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Which Pentium IIs can be underclocked?

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First post, by AdamP

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Hi,

I have a Compaq Deskpro EP 6400/10 with a Pentium II 400/100 SL2U6 CPU and 440BX motherboard which I tried to underclock using the motherboard switches, but nothing happened (it still reported it as 400/100 and I didn't notice any slowdown). I know that the clock and FSB speeds of Pentium IIs produced after a certain date are locked. I assume that's why I can't underclock it (I hope I'm not making a premature assumption, although I can't see why else), so I was wondering if anyone here knows which ones can be underclocked.

I wish to underclock it so some of my DOS games such as Theme Park can potentially run better (I hope. And by better, I mean not too fast.).

Thanks in advance.

Reply 1 of 202, by gerwin

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your pentium code has a core stepping of dB0. You will have better luck with dA0, thus an older stepping. Fortunately Pentium II CPU's are almost for free these days.

Gerwin wrote:
To summarize, we have these Deschutes Core Pentium II's (no celeron): The P-II 333/66MHz and 350/100MHz, with 5.5 ns L2 cache. T […]
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To summarize, we have these Deschutes Core Pentium II's (no celeron):
The P-II 333/66MHz and 350/100MHz, with 5.5 ns L2 cache.
The P-II 400/100MHz, with faster 5.0 ns L2 cache with a slower timing and heatsink connection.
The L2 cache is in the form of external chips which run at half the CPU speed.

The above three can potentially be found multiplier limited, at least when produced before week 34 1998, but maybe even afterwards. When multiplier limited they accept a multiplier of 2.0x and 2.5x, but disable the L2 cache at these settings. They run properly at multiplier 3.0x and up, until a certain limit which depends on the CPU type and the FSB speed. These CPU's are not too good for overclocking because their external L2 cache chips become the bottleneck.

source: Slowing a 440BX based PC

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Reply 2 of 202, by AdamP

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Thanks

My slot1 fsb speed is 100mhz. I think the slowest clock speed therefore, is 350mhz, which isn't that much of a difference. Would a 66mhz fsb Pentium II work?

Reply 3 of 202, by gerwin

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AdamP wrote:

Thanks

My slot1 fsb speed is 100mhz. I think the slowest clock speed therefore, is 350mhz, which isn't that much of a difference. Would a 66mhz fsb Pentium II work?

If you prefer that one, sure it will work. But the 400 or 350 MHz top speed should not be a problem as long as you can underclock to 200MHz or even 133Mhz within the mainboard BIOS. This depends just as much on your mainboard though: Some mainboards don't have the multiplier settings and some mainboards don't allow setting the FSB to 66MHz when a 100Mhz FSB CPU is installed.
Feel free to read the thread I linked to if you want to know more about the subject.

PS. in your initial post I read you cannot underclock the FSB speed to 66MHz? This is strange, because with almost any CPU the multiplier can be factory locked, but the FSB speed on the other hand is just a mainboard setting that the CPU has to accept. So you should be able to run your particular CPU at 266 MHz @ 66MHz FSB, and if not then that is because of your mainboard.

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Reply 5 of 202, by gerwin

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leileilol wrote:

I've underclocked a p2 233MHz (Klamath, 66MHz FSB) to 180MHz with H.Oda!'s SoftFSB and that's as far as I've ever gotten without a hard freeze.

133Mhz is possible when you change the multiplier to 2x whilst setting the FSB to 66MHz. That is if both CPU and mainboard support this. I suppose you pulled the FSB below 66... interesting

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Reply 6 of 202, by AdamP

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Unfortunately, my BIOS/CMOS setup (or whatever it's called) doesn't have any underclocking options. The only clocking options I have are the motherboard DIP switches. I think the clock speeds available depend on the fsb speed as well; a table on the motherboard shows the available frequencies for 100mhz fsb are; 350, 400, 450 and 500 mhz and 66mhz fsb are; 233, 266, 300 and 333mhz.

What about DA1 Pentium IIs? Are they underclockable?

Is there any way to tell precisely when my Pentium II was produced? My BIOS is dated 24th July 1998, so my Pentium II may well have been produced at around the time they locked the multiplier.

