It depends if it will even work reliably undervolted at default speed setting. You might even have to underclock the cpu. As Cyrix cpus were known to run on the limit - that's why they run so hot.

Perhaps you could look into the Cyrix MII, there are 2.2v parts which run up to 300MHz and are actually fairly cool. No need to underclock and you'll have better performance (and a real Cyrix!) as well 😀

Glitter,
Are you talking about the Socket 5/7 6x86 ones. I suppose you do.
There was the 6x86L model. The L meaning lower voltage. This processor is usually a split voltage model and therefor requires socket 7. This solved most of the heat problems with the original 6x86. The 6x86MX and MII are successors of the 6x86L and are also split voltage, normally 2.9V.

Edit: You already mentioned 2.9V yourself, must be an L, MX or MII already, so what Kixs and Tetrium said then. Just try it.

Tetrium wrote:

Perhaps you could look into the Cyrix MII, there are 2.2v parts which run up to 300MHz and are actually fairly cool. No need to underclock and you'll have better performance (and a real Cyrix!) as well 😀

Well, I would be very happy to find one these CPU, but either I'm not looking at the right place, either they are hard to find.

gerwin wrote:

Edit: You already mentioned 2.9V yourself, must be an L, MX or MII already, so what Kixs and Tetrium said then. Just try it.

Oh yes, sorry, I forgot that little detail.
I just received a MII-366 (2.5x 100 MHz) for only 15€, I just hope it's working fine.
Yep because I haven't the motherboard yet.

Glitter wrote:

Tetrium wrote:

Perhaps you could look into the Cyrix MII, there are 2.2v parts which run up to 300MHz and are actually fairly cool. No need to underclock and you'll have better performance (and a real Cyrix!) as well 😀

Well, I would be very happy to find one these CPU, but either I'm not looking at the right place, either they are hard to find.

They are very hard to find, I have not seen a single one for sale on Ebay the last month. I found one a bit over a year ago though, I think I paid about 25 euro + shipping for it and that was an untested CPU with bent pins.

It was a 4x66 MHz version, it did end up working fine and it is able to run at 300 MHz, it wont go much higher though.

Glitter wrote:

I just received a MII-366 (2.5x 100 MHz) for only 15€, I just hope it's working fine.
Yep because I haven't the motherboard yet.

Yes, that must be the one I saw for sale in ebay france. A 2.9V one. Looked like new. Earlier he had a MII-400GP there as well.

I wonder, are the 2.2V MII chips made with another process, or just selected from the normal 2.9V production? The strange thing is that there is nothing in between 2.2V and 2.9V.

Here I have one MIIv 266GP 2.0V and two different variants of MIIv 300GP 2.2V. The most expensive one was $16 incl. shipping (lucky?). The 266GP can run at 1.7V too, at 2.0x75=150MHz. That one does not support 4.0x though, 3.5x is the max.

The 6x86L 2.9V has a fun setting: 1.0x: In speedsys at 60MHz that is comparable to a 486DX/4-100. It can run 2.0x60=120MHz at 2.0V. But for 3.0x60=180MHz the voltage had to be increased to 2.7V.

gerwin wrote:

Yes, that must be the one I saw for sale in ebay france. A 2.9V one. Looked like new. Earlier he had a MII-400GP there as well.

I wonder, are the 2.2V MII chips made with another process, or just selected from the normal 2.9V production? The strange thing is that there is nothing in between 2.2V and 2.9V.

Damn, I missed a MII-400 ??? 🤣

About process, I remember that National Semi started to produce cpus using their own 0.25µ and the performance weren't better than the IBM 0.35µ but according to that website, only Media GXM were produced using it.
http://www.cpu-collection.de/?tn=&l0=md&l1=1999&l2=Cyrix

Because without a specific process, I don't understand how you can have two bins with such a gap with the voltage.

But about that website, there was some 0.25µ M2 and even 0.18µ
http://www.cpushack.com/2012/10/31/cyrix-josh … s-to-the-bible/

I got mine from CPU world, but that was years ago. They were actually quite easy to find and I obtained several of them that way. Most common was the silvery 4x66MHz one.

I think the 2.2v ones are made with a dieshrink, but it's not clear to me as to which exact dieshrinks were used with which chips. Apparently Cyrix used several different ones and all of these run at similar voltages (mostly 2.2v).

