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This Article was last modified in January, 1998.

The Cyrix 6x86 (originally codenamed the M1) was one of the first serious competitors to Intel's Pentium, and was able to outperform the Pentium in quite a few areas. One of the significant advantages of the 6x86 over the Pentium is the fact that the PR200+ brought with it a new bus speed of 75mhz to replace the old maximum of 66mhz. This bus speed can significantly increase the overall performance of a system, however, most motherboards with a Triton HX or HX chipset don't support this bus speed, so you'll need to buy a new motherboard if you plan to upgrade to this processor.

The 6x86 wasn't without its' problems though. The 6x86 was plagued by many software compatibility issues (many of which have been fixed, and patches can be obtained from Cyrix's website), of of the most serious ones being the NT writeback cache issue: the 6x86 is very sensitive to reflections on the different buses, so Microsoft decided to switch of the write back L1 cache on the 6x86 under NT in order to prevent crashes and instability. This disabling lead to serious performance decreases under NT. Owners of 6x86's which are revision 2.7 or higher won't be affected by this cache issue, but for people that don't have revision 2.7 or higher, a program called DirectNT will enable the writeback cache under NT. But please be cautious, this program may lead to crashes and instability.

Another problem with the 6x86 is its' extremely slow FP unit. The FPU in a 6x86 P166+ is roughly the equivalent of the FPU in a Pentium 90. This leads to very slow gameplay in Quake (believe me, I know) and other FPU intense applications. However, bear in mind that besides CAD programs and the odd Quake type game, not many applications rely heavily on the FPU performance of a processor.


Processor Performance Clock Speed
Bus Speed
PCI Bus Speed
6x86-P200+ 150 75 37.5 or 33 2
6x86-P166+ 133 66 33 2
6x86-P150+ 120 60 30 2
6x86-P133+ 110 55 27.5 2
6x86-P120+ 100 50 25 2
  • Clocking: 2x, 3x bus-to-core clock multiplier.
  • L1 Cache: 16-KByte; write-back; 4-way associative; unified instruction and data; dual-port address.
  • Bus: 64-bit external data bus; 32-bit address bus.
  • Pin/Socket: P54C socket compatible (296-pin PGA).
  • Fully compatible with x86 operating systems and software including Windows 95, Windows, Windows NT, OS/2, DOS, Solaris and UNIX.
  • Floating Point Unit: 80-bit with 64-bit interface; parallel execution; uses x87 instruction set; IEEE-754 compatible.
  • Voltage: 3.3V core with 5V I/O tolerance.
  • Power Management: System Management Mode (SMM); hardware suspend; FPU auto-idle.
  • Multiprocessing: Supports SLiC/MP(TM) and OpenPIC(TM) interrupt architecture.
  • Burst Order: 1-plus-4 or linear burst .


  • Superscalar architecture
  • Superpipelining
  • Register Renaming
  • Data Dependency Removal
  • Multi-Branch Prediction
  • Speculative Execution
  • Out-of-Order Completion
  • 80-bit Floating Point Unit (FPU)
  • 16-KByte Unified Write-Back Cache
  • Out-of-order processing
  • Data dependency solutions
  • Branch control

CPU-Central Scoring

Overclockability 50%
3D Performance 60%
Overall Performance 80%
Upgradability 80%
Compatibility 80%
Price 90%

Overclockability: In order for Cyrix to keep the prices of a 6x86 down, they had to be optimized for performance as much as possible, and therefore there is not much room left for improvement. In addition, these chips run extremely hot and might possibly overheat if they are overclocked for too long.

3D Performance: The FPU performance of a 6x86 is very weak. The FP unit in a 6x86 P166+ is roughly as fast as the FPU in a Pentium 90, and this leads to slower performance in FPU intense software, such as Quake.

Overall Performance: The 6x86 is quite a bit faster than it's equivalent Pentium Processor in business applications, however, in newer multimedia applications, the 6x86 performs adequately, but not near as well as the new batch of processors. Also, the 75mhz bus speed of the PR200+ helps significantly to increase overall performance.

Upgradability: These CPU's are just as upgradeable as any other Socket7 processor. However, newer CPU's that require split voltages may need a newer motherboard than the one's that are used with 6x86's.

Compatibility: Since almost all PC games and applications are fine tuned for Pentium CPU's, the 6x86 suffers some problems with some specific software. The most significant one (as I have mentioned earlier) being the writeback cache with NT issue. However, Cyrix has come out with patches to fix many of the problems, and newer 6x86's (rev. 2.7 or higher) do not have any problem running NT.

Price: 6x86 CPU's are much cheaper then their Pentium competitors, and the performance overall is about at a par.



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