Reply 7 of 202, by Old Thrashbarg

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Any PII/PIII can be underclocked just by setting the bus to 66mhz. Even if the motherboard doesn't allow you to set it in BIOS or with jumpers, you can just ground both BSEL pins manually and have it work.

Reply 8 of 202, by gerwin

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Old Thrashbarg wrote:

Even if the motherboard doesn't allow you to set it in BIOS or with jumpers, you can just ground both BSEL pins manually and have it work.

I have a CPU here I want to modify this way, as to make it default on 66 instead of 100 MHz FSB. But I could not quite decide on how exactly to do it, and do it without breaking the attached plastic cover...

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Reply 9 of 202, by swaaye

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All of the Klamath chips are multiplier unlocked and 66 MHz FSB. Last Intel chip like that until they brought Speedstep to the desktop, I think.

So I would think Klamath is the clear choice then? I've run one at 133 MHz to do my 133MHz challenge.

The only mobos without FSB selection in jumper or BIOS are OEM stuff, like what u have Adam. Annoying huh? 😁 Of course, the original Slot 1 boards may only have 60 and 66 MHz because they didn't officially support anything faster.

Reply 10 of 202, by Old Thrashbarg

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gerwin wrote:

But I could not quite decide on how exactly to do it, and do it without breaking the attached plastic cover...

Well, basically it would just involve connecting pin B21 to ground. If you don't want to crack apart the CPU casing, then... just off the top of my head... on the back of the motherboard itself, you could solder a wire between the B21 of the slot and a ground of your choice, perhaps with a switch in the middle so you can choose between bus speeds.

swaaye wrote:

All of the Klamath chips are multiplier unlocked

Not quite. AFAIK all the 233mhz ones were unlocked, and most (if not all) 266mhz ones as well. But a good many of the 300mhz ones were locked.

Last edited by Old Thrashbarg on 2010-04-13, 00:32. Edited 1 time in total.

Reply 11 of 202, by gerwin

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swaaye wrote:

All of the Klamath chips are multiplier unlocked and 66 MHz FSB. Last Intel chip like that until they brought Speedstep to the desktop, I think.

So I would think Klamath is the clear choice then? I've run one at 133 MHz to do my 133MHz challenge.

P-II Klamath is interesting, but P-II Deschutes is just as interesting. Deschutes goes a little higher up to 450MHz whilst running cool. The Klamath does not go there. But I must admit you proved it runs at 133MHz@66FSB with L2 cache working, whilst the Deschutes disables the L2 at 133MHz@66FSB. Now I am wondering how a Klamath responds to a 1.5x multiplier setting, or a multiplier above its factory default (I even heard one rumour that it will run 66MHz@66FSB under some circumstances?).

Old Thrashbarg wrote:

Well, basically it would just involve connecting pin B21 to ground. If you don't want to crack apart the CPU casing, then... just off the top of my head... on the back of the motherboard itself, you could solder a wire between the B21 of the slot and a ground of your choice, perhaps with a switch in the middle so you can choose between bus speeds.

Thanks I will try that.

Old Thrashbarg wrote:
swaaye wrote:

All of the Klamath chips are multiplier unlocked

Not quite. AFAIK all the 233mhz ones were unlocked, and most (if not all) 266mhz ones as well. But a good many of the 300mhz ones were locked.

well, I expect all P-II klamath cores to be multiplier limited at least, meaning they do accept a multiplier lower then the factory default.

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Reply 12 of 202, by Old Thrashbarg

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Well, the 300mhz ones I have don't accept any multiplier other than the default, same as the later PII/PIII chips.

But the unlocked ones are, to my knowledge, completely unlocked... the only limit with those is motherboard support. A lot of board makers didn't really anticipate people wanting to run 1X and 1.5X multipliers and such, so they didn't tend to include proper support for such low multi's.

Reply 13 of 202, by AdamP

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Would my 100mhz slot 1 accept a 66mhz fsb Pentium II? I'd rather not have to open the CPU and change anything and I can't see what good it'll do if my motherboard doesn't have an option to change the fsb speed (unless there's some BIOS update that has such an option?) and the fact that I can only underclock by 50mhz (350mhz is the lowest clock speed of 100mhz fsb Pentium IIs), makes the effort of opening it not worth it, and Pentium IIs are cheap so it's probably easier to just replace it anyway.

