Editing Pentium Pro (section)

===Performance===
Despite being advanced for the time, the Pentium Pro's out-of-order register renaming architecture had trouble running [[16-bit computing|16-bit]] code and mixed code ([[8-bit computing|8-bit]] with 16-bit (8/16), or 16-bit with [[32-bit computing|32-bit]] (16/32), as using partial registers cause frequent pipeline flushing.<ref>{{cite web |url=http://qcd.phys.cmu.edu/QCDcluster/intel/vtune/reference/LipsPro_Partial_Stall.htm |title=Partial Register Stall Warning |work=VTune Performance Analyzer online help |archive-url=https://web.archive.org/web/20170830055933/http://qcd.phys.cmu.edu/QCDcluster/intel/vtune/reference/LipsPro_Partial_Stall.htm |archive-date=August 30, 2017 |url-status=dead}}</ref> Specific use of partial registers was then a common performance optimization, as it incurred no performance penalty on pre-P6 Intel processors; also, the dominant operating systems at the time of the Pentium Pro's release were 16-bit [[DOS]], and mixed 16/32-bit [[Windows 3.1x]] and [[Windows 95]] (although the latter requires a 32-bit [[i386|80386]] CPU as a minimum, much of its code is still 16-bit for performance reasons, such as the 16-bit [[Windows USER]] [[dynamic link library]], [[Windows USER#Implementation|user.exe]]). This, along with the high cost of Pentium Pro systems, led to tepid sales among PC buyers at the time. To fully use the Pentium Pro's [[P6 (microarchitecture)|P6 microarchitecture]], a fully 32-bit operating system is needed, such as [[Windows NT]], [[Linux]], [[Unix]], or [[OS/2]]. The performance issues on legacy code were later partly mitigated by Intel with the Pentium II.

Compared to RISC microprocessors, the Pentium Pro, when introduced, slightly outperformed the fastest RISC microprocessors on integer performance when running the [[SPECint|SPECint95]] benchmark,{{r|MPR 1995-11-13|p=2}} but floating-point performance was significantly lower, half that of some RISC microprocessors.{{r|MPR 1995-11-13|p=3}} The Pentium Pro's integer performance lead disappeared rapidly, first overtaken by the [[MIPS Technologies]] [[R10000]] in January 1996, and then by [[Digital Equipment Corporation]]'s EV56 variant of the [[Alpha 21164]].<ref name="MPR 1996-07-08">{{cite magazine |last=Gwennap |first=Linley |date=July 8, 1996 |title=Digital's 21164 Reaches 500 MHz |magazine=[[Microprocessor Report]]}}</ref>

Reviewers quickly noted the very slow writes to video memory as the weak spot of the P6 platform, with performance here being as low as 10% of an identically clocked Pentium system in benchmarks such as VIDSPEED. Methods to circumvent this included setting VESA drawing to system memory instead of video memory in games such as ''[[Quake (video game)|Quake]]'',<ref>{{Cite web|url=https://github.com/id-Software/Quake/blob/master/WinQuake/data/TECHINFO.TXT|title=Quake/TECHINFO.TXT at master · id-Software/Quake|website=[[GitHub]]|date=November 25, 2022|access-date=February 10, 2019|archive-date=June 10, 2017|archive-url=https://web.archive.org/web/20170610101303/https://github.com/id-Software/Quake/blob/master/WinQuake/data/TECHINFO.TXT|url-status=live}}</ref> and later on utilities such as FASTVID emerged, which could double performance in certain games by enabling the [[write combining]] features of the CPU.<ref>{{Cite web|url=http://www.gamers.org/dEngine/quake/info/techinfo.09|title=Quake Technical Information file}}</ref><ref>{{Cite web|url=https://www.mdgx.com/umb.htm#FAS|title=MDGx Complete UMBPCI.SYS Guide|at=Fast Video|website=MDGx MAX Speed WinDOwS Tricks + Secrets|access-date=January 7, 2023|archive-date=January 7, 2023|archive-url=https://web.archive.org/web/20230107204339/https://www.mdgx.com/umb.htm#FAS|url-status=live}}</ref> [[Memory type range register]]s (MTRRs) are set automatically by Windows video drivers starting from 1997, and from there the improved cache/memory subsystem and FPU performance caused it to outclass the Pentium clock-for-clock in the emerging 3D games of the mid–to–late 1990s, particularly when using [[Windows NT 4.0]]. However, its lack of [[MMX (instruction set)|MMX]] implementation reduces performance in multimedia applications that made use of those instructions.