[FPUs and 3D] [Cyrix] [AMD] [Intel] [Centaur]

CPU Considerations


Believe me, I have no desire to be an Intel fan. I'm a firm believer in the benefits of competition.  However, as of this time, there is no good alternative for Quake, Quake II, or any game that makes heavy use of the FPU (floating-point unit).  John Carmack of id has been quoted as saying that the floating-point issue blindsided everyone but Intel.  An AMD VP was quoted in boot magazine as saying that AMD didn't have time to optimize their FPU because they had to get their MMX components to market in short order.

Here are my observations /opinions.  Consider these opinions to have no basis in fact if they upset you.


FPUs and 3D

[Cyrix] [AMD] [Intel] [Centaur]

There are basically two phases of 3D computation. Phase 1 is geometry calculation (triangle set-up). Phase 2 is rendering. Both phases are or can be floating-point intensive. All 3D accelerators help with Phase 2. Some, like the Rendition Vérité video cards, can help speed up Phase 1 also. Ideally, Phase 1 should happen on the CPU because when it is done on the video card, that info must pass along the PCI (or AGP) bus back to the CPU. That is why we will see the AMD K6+3D and MMX2 from Intel -- Phase 1 will occur on the CPU (and some of Phase 2 as well). In any case, a slow FPU can still hurt either phase, even with a 3D accelerator present.

FPU architecture is also important here. The Intel chips have pipelined FPUs. Unlike the AMD and Cyrix counterparts, the Intel chips can execute multiple instructions in parallel. Quake and Quake2 both take advantage of this pipelined architecture. If you see claims that the AMD K6 has a faster FPU than the Pentium, those claims are true only of non-pipelined instructions. However, that is irrelevant for Quake addicts like me.


Cyrix

[FPUs and 3D] [AMD] [Intel] [Centaur]

Cyrix chips are great for general business apps, especially in Win95.  Under the right conditions, they will give a Pentium II a run for the money.

However, in FPU intensive applications (i.e. 3D games), consider a Cyrix 6x86 to be about equal to a Pentium MMX (P55) of half the speed.  In other words, if you have a 6x86 PR200, consider it to be about equal to a Pentium 100, or at most, a Pentium 133. That doesn't mean that it will be only half as fast, however. A Pentium 100 will run Quake or Quake2 at more than half the speed of a Pentium.

A Rendition Vérité card can help matters somewhat because the card does the triangle setup, which reduces the load on the CPU's FPU. How much does this help a Cyrix? I don't know. But here are my test results.

CPU Description FPS
Intel P200MMX GLQuake2 baseline listed on the Single-player tuning page

16.8
Cyrix 6x86L PR200+ Only the CPU has changed

12.3

CPU Description FPS
Intel P200MMX VQuake2 with "v1k_antialias 7" and "v1k_mode 3" enabled 14.2
Cyrix 6x86L PR200+ Only the CPU has changed 11.5

A 6x86MX (M2) FPU may be a little faster than the non-MX counterpart, but it still doesn't have the dual-pipelined FPU of a Pentium.

The good news is that Cyrix has announced that their next chip will have a dual-pipelined FPU like the Pentium. Me, I can't wait that long.

One other potential problem with the Cyrix chips:
Cyrix CPU + Rendition Verite + VIA chipset motherboard = Potential Disaster.

This is not Cyrix's fault, and it doesn't seem to affect everyone.  This is unfortunate, however, because the VIA2 (and VIA3, I presume) is present on the fastest Socket 7 motherboards.

One Caveat:  if buying a Cyrix will save you enough money to buy a 3D accelerator that you couldn't otherwise afford, then go for it. 3D accelerators make a much bigger difference than raw CPU power for 3D games.


AMD

[FPUs and 3D] [Cyrix] [Intel] [Centaur]

The AMD K6 is a great all-around CPU.  As noted earlier, however, the FPU is not up to Pentium standards as it lacks to pipelined architecture.  However, it is faster than a Cyrix.

I cut the CPU by about 20-25% when comparing to a Pentium MMX in FPU intensive games.  So if you have an AMD K6-200, I'd consider it about equal to a Pentium 150 (166 if you're really optimistic) for games like Quake. Just my opinion...

There is, however, a point of diminishing returns when it comes to FPU power, especially with a 3D accelerator. This will differ from game to game, and from 3D card to 3D card. With Quake and the v2x00 series, this point seems to be around a Pentium 166MMX and for Quake2, it seems to be around a Pentium 200MMX. It is therefore likely that the AMD (or IBM) K6-233 would do just as well as any Pentium in Quake (when using a v2x00), and a K6-266 would probably do just as well in Quake2.

The good news is that AMD has a K6+3D chip on the way.  It holds a lot of promise (or hype).  Only time will tell.

The 100MHz bus may even bring these Socket 7 CPUs to the top tier.


Intel

[FPUs and 3D] [Cyrix] [AMD] [Centaur]

It's the standard.  Consider MMX chips to be about equal to the next higher bin of the non-MMX variety.  In other words, a Pentium 166 MMX  (P55) will be about equal to a Pentium 200 non-MMX (P54).  Why?  Because the MMX chip has twice the internal cache, which can help to speed up things considerably.  

The Pentium 166 MMX has been the bargain buy of the year, especially because it is so overclockable.  A lot of people have theirs running at 187MHz, 200MHz, 225MHz, or even 250MHz.  However, the newest batch from Intel has the highest multipliers disabled, so 187  (75x2.5) or 208 (83x2.5) is the most you can get, depending on whether your motherboard supports the 75MHz or 83MHz bus speeds.

Pentium Pro's and Pentium II's have beefiest FPU's available for the x86 platform.  At the same clockspeed, I consider these monsters to be about 50% higher than the equivalent Pentium MMX.  So if you have a Pentium Pro 200, it's about equal to a Pentium 300MMX (I know, a fictional beast) for Quake.  Pentium Pro's can be slightly faster than the equivalent Pentium II because of the on-chip L2 cache.  However, the Pentium II's are available in much higher clock speeds and the Pentium II-233 is priced to kill the Socket 7 market. You can get one now (Jan 23, 1998) for about the same price as a Pentium (P55) of the same clock speed.

100MHz bus speeds may make the Socket 7 CPUs more competitive, but Intel's Slot 1 100MHz bus is just around the corner with the BX chipset.

I really don't want to be such an Intel fan...


Centaur

[FPUs and 3D] [Cyrix] [AMD] [Intel]

Consider these puppies to be in the same boat as the Cyrix.  Plus, they're more expensive.