Intel Mobile Pentium 4 M Exploration

Mobile P4M logoThis is pretty much the only thing I feel bad about just getting rid of from my old site so thought the least I could do is just copy it across. It’s a bit outdated but I hope it’s of use to some people. Cheers.

I came across the possibility of using Mobile Intel Pentium 4 Processors – M (to give them the official title) in desktops while sourcing components for the company I worked for. My boss wanted these tiny cube systems to sit looking pretty on the desks of our office. However, he also wanted top of the range performance and didn’t even think about the noise. As a quiet PC enthusiast I said he would need some impressive cooling system if he didn’t want the CPU fan to sound like a jet engine right next to his ear!

So I set about researching and stumbled across a few threads in various forums (huge thanks to the people at Silent PC Review and AnandTech) which explained how I could use these CPU’s in desktops. Although the CPU’s didn’t turn up on eBay often, I got hold of 2, tested them out, got very excited and tried to source more! When the chance came around to buy a large quantity of them I decided to grab them all, put some effort in and resell them. This page is here for the people who want to know more about these great CPU’s, although I no longer have any left for sale I’m afraid.

Mobile P4M waferThe Mobile Intel Pentium 4-M is based on the same Northwood core as it’s desktop equivalent. Indeed they most likely came the same wafer and are essentially identical in design. However, processors destined for mobile use are ‘hand picked’, almost certainly from the middle of the wafer where the silicon is best. They are the cream of the crop, able to match or exceed the performance of their desktop siblings while running at a lower voltage. If you look at it from this point of you, standard desktop Pentium 4’s are the rejects that don’t quite make the grade!

Where the two processors differ is that the mobile P4-M makes use of Enhanced Intel SpeedStep technology, which is designed to reduce the clock speed of the processor when not under heavy use, thus preserving a laptop’s battery. This basically involves reducing the default bus speed multiplier (17 in the case of a 1.7 GHz Mobile P4-M) to 12, resulting in a clock speed of 1.2 GHz. When placed in a desktop motherboard which doesn’t understand SpeedStep technology, the processors run at the default 12 multiplier with a bus speed of 100 MHz (1.2 GHz). I’m unaware of any consumer orientated motherboards with support for SpeedStep.

Now you may think this SpeedStep technology is a bad thing for anyone wanting to use these processors in a desktop. However, by simply increasing the FSB (Front Side Bus) from 100 MHz to 200 MHz the cpu will easily run at 2.4 GHz and benefit from a substantially faster bus speed.

A point worth making is that Mobile Celeron processors don’t feature SpeedStep technology and therefore retain their original clock multiplier. However, with modern motherboards officially supporting an 800MHz FSB (200MHz quad-pumped) and many enthusiast targeted motherboards allowing even greater bus speeds it seems silly to hamper the speed of you system by sticking to the slow, old 100MHz bus speed.

An important physical difference anyone can spot is that the Pentium 4-M doesn’t have a heat spreader. This has a significant impact on reducing core temperatures as the heat has one less layer to dissipate through. This is not only of interest to quiet pc enthusiasts but anyone who likes to keep their CPU’s core cool and healthy.

Gigabyte GA-8I875The rig I’ve set up for testing all the processors I’ve bought is based around a Gigabyte GA-8I875 (Gigabyte are one of the few manufactures to allow VCore undervolting). I picked it up as B-Stock from for a very reasonable price. I’ve got a couple of 256mb sticks of Samsung DDR400 (nothing to write home about) running in dual channel configuration. When I eventually make a proper PC out of this motherboard I will invest in some Kingmax DDR500 😉

Based on the Intel 875P north bridge and ICH5R south bridge, in terms of features the only thing slightly disappointing is that the Gigabit ethernet is connected over the PCI bus instead of via CSA but like I’m ever gonna tell the difference!

Something which I would really have liked to see is the ability to select a 1:1 FSB/RAM ratio even when the CPU doesn’t have a 800 (200 quad-pumped) MHz FSB. This would have allowed a us more flexibility when overclocking, or in this case, bringing the processor back up to stock speed.

The Mobile Pentium 4-M has a 400 (100 quad-pumped) MHz FSB. Therefore it is necessary to do a bit of a modification to the processor in order to fool the motherboard into thinking we’ve inserted a 800 FSB CPU.

The modification is basically breaking the BSEL1 pin from the bottom of the CPU. This is not an easy task; it requires a steady hand, good eyesight even when using a ‘third-hand’ magnifying glass and lots of patience, taking care not to damage any of the surrounding pins.

Once performed successfully the processor will show up as a 2.4 GHz (200 x 12) CPU by default in 800 FSB motherboards, allowing much more flexibility with memory speeds as well as flawless operation in motherboards which don’t offer a manual bus speed selection. Click here for a pin layout of the Mobile Intel Pentium 4-M which shows which pin you need to break off.

Anyway, here are some preliminary testing results of the 1.8GHz Mobile Intel Pentium 4-M Processor (SL6FH). The testing was performed on the bench with an ambient temp of around 20C. The testing is by no means deadly accurate or scientific, just a rough guide. The power supply I used was just a cheap and nasty thing and I’m sure a higher quality PSU would help stability. I used a Thermalright SKU-800 heatsink with a Zalman 92mm fan running at full pelt. This is certainly an overkill at lower speeds but it made testing consistent across the board.

Speed (GHz) BIOS VCore (Volts) Idle Temp (Degrees Centigrade) Prime95 Temp (Degrees Centigrade) Comments
1.20 0.8375 28 30 Will run fanless at 35 when idle!
1.60 0.8375 30 32 Performance increase makes this speed worthwhile over 1.2 GHz
2.26 0.8375 31 34 Fastest speed achievable without increasing the voltage from 0.8375
2.40 1.3375 32 38 An excellent compromise between heat and speed
3.20 1.6800 34 49 I reckon there’s more speed in there too!

