapd reg,reg" take 6 clocks and should be avoided, whereas on Core 2 the same instruction uses just 1 clock cycle and improves scheduling. The 32-bit version may be slightly faster or slower on the Pentium 4, but not enough to worry about. Preliminary benchmarks indicate:
  • 32-bit Pentium 4: between 2% slower and 1% faster
  • 32-bit Core 2: between 2% and 6% faster
  • 64-bit Pentium 4: between 2% and 4% faster
  • 64-bit Core 2: between 10% and 13% faster
  • 64-bit Core i7: between 7% and 12% faster
  • 64-bit Phenom: between 10% and 13% faster
  • AMD64: between 1% slower and 1% faster
• Intermediate save files were overhauled. Trial factoring, P-1, and ECM now support multiple save files. The backup save file uses a .bu extension. While a new save file is being written a .write extension is used and renamed when successfully written (thus, briefly there are 3 save files). NOTE #1: LL save files no longer use an initial letter of q and r. The program will automatically use these obsolete files if necessary and delete them when no longer needed. NOTE #2: ECM and P-1 save files on k*2^n+1 no longer use file names beginning with d and l. The -An command line argument no longer appends an extension to save file names. NOTE #3: The client now supports keeping three save files.
• Miscellaneous minor bug fixes.