Running a legacy Intel CPU at clock speeds lower than spec.

aoeuin

Junior Member
Sep 14, 2018
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#1
So the general question is:

Can legacy Intel Xeon processors that are built to operate at a specific "Processor Base Frequency" be successfully setup to run in motherboards that have their max clock speeds lower than the "Processor Base Frequency".

Specifically, I'm interested in installing two "Intel Pentium III Xeon Processor 700 MHz, 2M Cache, 100 MHz FSB" in a Supermicro S2DGR which can only go up to 600 MHz (might be 550 Mhz).

Would this even work?

Any insight would be appreciated.
 
Apr 27, 2000
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#2
Conservatively, I would say no. The only literature I can find regarding CPU support for this board is that it won't support anything newer than 550 MHz Xeons. Such old tech should be dirt cheap, so if you have a chance to try it, just pop it in there and see what happens.

Worst case scenario, the BIOS detects the CPU wrong and won't support multipliers high enough for the chip to run at full speed. Or it just won't POST.

But anyway it does not look like the launch BIOS for this board supports that CPU. Maybe later BIOS updates added support.
 

bbhaag

Diamond Member
Jul 2, 2011
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#3
Is this at all helpful? My knowledge of early systems is limited but I saw it and thought it might be worth checking out.
 
Apr 27, 2000
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#4
Interesting. SuperMicro didn't use software controls to set the CPU multi. So your max clockspeed with any chip is going to be 600 MHz per the jumper diagram.
 

bbhaag

Diamond Member
Jul 2, 2011
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#5
Interesting. SuperMicro didn't use software controls to set the CPU multi. So your max clockspeed with any chip is going to be 600 MHz per the jumper diagram.
Yeah I thought so to but a lot of those old mainboards still used jumpers. I don't know if the OP will ever be back but here is a diagram layout I found of the mainboard in case anyone else ever stumbles across this thread and wants to know the jumper layout.
 

bbhaag

Diamond Member
Jul 2, 2011
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#7
So I got to thinking about the OPs question and if whether or not a 700mhz cpu would work in the S2DGR. Theoretically couldn't he set the jumper pins to 5.5, 6.0, or even 4.5 and when the pc is booted it will down clock the 700mhz cpu to the lower frequency? That would then theoretically allow the pc to load an OS correct?
 
Aug 25, 2001
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#8
Correct, AFAIK. You should be able to run a 700Mhz clock / 100Mhz bus CPU at 500Mhz (5.0) or 600Mhz (6.0). I would try it, it wouldn't really hurt.

The only issue I can see, is microcode update revision levels, newer faster CPUs might require newer microcode than what's in the BIOS.
 

bbhaag

Diamond Member
Jul 2, 2011
4,136
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#9
Correct, AFAIK. You should be able to run a 700Mhz clock / 100Mhz bus CPU at 500Mhz (5.0) or 600Mhz (6.0). I would try it, it wouldn't really hurt.

The only issue I can see, is microcode update revision levels, newer faster CPUs might require newer microcode than what's in the BIOS.
That's a good point I didn't think of that. Come to think of it even my ASRock Z170M Pro4S needed a BIOS revision to be able to recognize Skylake cpus. Without that updated BIOS only Kabylake cpus would be recognized.
Anyway, I know it is highly unlikely that the OP will ever come back and post an update but it would be interesting if he did just to see how it went.
 
Apr 27, 2000
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#10
If memory serves, older CPUS would basically run as long as the pinouts lined up correctly, the bus protocols matched up, and they were electrically compatible. Older CPUs were a lot less sensitive to voltage/current at the socket.

The only reason you would need a major update to BIOS in CPUs of that generation is if something drastic changed, such as cache layout. For example, my old AM2 board (Abit NF-M2 nView) never truly became Phenom-compatible despite AMD saying that it would be possible. Phenom (Agena, specifically) was too different from anything x2-based. It had L3 cache and all kinds of other funkiness.

Regardless I concur with @VirtualLarry that OP should just try it and see. Hardware that old should be pretty expensive, especially the CPUs. The chips should run at 600 MHz, no problem, since the processor layout is otherwise similar/identical to the Xeons that were officially-supported by the board.
 

aoeuin

Junior Member
Sep 14, 2018
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#11

Markfw

CPU Moderator, VC&G Moderator, Elite Member
Super Moderator
May 16, 2002
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#12
I got 2 quad-core 3.2 ghz Xeons with HT (16 threads total) for $20 (10 each) and $44 for a motherboard with heatsinks just last month off ebay.. I have no idea why you would even bother with something this old.
 

aoeuin

Junior Member
Sep 14, 2018
5
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#13
You have a point...its old, but it runs and I don't have a reason to retire it yet, or worse discard it into landfill. Have a removable SCSI setup on this where I can boot a number of OSs as needed.
 
