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Prime95 / Mprime
Workload type LL, PRP, TF, P-1, ECM
First release 1996
Latest version 29.4
MPrime redirects to this. For other uses, see Mprime (disambig).

Prime95 is the freeware computer program written by George Woltman that is used by GIMPS, a distributed computing project dedicated to finding new Mersenne prime numbers. More specifically, Prime95 refers to the Windows and Mac OS X versions of the software.

MPrime is the Linux command-line interface version of Prime95, to be run in a text terminal or in a terminal emulator window as a remote shell client. It is identical to Prime95 in functionality, except it lacks a graphical user interface.

As of April 2018, Prime95 was used to discover all 16 Mersenne primes found by GIMPS.


Although most of the GIMPS software's source code is publicly available, it is technically not free software as users must abide by the project's distribution terms if the software is used to discover a prime number with at least 100,000,000 decimal digits and wins the $150,000 bounty offered by the EFF (Cooperative Computing Awards). As such, a user who uses Prime95 to discover a qualifying prime number would not be able to claim the prize directly. A free software package would not have this restriction.

The code that is used to generate checksums is not publicly available due to security reasons. The rewritten FFT assembly code in the versions since 27 (May 2012) uses AVX (Advanced Vector Extensions) instructions of Intel's Sandy Bridge and Ivy Bridge CPUs (Core i3/i5/i7-2xxx and 3xxx models), resulting in a huge performance increase compared to previous versions.

Prime95 currently does not have GPU support, although Woltman has indicated that it is under development. However, there are third-party programs, such as CUDALucas, gpuOwL and clLucas, that make use of the processing power of GPUs.

Finding Mersenne primes by distributed computing

As of Mai 2017, 15 new Mersenne primes have been found by the network of participants, and, on average, a new Mersenne prime is discovered approximately every year. Scott Kurowski wrote the Internet PrimeNet server that supports the Prime95/MPrime software on GIMPS, one of the earliest grid computing projects, researching Mersenne prime numbers, to demonstrate distributed computing software of Entropia, a company he founded in 1997.

Processing power

A table of selected benchmarks is provided below. The complete list can be found at the official GIMPS website.

Comparison of CPU core power Frequency Cores FFT Trial factoring TDP
Prime95 benchmark (per core) 2048k 4096k 65-bit
Platform CPU model MHz ms ms ms Watts
Intel Atom 330 1600 2 621 1166 46 8
Intel Atom D510 1664 2 585.91 1954.40 25.65 13
Intel Pentium III 1151 1 438.10 922.58 50.59 30
AMD Athlon 1054 1 457.40 774.49 56.08 ?
AMD Fusion E-350 1596 2 222.03 491.02 15.18 18
AMD Athlon XP 2000+ 1640 1 201.21 448.28 32.80 ~60
Intel Pentium 4 3078 1 72.40 162.02 14.91 86
AMD Phenom II X4 3414 4 34.86 76.27 4.59 125
Intel Core2 Duo E8600 3334 2 34.15 73.07 4.89 65
Sandy Bridge Pentium G620T 2159 2 41.09 72.53 4.99 35
AMD Phenom II X6 1100T 3310 6 32.68 69.54 3.85 125
Intel Core i5-2500K 3330 4 23.94 53.24 3.49 95
Intel Core i7-2600K 3463 4 21.75 45.35 3.67 95

Use for stress testing

| Over the years, Prime95 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. This is important because each iteration of the Lucas-Lehmer depends on the previous one; if one iteration is incorrect, so will be the entire primality test.

The stress-test feature in Prime95 can be configured to better test various components of the computer by changing the fast fourier transform (FFT) size. Three pre-set configurations are available: Small FFTs and In-place FFTs, and Blend. Small and In-place modes primarily test the FPU and the caches of the CPU, whereas the Blend mode tests everything, including the memory.

By selecting Custom, the user can gain further control of the configuration. For example, by selecting 8-8 kB as the FFT size, the program stresses primarily the CPU. By selecting 2048-4096 kB and unchecking the "Run FFTs in-place" checkbox, providing the maximum amount of RAM free in the system, the program tests the memory and the chipset. If the amount of memory to use option is set too high, then the system will start using the paging file and the test will not stress the memory.

