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Advice needed on CPU-Motherboard compatibility

BillDL asked
Last Modified: 2008-01-09
Hello Experts

I bought an old PIII Slot 1 CPU from ebay for absolute buttons.  I wanted to upgrade an old system that I use for testing out installations on, but I have now notices a mismatch that probably means that I can't use the CPU in the board. The postage was double what I paid, and that was just as it really cost to post a local package, so it's not going to break the bank if I can't use it.

My current system:

SuperMicro P6SBA Slot 1 (SECC2) motherboard Revision 2.00 Model ST371313 04/99. Based on Intel 82440BX Revision 3.
Motherboard ID (from AIDA32):
62-0101-006389-00101111-071595-440BX/GX$0AAMH301 1121001130 SUPER P6SBA Motherboard BI

It doesn't show what I recognise as a standard BIOS String at POST, but just a section of the Motherboard ID above, ie. 1121001130 SUPER P6SBA Rev. 3.1.
I believe this is the "Date Code".  See page 68 of the manual:

Pentium III 450 MHz sSpec SL35D
Katmai 0.25 Micron technology
Stepping kC0
Core Voltage 2.00 -> 2.05 Volt <----------- ** NOTE **
I/O Voltage 3.3/2.9  Volt
Bus Speed: 100 MHz <------------------- ** NOTE **
Bus/Core ratio 4.5 (ie. 100 MHz x 4.5 = 450 MHz)

AMIBIOS: Sticker A719312 SBA1192
BIOS Date: 01/15/99
BIOS ID (from AMIBIOS ID Utility):   62-0101-006389-00101111-071595-440BX/GX

The new Pentium III CPU I bought has the following specification:


sSpec Number: SL3XN  
Frequency: 733 MHz
Core Voltage: 1.650 <---------- ** NOTE ** (quoted in other references as 1.7 Volt)
Bus Speed: 133 MHz <--------- ** NOTE **
Bus/Core Ratio: 5.5 (ie. 133 MHz x 5.5 = 731.5 MHz)
Core Stepping: cB0
Coppermine Technology: 0.18 micron
(No mention of I/O Voltage in that Intel spec).

The important point here is the Core Voltage.  The Coppermine (0.18 Micron) Pentium III CPU's have a Core Voltage of 1.65 or 1.7 Volt, while the older Katmai (0.25 Micron) ones have a Core Voltage of 2.0 or 2.05 Volts.  The differences are due to different "stepping" versions in this "family" as the technolgy changed during the manufacturing period quite a lot.

The core voltage for the 450, 500 and 550 MHz Slot 1 Pentium III CPU's is 2.05 volts for the stepping models: kB0, kC0, and is the same for 600 and 600B MHz stepping kC0 models. (My 450 MHz version is kC0).

For stepping models cA2, cB0 in speeds 533EB, 550E, 600E, 600EB, 650, 667, 700, 733, 750, 800, 800EB, 850, and 866, the core voltage is 1.65 Volts (My new 733 MHz version is cB0).

As far as the motherboard goes, setting the Core/Bus Ratio is done from jumpers (JB 1 to 4) that allow the range between x3.0 to x7.5 (x8.0 for the later Revision 4.0 board).
Bus Speed Selection by jumpers (JP 11) only offers up to the Motherboard's limitation (ie. 66 MHz or 100 MHz), but also has an "Auto" setting which states that speed is "Determined by the CPU".
Pages 25 and 47 of the manual below.

P6SBA Model layout Page 25.
Common motherbard family features Page 29.

From Page 33:
"Auto-Switching Voltage Regulator for the CPU Core
The switching voltage regulator for the CPU core can support up to 20A
current, with the auto-sensing voltage ID ranging from 1.4V to 3.5V. This
will allow the regulator to run cooler and thus make the system more stable".

From Pages 3 and 29:
"Currently, the P6SBA motherboard supports CPU speeds up to 1GHz with 100 MHz FSB speed".

