Pentium MMXs were made with a margin in frequency, they are very overclockable. Also, due to the older process technology, they are much more durable than the newer CPUs.
In the other hand if Pentium MMX 200 isn't enough fast for you, it will make much more sense to use Pentium 3 or K6-3 and get a real performance leap instead of struggling with Pentium MMX, even overclocked. In those tasks for which it was created, it is fast and does not need overclocking.

Kruton 9000 wrote on 2022-11-29, 09:59:

Pentium MMXs were made with a margin in frequency, they are very overclockable. Also, due to the older process technology, they are much more durable than the newer CPUs.
In the other hand if Pentium MMX 200 isn't enough fast for you, it will make much more sense to use Pentium 3 or K6-3 and get a real performance leap instead of struggling with Pentium MMX, even overclocked. In those tasks for which it was created, it is fast and does not need overclocking.

I just wanted to see how fast/stable I could get that P200, no other reason besides that, I mentioned quake because it is a good way to measure performance.
I also wanted to see how well games could run on period correct hardware, see how spoiled we got over the years.

Almost all Pentium MMXs 166-200 MHz can be easily overclocked to 233 MHz. In 90's a lot of people run them overclocked for years without problems.

_tk wrote on 2022-11-29, 06:54:

As was mentioned, overclocking a Pentium MMX was pretty common back in the day. We all know that it works and what can be done. […]
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Paadam wrote on 2022-11-29, 06:38:

Funny to see that most people who say don't do it only say it based on nothing than theory (which isn't wrong in itself but certainly does not apply to specific case). But I guess it is always that way: there are people who talk and there are people who do 😀

As was mentioned, overclocking a Pentium MMX was pretty common back in the day. We all know that it works and what can be done. We've done it, period.

However, it's now 25+ years later and thus those same electronics are 25+ years old. Will it still work? Likely, assuming all parts are still within spec. Is it a good idea? Debatable. I'm not going to run any of my MMX processors at an overclock anymore because, as said, PII/PIII parts from that era are still relatively cheap and faster than an overclocked MMX will ever be. And I've owned my MMX processors since new and frying them doing a silly overclock is not something I want to do.

Otherwise, do you have any experience or knowledge to add to this?

There is no theory about this. Chips are designed with margin for manufacturing purposes, first, and a max speed spec, likely as advertised. They can also tolerate small swings in voltage too. Whether it was designed for max 233 mhz, 250 mhz, who knows. But running both higher voltage and higher speed that the public spec could be not intended. If they did, then they would have sold a 100mhz fsb capable chip.

I made a bit of a mistake though when I was thinking about it earlier. I was speaking about 2.8V IO, but the chip is obviously 3.3V IO. It's the core voltage being overvolted to support 100mhz fsb and giving an overclock. I wouldn't do that, and that is because of the physical design of the gates inside. If 2.8V was the max core spec, then I wouldn't for sure go above 2.92V, which who knows if the voltage regulator and demands on that core VDD will give you a perfect 2.9V. And pushing the chip, you may eventually burn out the large drivers inside the chip due to electromigration, or overheat just a part of the chip that produces more heat than normal. So if some people have chips that aren't exactly stable at 2.9V with 100mhz fsb, then you know these chips are at their limit.

I do admit that overclocking was pretty common on these chips back in the day.

I think I wasn't clear on what I'm doing, sorry.
I'm using the CPU at 100x2.0 at 2.8v, I did not have to increase the core voltage at all for that.
I only have to increase the core voltage if I try to use 100x2.5, which I'm not going to.
66x3.5 I cannot do because my motherboard does not allow it.

_tk wrote on 2022-11-29, 06:54:

Otherwise, do you have any experience or knowledge to add to this?

I have also a 233 MMX running at 3x100 on Asus P5A, for ~6 years. And yes, I have lots and lots of other platforms so it is not matter of why would I need to do this. It is because I want and I can. And it works.

gaffa2002 wrote on 2022-11-29, 15:49:

I think I wasn't clear on what I'm doing, sorry. I'm using the CPU at 100x2.0 at 2.8v, I did not have to increase the core volta […]
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I think I wasn't clear on what I'm doing, sorry.
I'm using the CPU at 100x2.0 at 2.8v, I did not have to increase the core voltage at all for that.
I only have to increase the core voltage if I try to use 100x2.5, which I'm not going to.
66x3.5 I cannot do because my motherboard does not allow it.

It's the CPU that doesn't allow it. 1.5x and 3.5x use the same motherboard jumper settings, and the CPU determines what to do.

debs3759 wrote on 2022-11-29, 18:15:

gaffa2002 wrote on 2022-11-29, 15:49:

I think I wasn't clear on what I'm doing, sorry. I'm using the CPU at 100x2.0 at 2.8v, I did not have to increase the core volta […]
Show full quote

I think I wasn't clear on what I'm doing, sorry.
I'm using the CPU at 100x2.0 at 2.8v, I did not have to increase the core voltage at all for that.
I only have to increase the core voltage if I try to use 100x2.5, which I'm not going to.
66x3.5 I cannot do because my motherboard does not allow it.

It's the CPU that doesn't allow it. 1.5x and 3.5x use the same motherboard jumper settings, and the CPU determines what to do.