On an absolutely stable system, Prime95 would run indefinitely. If an error occurs, at which point the stress test would terminate, this would indicate that the system may be unstable. There is an ongoing debate about terms "stable" and "Prime-stable", as Prime95 often fails before the system becomes unstable or crashes in any other application. This is because Prime95 is designed to subject the CPU to an incredibly intense workload, and to halt when it encounters even one minor error, whereas most normal applications do not stress the CPU anywhere near as much, and will continue to operate unless they encounter a fatal error.

In the overclocking community, a rule of thumb is often used to determine how long to run Prime95: test the CPU (8 kB FFT) for 10 hours and the memory (4096 kB FFT) for 10 hours, and if the system passes, there is a high chance that it is stable. Twenty-four hours of testing is recommended to be sure, as errors may show up after 16 or more hours of testing (compared to, say, just four hours of testing). Moreover, a large proportion of system overclockers and enthusiasts favor Prime95 over other benchmarking suites because Prime95 pushes the CPU's floating point units extremely hard, causing the CPU to become extremely hot. In addition, Prime95 stresses a computer far more than the majority of software based torture-suites. The nature of this is because the operating system usually shuts down the floating-point unit when unused by other programs, whereas Prime95 is well-optimized to continuously and effectively thread the FPU, causing it to be deeply pipelined, thereby generating significantly more heat because of elevated power consumption under the massive workload conditions. In CPUs which are not adequately cooled, errors are likely to occur. Prime95 also constantly accesses main memory at up to 60 MB per second. This constant activity will detect memory problems that other programs will not.

Lastly, power supply units of any machine running Prime95 are subject to the consistent ramifications of such harsh conditions. Power must be maintained clean, while providing adequate voltage, particularly to the CPU, RAM, and chipsets (mainboard chipsets such as the Northbridge where the memory controller may or may not reside; see Athlon 64 or Intel Core i7 for on-die memory controllers) to provide peak performance while maintaining stability. Cray Research used programs similar to Prime95 for over a decade for the purpose of stability testing.


Version 24 and older of Prime95 cannot test Mersenne numbers beyond <math>2^{79,300,000} -1</math> (Web Archive). This is slightly shorter than a 24 million digit number. Newer versions of Prime95 (version 25, 26 and 27) can handle Mersenne numbers up to the limit <math> 2^{596,000,000} -1</math> (see here).

Prime95 does not fully stress all processor threads when the threads number is more than 64 in Windows, or 32 for the 32-bit version. Windows will manage the processors in groups when the number beyond 64. Each group will only have maximum of 64. Prime95 will only load into one processor group.

Prime 95 and MPrime Release history

More details are located in the whatsnew.txt file.

Color Meaning
Red Former release
Pink Former test release
Green Current stable release
Gold Current test release
Blue Future release

