Karre´s UM 8810 PAI-O

From the graph shown above, it is clear that BTB had the largest impact for ALU-focused processes, with a 22% boost. BTB, or branch prediction, on a Cyrix 5x86 is generally not considered a stable setting in Windows except possibly with Stepping 1, Revision 3 CPUs. To get BTB working on Stepping 1, Revision 3 CPUs, it is necessary to disable LOOP, BWRT, and possibly RSTK. The CPU used in this study was Stepping 0, Revision 5. It was possible to run the noted Windows benchmark tests with LOOP, BWRT, and RSTK disabled, however the only way to boot into Windows was to first boot into DOS, then type win at the command console to enter Windows. BTB appears stable in DOS with both revisions of the CPU. To date, no other CPU revisions have been encountered. The Cyrix 5x86-80 and 5x86-100 came in Stepping 1, Revision 3 editions, whereas Stepping 0, Revision 5 CPUs came in 100, 120, and 133 MHz flavours.

It was recently discovered that Windows NT4/98SE/2000 all initially appear usable with a Stepping 0, Revision 5 CPU and all Cyrix features enabled (including BTB) except for LOOP, RSTK, BWRT, and DTE. Stability, however, had the tendency to decrease as the CPU was run longer (and began to heat up). This effect may be more of a consequence of running the CPU overclocked and above thermal spec for core voltage than with a broken feature. Some Cyrix features may be more frequency and/or thermal sensitive than others.

Referring to Appendix 6, we see that BTB did not have such a magnificent impact for DOS-only ALU tests; in DOS, performance boost dropped to only 5%. An interesting point to note from the DOS-only ALU chart is that LOOP yielded a performance gain only when used in combination with BTB, thereby bumping the results up another 1%. Also surprising was that RSTK seemed to have no effect on its own accord. Unfortunately, LOOP and BTB together were not very stable. They may only work together in 16-bit mode since Windows would not boot with this setting and 3Dbench, Doom, Pcpbench, and Quake wouldn’t run. It may be that BTB and LOOP work well together with Stepping 1, Revision 3 CPUs, however this configuration wasn’t been tested.

Next in line for performance was LSSER at 7.4%. It was previously thought that LSSER needed to be disabled (set to 1) for motherboards which contained in-use PCI slots, however the author has had LSSER enabled (set to 0) for years with 3 filled PCI slots and hasn’t had issues. From personal experience, the Biostar MB8433-UUD and PC Chips M919 both functioned with LSSER enabled.