Mar 24, 2017
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#14
I don't see the point getting something this old back up and running. It may still have a purpose for your task but, think first of how many hours you are going to invest in something that may die at anytime.
You are better off spending $100 and getting a Dell I7 tower with 8gb of memory. If you want to boot to a number of operating systems just install windows 10 and install hyperv or install esxi.
Anything below a i5 2500k should be in the dumpster.
 

bbhaag

Diamond Member
Jul 2, 2011
4,136
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#15
You have a point...its old, but it runs and I don't have a reason to retire it yet, or worse discard it into landfill. Have a removable SCSI setup on this where I can boot a number of OSs as needed.
@aoeuin NO don't ever throw old tech like that into the landfill always try and sell it first!!! Don't take this the wrong way but it sounds like English is not your first language. If that is the case try www.amibay.com first or www.ebay.com as a second. Items like CPUs are a dime a dozen and go cheap but working mainboards and AIBs can demand a premium the older they get.

Anyway, let us know if your mainboard boots proper with those newer Xenons installed. I'm a casual vintage computer geek/collector and would be interested in knowing if it worked out ok for you.
 
Last edited:

Charlie22911

Senior member
Mar 19, 2005
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#16
Back when I had my PC Chips M599LMR socket 7 rig I upgraded it to a K6-III+. IIRC (it’s been many years ago) you could set the multiplier to x2 and the CPU would internally interpret it as x6 since socket 7 boards didn’t support those multipliers natively.
Did Intel chips have any such functionality?
 

SPBHM

Diamond Member
Sep 12, 2012
4,774
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#17
Back when I had my PC Chips M599LMR socket 7 rig I upgraded it to a K6-III+. IIRC (it’s been many years ago) you could set the multiplier to x2 and the CPU would internally interpret it as x6 since socket 7 boards didn’t support those multipliers natively.
Did Intel chips have any such functionality?
in terms of desktop CPUs Intel had the multipliers locked at that time for Pentium II/Celeron and newer, you couldn't underclock via the multiplier, just FSB when possible, like using a 440BX board you could underclock a 100Mhz fsb CPU to 66, but didn't really have options for a 66MHz FSB CPU for underclocking, and the multiplier jumpers on my boards had 0 effect up or down, the multiplier was locked.

now I've never used Xeons from this era, so I don't know if they are multiplier locked, I would think so? in that case underclocking is probably not useful, unless you had a FSB 133 one running at 100 or something;


looking at this list
https://en.wikipedia.org/wiki/List_of_Intel_Xeon_microprocessors#"Cascades"_(180_nm)
550 is where the 250nm Xeons ends, that might mean your motherboard can't handle the 180nm ones, so no higher clocked models, and underclocking wouldn't help at all.

the 180nm ones probably run with lower voltage, and it might be that your board can't handle it, or it might be more of a bios compatibility problem...
(common problem for Pentium 3, older P3 Katmai boards that couldn't handle Coppermine because the voltage regulator couldn't run a vcore under 1.80v)



but the clock is not really the problem, the problem is knowing if the S2DGR can run the 180nm CPUs or just the 250nm ones,
 

aoeuin

Junior Member
Sep 14, 2018
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#18
So, after experimenting a bit with two different Pentium III Xeon vintages - I have two types sSpec SL49R and SL49S, here are some preliminary findings.

The Supermicro S2DGR motherboard set to 600 MHz (via the jumpers) running a pair of sSpec: SL49Rs booted up Linux and did report information on both CPUs accurately though it show a microcode error during bootup that flashed by too quickly for me to capture the exact error.

On the other hand the SL49Ss did not boot, the display wouldn't even get activated with the newer CPUs.

I'm going to look into the differences between the CPU types next....
 

aoeuin

Junior Member
Sep 14, 2018
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#19
The PIII Xeon Spec Update provides some insight.

 

daxzy

Senior member
Dec 22, 2013
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#20
First of all, if you're going to throw it away, I'll buy the board from you, as I collect retro stuff.

Second, the reason there may be a hard limit of 550Mhz is because 550Mhz slot-2 Xeons were 250nm (Katmai based). Anything newer are 180nm (Coppermine based) cores. So there may be a VRM limitation. I have an Asus P2B v1.04 slot 1 motherboard that can only accept Katmai Pentium III's due to VRM limitations.

The clock multiplier is actually locked, so it doesn't really matter what you select in the jumpers.
 

JWade

Diamond Member
Oct 9, 1999
3,037
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#21
try different multiplier settings, some P3's (not sure about the XEONs though) remapped multipliers. like 2.5x would actually be 6.5x (dont quote me on that)
 

bbhaag

Diamond Member
Jul 2, 2011
4,136
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#22
Not sure if this helpful but I did stumble across some older posts from Vogons and Thinkpad forums. They unfortunately reference the S2DG2 not your S2DGR. They appear to be very similar though. Even the manual I referenced above note both boards in it.

Anyway, the first thread is from Vogons IMO it is THE place to be when it comes to vintage hardware. To quote from post #10
Paadam said:
Found this topic, I am Raceboy in Thinkpad forums. I sold the dual Xeon 900 MHz rig in 2010, now looking at buying it back
S2DG2 supported only 2.8v CPU's though, I found a pair in ebay back then.
https://www.vogons.org/viewtopic.php?f=46&t=46816

This above is in reference to an older thread dating back to 2008 on the Thinkpad forums where he owned a "Dual Xeon 900 Mhz (2 MB L2) on Supermicro S2dg2, 2x73 GB Atlas 10K rpm SCSI as main desktop: movies, photos, internet, very nice machine, still have it."
Post #21.
https://forum.thinkpads.com//viewtopic.php?t=59702
 


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