If I go for a Klamath, would I need to get any additional cooling?

Reply 14 of 202, by Old Thrashbarg

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Come to think of it, I don't know if that Compaq will take a Klamath. They're not really known for wide ranges of CPU support in their boards, and it wouldn't surprise me a bit if they dropped support for the older cores in order to save a few cents on the voltage regulator and such.

A 266mhz Deschutes would probably work fine, though, if you can find one. It was step code SL2W7, if I remember correctly.

Reply 15 of 202, by swaaye

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I've run my current PII Klamath 233 at 2x66, 3.5x66, 4x66, and 4.5x66.

If you can get a Klamath cheap, why not try it. They don't run exceptionally hot, but they are somewhat hotter than the Deschutes chips. I'd just get a CPU that has a cooler included to keep things simpler. There are a couple of different cartridge designs so you can't use late model Deschutes or Katmai coolers on Klamath or most Deschutes PIIs.

Sounds like it's best to stick with the 233 or 266 CPUs to make sure it's unlocked.

Reply 16 of 202, by gerwin

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Old Thrashbarg wrote:

Well, the 300mhz ones I have don't accept any multiplier other than the default, same as the later PII/PIII chips.

That is unfortunate...did they produce these klamath's after week 34 1998 or something?

Old Thrashbarg wrote:

Come to think of it, I don't know if that Compaq will take a Klamath. They're not really known for wide ranges of CPU support in their boards, and it wouldn't surprise me a bit if they dropped support for the older cores in order to save a few cents on the voltage regulator and such.

A 266mhz Deschutes would probably work fine, though, if you can find one. It was step code SL2W7, if I remember correctly.

I never tried it but shouldn't a compaq slot-1 board be prepared to take a slot-1 celeron with 66MHz FSB? Either way an OEM mainboard usually lacks all the fancy tweaking options.

Don't just take any 266 or 300MHz Deschutes model, if core stepping is dB0 it will certainly be completely multiplier locked. I think the 333MHz is the easiest Deschutes for underclocking, many of these are stepping dA0, dA1 should also be OK, but I am not 100% sure about dA1. I have a SL2QF with dA0 stepping and it takes multiplier 2.0x to 5.0x, it also works fine on a 100MHz FSB. (but again, no L2 cache at multiplier 2.0x and 2.5x)

swaaye wrote:

If you can get a Klamath cheap, why not try it...
Sounds like it's best to stick with the 233 or 266 CPUs to make sure it's unlocked.

I intent to try a klamath, but I will be going abroad next week already... so me should either hurry or postpone it. I have this Aopen mainboard with a 1.5x multiplier option, maybe it works for klamath CPU's.
hmm, 233/266 seems the safest choice indeed. But is there any other indication such as stepping or manufacturing date? E.G. which product numbers are verified to be locked/unlocked/limited?

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Reply 17 of 202, by swaaye

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gerwin wrote:

But is there any other indication such as stepping or manufacturing date? E.G. which product numbers are verified to be locked/unlocked/limited?

I really don't know. I was unaware of the 300 MHz Klamath chips being locked. I always thought that the Klamaths were all like the older Pentium CPUs with being fully unlocked.

Reply 18 of 202, by Old Thrashbarg

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gerwin wrote:

I never tried it but shouldn't a compaq slot-1 board be prepared to take a slot-1 celeron with 66MHz FSB? Either way an OEM mainboard usually lacks all the fancy tweaking options.

Y'know, for some reason I completely forgot about the Celerons. Yeah, they should work without troubles.

As far as the locked Klamaths, I dunno when mine were produced... the machine I have them installed in is buried in a closet and I CBA to dig it out, but IIRC they're SL2HA chips. (Which, BTW, is a bit odd, since I've heard of unlocked SL2HA's as well.)

Reply 19 of 202, by leileilol

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gerwin wrote:

I suppose you pulled the FSB below 66... interesting

Yeah, to 50. I was too lazy to pop open the case and set the widdle jumpers to do it for REAL 🙁

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