Thanks for the correction. Anyway, I guess I'll stay with 100x2 instead 66x3. It's a bit faster and apparently stable.

Yeah, 100 FSB on an MMX processor is pretty great. I know when I boosted my 233 from 3.5 x 66 to 2.5 x 100, it boosted my Quake 2 FPS from about 20 to 30 in the first timedemo on a Riva 128. It's remarkable what a bottleneck the memory must have been.

Namrok wrote on 2022-11-29, 20:01:

Yeah, 100 FSB on an MMX processor is pretty great. I know when I boosted my 233 from 3.5 x 66 to 2.5 x 100, it boosted my Quake 2 FPS from about 20 to 30 in the first timedemo on a Riva 128. It's remarkable what a bottleneck the memory must have been.

Yeah, I got something around 5fps boost in quake (DOS version) at 340x240 resolution, it jumped from 34 to 38.3 to be more precise.

These chips were tough little guys. I've seen many a stock pentium with no cooling or just a thin aluminum heatsink. The OC'd p233 should be just fine with a proper heatsink and fan.

And if you want to be extra safe, buy a backup CPU for $10.

100fsb is fine.

Staying overclocked is okay. Just test for stability

Voltage is what kills. Keep it stock if you can

You could try setting SW3[4..8]=OFF,ON,OFF,ON,OFF. This should give you the missing/undocumented 3.0V for Vcore.
Test this with a multimeter at hand and WITHOUT CPU first since it's a little guess based on the other combinations for Vcore. But it should work.
Personally this would be the maximum I would feed a standard P-MMX with.

Then you could try setting the FSB to 75MHz and the multi to 3.0 and see if 225 MHz are stable (should be at stock Vcore of 2.8V with any MMX-200).
If this works try setting the FSB to 95 MHz and the multi to 2.5 for 238 MHz. This should also be reachable with most P-MMX at 2.8 or maybe 2.9V.
Last step, repeat 2.5 x 100 MHz at 3.0V Vcore. Maybe this is the little bit it needs to run stable if 2.9V isn't enough for this. If it needs 3.0V (or more) you know you have reached the limit of your chip/system. Then you can opt to go back to the 95 MHz setting for increased memory throughput but without the risk of stressing you CPU too much.

Just buy a bunch of pmmx cpus and test them for optimal voltage/clock ratio if you're concerned.
The bad samples can be kept as "burner cpus".
I have one pmmx200 running at 310mhz(124 x 2.5) which, at 3.1V, happily tics towards its demise.

The fsb doesn’t have much to do with the cpu. If the mobo can handle 100fsb (any ss7 should) then just set that.

As far as core, most pentium mmx chips can do 250mhz pretty easily at 2.8v so 100x2.5

Mandrew wrote on 2022-11-29, 08:40:

It will decrease the lifespan so it will die within 2 hours - 300 years, give or take a century. Be careful!

This.

While the lifespan reduction is definitely real, depending on the overvoltage or overheating (or both), the lifespan you loose might be negligible. Someone I know found a Pmmx in the dumpster which had its fan that failed and was powered on 3.3V. The color of the PCB changed, but it still works fine. I myself have found a IBM aptiva which originally came with a pmmx 233, but when I removed the heatsink, it wasn't there anymore, instead there was a K6-2 500 ... still configured as a pmmx 233, so it was overvolted by 0.6V and downclocked to 233MHz for many years and still worked ok.
These are extreme situations that shouldn't be reproduced, but it shows how sturdy these chips are. It's still hard to tell how long these chips will truly last (overclocked or not), but they seem to be doing very well so far.

It’s funny,

People always come onto these forums thinking that their cpus have a ticking timer in them that accelerates soon as they overclock them a little bit but here we are playing with 486 chips some 30?40? years later and basically all of them work. Many have had serious fan trouble also.

And those of us that have seen chips fry we know what happened, they were put in backwards, went nuclear because of no heatsink and super high tdp, were cracked, over volted.

Yeah, don’t worry.

Quick update, switched back to 3x66 as the computer was restarting randomly at 100x2. Maybe changing voltage to 2.9 could solve the issue but I rather not mess with it.
Well, never had any luck overclocking CPUs or GPUs anyway...

gaffa2002 wrote on 2022-12-05, 16:43:

Quick update, switched back to 3x66 as the computer was restarting randomly at 100x2. Maybe changing voltage to 2.9 could solve the issue but I rather not mess with it.
Well, never had any luck overclocking CPUs or GPUs anyway...

Yeah, stay away from 2.9V, because it will cause a chain reaction that would unravel the very fabric of the space-time continuum and destroy the entire universe! 😁

Seriously, though, I've ran these chips at 3.1V / 292 MHz for EXTENSIVE periods of time (1 - 2 months / 3 - 4 hours a day). As long as you have adequate cooling (a bigger socket 370 cooler would be perfect), then you have absolutely nothing to worry about at 3.1V.
And as for 2.9V... yeah... any socket 7 heatsink will do and there is absolutely NO risk of damaging your CPU.

In other news... am I missing something? 😀 These CPUs are not rare, you can usually get a Pentium MMX for ~ $10 - $15 (sometimes for less/free). So even if you were to destroy one, it's not the end of the world. 😀

If 100 isn’t stable it is probably a mobo problem