CMOS Setup starts on page 68. Nothing aparently relevant except on page 88, ie. CPU Temperature threshold alarms.  Currently set at failsafe.

BIOS Update:
Readme docs in download give no indication of version history.

OK, so hopefully that gives sufficient detail of the setup and allows you to provide me with the best advice, so here's what I am asking:

1. What is likely to happen if I set the Bus Speed jumper (J11) to "Auto" and set the Core/Bus Ratio jumpers (JB 1 to 4) to 5.5 (ie. 133 MHz x 5.5) and boot to the 733 MHz CPU with the lower Core Voltage (ie. 1.65 v rather than the previous CPU's 2.0 v)?

2. Would it be better if I set the Bus Speed Jumper to 100 MHz and the Core/Bus Ratio to x7.0 or x7.5 (ie. 100 MHz x 7.5)?

3. What is the significance of the "I/O Voltage 3.3/2.9  Volt" statement in the spec for the old processor on the Intel sSpec Finder page and the statement from the motherboard's pdf manual on Page 33:
"The auto-switching voltage regulator for the CPU core can support up to 20A
current, with the auto-sensing voltage ID ranging from 1.4V to 3.5V. This
will allow the regulator to run cooler and thus make the system more stable"?

Does this mean that the processor will only draw what it needs to run on, and the 2.0 Volt Core Voltage demanded by the older 450 MHz CPU is insignificant?

Hopefully you can shed some light on these factors so I can run this sentimental old base unit at a more respectable speed without it overheating and burning up.  Hardware at this level isn't really my forté, fixing software problems is :-)

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Thank you for that in-depth explanation, RoadWarrior.

That's true about the CPU cache.  The old 450 MHz (Katmai) one has 512 KB of L2 cache, but it runs at half speed.  The new 733 MHz (Coppermine) one has 256 KB of L2 cache, but it's full speed.  With the marginal increase in CPU speed, there might be some improvement.

At present, the board has a PC100 SDRAM, but I've got some PC133 around here somewhere that I can use.

When you say "AGP Divider", is that something that could be balanced out by adjusting the "AGP Aperture"?  At present that is set at the default 64 MB.  I assume that I would have to lower it.  It increments from 32 MB to 64 MB, so I would have to drop to 32 MB.  There's no apparent way of manipulating the interface speed of the AGP socket.

The AGP slot is the Type 1 which only supports 1x and 2x speeds and the motherboard manual states that it has a maximum transfer rate in 2x mode of 532 MB/s (133 MHz). It's got a cheapo Sparkle SP6326 card based on the nVidia GeForce2 MX 400 chipset with 32 MB of RAM, but I don't know how tolerant that would be to overclocking.  In fact, overclocking isn't something I've ever experimented with at all.

From an AIDA report, I see some settings relating to the Graphics card, but I'm not absolutely sure that I know what all of them mean:

GPU Clock:  200 MHz
RAMDAC Clock:  350 MHz
Bus Width: 128-bit
Real Clock: 166 MHz  
Effective Clock: 166 MHz  
Bandwidth: 2656 MB/s

Is it coincidence that the bandwidth of 2,656 MB/s is exactly 5 x the bandwidth of the AGP Slot?
AGP apeture is the amount of main memory that can be used to buffer textures etc when the AGP memory is full.

AGP divider is the proportion of the FSB clock the APG bus runs at. Typically it can be 1/1 or 2/3 on a BX motherboard. 1/1 is only required on a 66Mhz FSB, for 66Mhz, AGP speed. At 100 Mhz, 2/3 is required for 66Mhz AGP speed, at 133, the AGP is overclocked at 88.7Mhz, some cards deal with this, some don't. Geforce 2 series cards are usually pretty tolerant of it.

I think the bandwidth there is the onboard memory bandwidth of the AGP card, it appears to be a 128bit interface, which is 16x8bits, and 16x166Mhz=2656MB/sec.


Thanks a lot for that explanation, RoadWarrior.  It's all in context now, so I know what I'm dealing with.
Much appreciated.
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