As you can see it seems feasible to run this CPU fanless at 1.2 or 1.6 GHz. 15 minutes of Prime95 gradually built the temperature up to 65 but for everyday average use with a very slow fan or good case airflow you could have a very quiet system. I personally would rather run a little faster and reduce the bus speed using ClockGen when not in use (I leave mine on 24/7 usually). Amazingly this CPU will go all the way up to 3 GHz and beyond! I’m not sure but I think those temps are less than the equivalently clocked desktop processor which just shows that even when stretched to the extreme these CPU’s are truely “The Daddy”!

Perhaps someone can help me with this but I suspect the 8I875 does not allow VCore to go all the way down to 0.8375. The diagnostic utilities I used, SpeedFan and CPU-Z, reported it as 1.25v, which I could put down to the voltage being out of the sensors range. However, I seriously doubt I would be able to hit 2.26GHz on such a low voltage. Given that the default VCore when used in a laptop is 1.2/1.3v it seems that Gigabyte have only implemented voltage adjustment to 1.25. Maybe it’s a bug in the BIOS, I’m using the latest version (F8). Someone else’s opinion on this would be most welcome. I’m not sure how I could test for sure.

Soltek SL-865PE-LFinally got a new motherboard, or rather two. One to keep on the bench for testing and one to replace my Gigabyte GA-8I875 as my main computer. I was less than impressed with the Gigabyte’s stubbornness to change the actual VCore despite allowing a vast range of low voltages to be set in the BIOS. So despite its many appealing features it will soon be heading to an eBay near you 🙂

So the motherboard in question is the Soltek SL-865PE-L (the L on the end means it has onboard LAN). If you want to check out the detailed specification of this motherboard check out here. Yes it does have the most hideous colours on a motherboard you’ve ever seen but then I’m not one to show off my motherboard through a case side panel. More to the point, the can be picked up for £35 from eBuyer UK hence the reason I’ve got my mittens on two of them!

Just to summarise briefly, the board is based on the Intel 865PE north bridge and ICH5 south bridge (no RAID unfortunately). The heat sink on the north bridge is reasonably sized and importantly doesn’t use a fan. It does get quite hot but I haven’t yet encountered a problem with this. There a several aspects of this motherboard which make it ideal for use with Mobile Pentium 4 M processors. The first is that the VCore can be reduced right down to 1.1v. While on paper this isn’t as good as Gigabyte boards I am quite confident that the board is actually supplying what it says it is!

Secondly, you can set the default bus speed using jumpers. Normally motherboards automatically detect the correct FSB and you have to over/underclock in the BIOS. Of course with the jumpers left in their default position this is also the case with the Soltek but you can change this to force a 100, 133 or 200 MHz bus speed. This effectively does the same as the cutting the BSEL0/1 pin without making an irreversible change to your processor. Top Dorris!

It’s fair to say this isn’t a hardcore overclockers board. The fastest I got out of it was a FSB of 235.5 (2.82 GHz). Not bad but I know there’s would still be room in the processor and this is with a massive Zalman 92mm fan pumping air around the north bridge to see if that was the weakest link. I also attempted to do the trick of setting the FSB with jumpers to 133 MHz but increasing it to 200MHz (or whatever) in the BIOS. This enables PAT (the main difference between the 875 and 865PE chipsets) and vastly improves memory performance. The sweet spot I’ve found is running around 190MHz host clock which allows me to use a 1:1 FSB:RAM ratio and yet have my Geil value RAM, which was very cheap and is rated 2.5-4-4-8, running at 2-3-3-6. Not bad and certainly is better than overclocking the FSB to improve memory performance but at the cost of relaxed CAS timings.

So I guess what you’re all wanting to know is the temps Im getting now that the VCore is actually being altered, huh? Well…

1844.2 mhz

1844.2 mhz

Click here for the complete screenshot which will show that these temps are after running once through Prime95 to max out the core temp. Yes that really is an 1844.2 MHz Mobile Pentium 4M at a peak temperature of 33C! To put the data in to context, this is with my new Zalman 7000A-Cu with the fan at its slowest setting using the fan mate. At 1383 rpm it is very quite, although not silent. I figured that this would be a common setup and so a good indication of what you can expect.

This is the fastest I could get the processor to run without raising the VCore in the BIOS. You may notice that the reported VCore is actually 1.072v and it did jump around a bit. I’m still using that cheap PSU on the bench until a replacement arrives so I’m sure you could get better results with something that cost more than £5!

Note: I’ve since tested with my Q-Technology PSU which is far superior and I could push it faster at1.1v with this supply however I have not had chance to do any formal testing.

I should really round this with more recent testing and results but I’m not actually sure how much of this information is of any value to anyone anyway. Leave some comments if there are people out there who want more info.

I’m now using a Foxconn 865A01-PE-6EKRS which doesn’t actually allow for any VCore alterations in the BIOS. So I did another little pin mod trick on a 1.7GHz MP4M to hard undervolt it to approximately 1.15v. See this topic for more info.

It involves breaking or bending the AE1 pin so that it doesn’t make contact. With the VCore fixed I just pushed the FSB as far as I could before I encountered stability issues. I was surprised to see it went as far as 2.0GHz which is where it rests now, with an Arctic Cooling AC-FRZ-4 Freezer 4 running completely fanless! It’s an amazing heat sink for the price that’s for sure and the whole set up is doing me just fine for now.

28 thoughts on “Intel Mobile Pentium 4 M Exploration”

  1. this is cool stuff. i have an old non-functional laptop of the same processor but will need a spare desktop mobo to carry on with this experiment. i hope i find one. btw, Speedstep technology has been around since Pentium 3.

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