I did find a website a couple months ago that explained a bit more, but info seems to be a bit sketchy.

Glitter: What is the intended machine of use and target software?

I have been able to run the silver Cyrix MII-333's 2.2V chips at 300 MHz. There are also 2.2V MII-366, 400, and 433's. Nearly all MII-366's were 2.9V, although I do have one marked at 2.2V. I'm guessing the MII-366 chips marked for 2.2V failed qualification as MII-400 chips. All the MII-400 chips I tested ran well at 300 MHz. I wish I knew which qualification software Cyrix was using and which test failed for rating them for 300 MHz.

Luckily, the MII-400's aren't too difficult to find as they are frequently mixed in with scrap gold lots. I found most of my MII-400's from scrap gold lots whereby the eBay seller was kind enough to remove it from the lot and sell it separately. The MII-433, on the other hand, is nearly impossible to find. I bought mine from a CPU collector many years ago. Some MII-400's will run respectably at 333 MHz, however at 350 MHz, more aggressive benchmarks failed.

feipoa wrote:

Glitter: What is the intended machine of use and target software?

I will try to set a dual boot, Win98 to play some games and BeOS/Haiku.

feipoa wrote:

Luckily, the MII-400's aren't too difficult to find as they are frequently mixed in with scrap gold lots. I found most of my MII-400's from scrap gold lots whereby the eBay seller was kind enough to remove it from the lot and sell it separately.

Wait a second, when someone is selling with written "scrap gold" those CPU might still be functionnal ?
I thought it was only defected units.

Glitter wrote:

Wait a second, when someone is selling with written "scrap gold" those CPU might still be functionnal ?
I thought it was only defected units.

At best they have bent pins. But they could be missing pins and in the end could be just dead. You never know what you'll get. Buy only if it's really cheap.

Intel and AMD 486 CPUs and Intel Pentium and Pentium MMX socket 7 CPUs have a very very good chance of working and rarly miss pins as the pins can really take a beating without snapping. I have saved CPUs with all pins flat against the underside of the CPU as if someone stepped on it. 😀

With many other CPUS it's a gamble and with some it's a snowballs chance in hell. Socket 478 and S754/940/939/AM2/AM3 CPUs with lots of bent pins are a total waste of time as the pins are very very easy to break off.

Win98 with BeOS - oh you are a rebel! I like it!

I liberated a MII-400 from a scrap lot which had the crap beaten out of it pin-wise. I spent more than an hour to straighten all the pins and the chip worked. Here is an image of it Re: Cyrix MII-433GP Build That is actually my worst looking MII-400, but isn't all that bad for a salvage.

Glitter wrote:

But about that website, there was some 0.25µ M2 and even 0.18µ
http://www.cpushack.com/2012/10/31/cyrix-josh … s-to-the-bible/

I read that too. Interesting article.
0.18µ process is also the one used to make coppermine core Pentium IIIs. A coppermine, at conservative speeds, can run on just 1.3V. But the lowest my CT-5RSA0 Super Socket 7 Motherboard can set is 1.7V. The Asus P5A bottoms out at 2.0V.

feipoa wrote:

Luckily, the MII-400's aren't too difficult to find as they are frequently mixed in with scrap gold lots. I found most of my MII-400's from scrap gold lots whereby the eBay seller was kind enough to remove it from the lot and sell it separately. The MII-433, on the other hand, is nearly impossible to find. I bought mine from a CPU collector many years ago. Some MII-400's will run respectably at 333 MHz, however at 350 MHz, more aggressive benchmarks failed.

A few hours ago I bough one of these too, a working MII-433GP 2.2V that is! 3x100MHz. It was reasonably priced I guess, considering your remark on rarity. But now I will have to take care not to wear out the markings any further when connecting a heatsink, to prevent it becoming an unmarked Cyrix. Instead better to use one drop of water and let gravity hold the heatsink.

gerwin wrote:

Glitter wrote:

But about that website, there was some 0.25µ M2 and even 0.18µ
http://www.cpushack.com/2012/10/31/cyrix-josh … s-to-the-bible/

I read that too. Interesting article.
0.18µ process is also the one used to make coppermine core Pentium IIIs. A coppermine, at conservative speeds, can run on just 1.3V. But the lowest my CT-5RSA0 Super Socket 7 Motherboard can set is 1.7V. The Asus P5A bottoms out at 2.0V.