Version Release date Significant changes
1 January 3, 1996
8 April 12, 1996 Trial factoring speeds improved
10 Self-test added, various changes to menus
12 May 22, 1996 Speed increased, memory usage decreased, more self-tests and error checks added, exponent limit increased to 2,630,000
12.1 Command line arguments added
12.3 Backup files are now generated every 30 minutes
12.4 Improvements in trial factoring and backup file generation
13.0 November 1, 1996 The Lucas-Lehmer assembly code was almost completely rewritten to increase modularity and readability.
13.1 More FFTs added
14.0 March 3, 1997 Exponent limit increased to 5,250,000
14.1 Pentium code improved
14.2 Trial factoring improved
14.3 Pentium Pro optimizations added
15.0 February 2, 1998 Prime95 can now automatically obtain assignments, configuration file system reworked
15.2 New HTTP protocol, support for Windows 95 services
15.4 More support for firewalls and proxies
16.1 May 22, 1998 Exponent limit increased to 20,500,000, factoring limit increased to 64 bits
16.3 Safety check added to prevent accidental credit transfer
16.4 Prime95 now shows progress in percentage
16.5 Networking code updated
17.0 Support for double checking
17.1 Prime95 can now do ECM factoring for numbers of the form 2n + 1
17.2 ECM speeds improved for 2n + 1
18.0 April 3, 1999
18.1 April 13, 1999 Change added to allow server to distinguish versions
19.0 October 10, 1999 Exponents of up to 79,300,000 supported, P-1 factoring added, save files for ECM factoring
19.1 AMD K6 optimizations
19.2 December 17, 1999
20.0 ECM and P-1 improvements
20.1 P-1 improvements
20.1 P-1 improvements
20.6 June 16, 2000
21.2 SSE2 support
21.3 Ability to skip redundant P-1 factoring
21.4 September 23, 2001 Torture test improved
22.1 Server and network code improved
22.2 Assignment optimizations
22.3 More FFT sizes, better error checking
22.6 Windows service support
22.7 SSE2 support for trial factoring on Pentium 4
22.8 August 15, 2002 Crossover FFTs
22.9 Windows tray UI improvements
22.10 October 8, 2002 More support for automatic start at bootup
22.12 November 9, 2002
22.13 December 29, 2002
23.1 February 10, 2003 SSE2 optimizations
23.2 February 20, 2003 More SSE2 optimizations
23.3 April 29, 2003 More SSE2 optimizations
23.4 More SSE2 optimizations
23.5 July 3, 2003 More error-checking
23.6 July 15, 2003
23.7 September 6, 2003 Changes in memory settings
23.8 February 17, 2004
23.9 September 16, 2003 Reduced memory requirement in torture tests
24.6 December 7, 2004 AMD Athlon CPU optimizations
24.10 Support for 64-bit Windows
24.11 March 30, 2005 AMD CPU optimizations
24.12 June 8, 2005 SSE2 optimizations
24.13 July 5, 2005 Different FFT implementations for certain processors
24.14 August 5, 2005
25.3 June 16, 2007 True multi-core support, introduced probable prime (PRP) tests, various other changes
25.4 August 10, 2007
25.5 September 24, 2007 PRP tests of (k*bn+c)/f
25.6 December 22, 2007
25.7 October 7, 2008
25.8 December 14, 2008
25.9 March 16, 2009 Speed improvements
25.10 May 3, 2009 Mac OS X and FreeBSD GUI support
25.11 July 14, 2009 Faster PRP tests
25.12 July 29, 2009
25.13 October 6, 2009
25.14 March 22, 2010
26.2 September 14, 2010 Optimized FFT code for many architectures; file size is greatly increased as a result
26.3 October 13, 2010
26.4 November 15, 2010 Support of up to 64 cores (for 64-bit systems)
26.5 February 23, 2011
26.6 April 8, 2011
27.1 alpha December 5, 2011 Intel AVX support (32 bit only)
27.2 alpha December 19, 2011 Includes all FFT lengths; first Linux/MPrime test version
27.3 beta February 16, 2012 64 bit AVX support
27.4 beta March 9, 2012
27.5 March 9, 2012 v27.5 was skipped
27.6 beta April 24, 2012 Bug fixes; the underlying math libraries went through a revision, but Prime95 was not modified.)
27.7 beta May 3, 2012 More bug fixes
27.7 May 15, 2012 Stable AVX support (for Sandy Bridge and Ivy Bridge CPUs, Core i3/i5/i7-2xxx and 3xxx models) totaling a ~30% performance increase relative to version 26.
27.9 December 12, 2012 Minor bug fixes.
28.5 May 30, 2014 AVX2 and FMA3 support for Haswell-based CPUs (Core i3/i5/i7-4xxx models), additional memory caching optimizations for Sandy Bridge and Ivy Bridge-based CPUs
28.9 Multi-threaded trial factoring in 64-bit executables
29.1 March 19, 2017 AVX-512 support for trial factoring, faster trial factoring for machines that support FMA
29.3 September 8, 2017 Jacobi error checking have been added to LL tests with 50% chance of detecting hardware errors between checks
29.4 November 3, 2017 Gerbicz error checking added to the PRP tests that almost guarantees correct results even on flaky hardware
Option to do PRP tests instead of LL tests with 4 new work preferences
New subproject: finding (probable) prime Mersenne cofactors
PRP tests now also support shift counts.
TBD TBD GPU support (however, there are other free programs created by community volunteers that use GPUs)


Main article: worktodo.txt

File worktodo.txt takes Lucas-Lehmer test assignments in the following formats:[1]



  • <ASSIGNMENT ID> - unique assignment ID generated by the PrimeNet v5 server as an anti-poaching measure
  • <EXPONENT> - Mersenne number exponent
  • <TRIAL FACTORING EXPONENT> - indicates power of 2 to which trial factoring had been attempted
  • <P-1 FACTORING>:
    • 0 indicates that p-1 factoring still needs to be done
    • 1 indicates that p-1 factoring attempted with no small factors found



See also

External links