Would it be possible to set the CPU voltage on the motherboard really low and on top of that add one of those s7 CPU adapters with their own VRM and set the CPU adapter really low as well so it lowers the CPU voltage even further?

gerwin wrote:

feipoa wrote:

Luckily, the MII-400's aren't too difficult to find as they are frequently mixed in with scrap gold lots. I found most of my MII-400's from scrap gold lots whereby the eBay seller was kind enough to remove it from the lot and sell it separately. The MII-433, on the other hand, is nearly impossible to find. I bought mine from a CPU collector many years ago. Some MII-400's will run respectably at 333 MHz, however at 350 MHz, more aggressive benchmarks failed.

A few hours ago I bough one of these too, a working MII-433GP 2.2V that is! 3x100MHz. It was reasonably priced I guess, considering your remark on rarity. But now I will have to take care not to wear out the markings any further when connecting a heatsink, to prevent it becoming an unmarked Cyrix. Instead better to use one drop of water and let gravity hold the heatsink.

You got the one for 35 euro off eBay? That is a great price. I guess the seller didn't know what he had. Yours has the small MII, while my 433 has the big MIIv. Personally, I really like the look of the small MII. Anyway, nice find! You planning to setup a system around it? Due to the rarity of the MII-433, I am using the MII-400 at 300 MHz in a Tyan super7 board.

Indeed I have 😀 . The description said: "The best Cyrix MII (Very Rare)". To compare: In 2015 I noted that an auction for a 366GP 2.2V ended up at E60,-. The good thing is that from now on I won't waste my time looking at photos of gold scrap lots...oh wait, I still want a Cx5x86-133 4x. 🤣

That 433GP will first be put on the test bench. Not to overclock, but to see what multipliers it supports and to find how it fares with voltages below 2.2V. The problem and fun with Socket 7 processors is that each model has particular strenghts and advantages whilst lacking in other areas. It is hard to decide between Cyrix 6x86L, Cyrix MII 2.2V, AMD K6-III+ or maybe Pentium Classic/MMX with Test Register support.

Tetrium wrote:

Would it be possible to set the CPU voltage on the motherboard really low and on top of that add one of those s7 CPU adapters with their own VRM and set the CPU adapter really low as well so it lowers the CPU voltage even further?

That is a funny idea, but I don't think it works that way. I do know that diodes can cut 0,6 to 0,7V regardless of the input voltage.

60 EU for MII-366 2.2 V? WOW! I should sell mine to a collector then. I couldn't get my MII-366 2.2V to show any sign of life, so it is useless for my purposes, but most collectors don't seem to care about using the chips, so...

I haven't yet put my 433GP on the test bench. I am overwhelmed with computer ambitions but time isn't on my side. I'm looking forward to your testing.

There was a Cyrix cx5x86-133/4x in a scrap lot on eBay within the last 6 months.

If you need a little less voltage drop than a standard 0.7V diode, then a typical schottky diodes drop 0.3V +-0.1V. There are also zener diodes which can be operated reverse bias and have all sorts of voltage drops you can select when ordering, however voltage drops are typically in the 2-40 V range.

feipoa wrote:

60 EU for MII-366 2.2 V? WOW! I should sell mine to a collector then. I couldn't get my MII-366 2.2V to show any sign of life […]
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60 EU for MII-366 2.2 V? WOW! I should sell mine to a collector then. I couldn't get my MII-366 2.2V to show any sign of life, so it is useless for my purposes, but most collectors don't seem to care about using the chips, so...

I haven't yet put my 433GP on the test bench. I am overwhelmed with computer ambitions but time isn't on my side. I'm looking forward to your testing.

There was a Cyrix cx5x86-133/4x in a scrap lot on eBay within the last 6 months.

If you need a little less voltage drop than a standard 0.7V diode, then a typical schottky diodes drop 0.3V +-0.1V. There are also zener diodes which can be operated reverse bias and have all sorts of voltage drops you can select when ordering, however voltage drops are typically in the 2-40 V range.

There was a lot of (I think) 5 IBM 5x86 from 1996 on Ebay recently and all of them were purchased by at least 2 Vogoners. These may or may not have x4 and may or may not be able to run at 133MHz.