Setting ASUS P3B-F AGP Multiplier To 1 At 133 MHz FSB Speed May Be Even Faster

[Johnny GHz]

Tom's Hardware Guide showed that an ASUS P3B-F 440 BX motherboard with an 800MHz PIII Coppermine running at 133 MHz FSB speed with PC133(CAS 2)memmory beat even Intels latest chipsets running same CPU with Rambus memmory in most of his benchmark tests (http://www.tomshardware.com/mainboard/00q1/000308/index.html)

In a later report, Tom's hardware guide ran tests showing that only some graphics cards would run on the P3B-F in this configuration because the AGP bus which is supposed to run at 66MHz runs at 88&8/9ths MHz when the 440BX chipset is run at 133MHz FSB speed. (http://www.tomshardware.com/graphic/00q1/000321/index.html)

The AGP 2X Spec bus speed is 66 MHz. The 440BX chipset sets the AGP as a multiple of the FSB speed. Since the 440BX chipset was designed to run at FSB speeds of either 66MHz or 100MHz, the AGP Multiple can be set at "1" or "2/3"(1*66MHz FSB speed = 66MHz AGP bus speed) and (2/3*100MHz FSB speed = 66MHz AGP bus speed).

When you set the FSB speed to 133 MHz and the AGP multiplier is at "2/3", you get 88&8/9ths MHz (133&1/3MHz)x(2/3)=88&8/9ths MHz.

Let Me Pose The Question: ....

What if you run Tom's ASUS P3B-F above configuration, but set the AGP bus multipier to "1" causing the AGP Bus to run at 133 MHz (Isn't this AGP 4X) ...

Would many of the AGP 4X compliant Graphics cards that wouldn't run at the odd 88&8/9ths MHz AGP bus speed run just fine (!!! and faster !!!) on the 133 MHz AGP 4X bus speed ?

Has anyone out there tried this ???

I looked in the manual (on ASUS web page) and there's no setting in the BIOS setup for the AGP multiplier, but you can set it manually using the Dip switch on the motherboard (Page 17 of ASUS P3B-F motherboard manual) !!!




[This message has been edited by Johnny GHz (edited 03-25-2000).]

[Maxout]

4X AGP is a data transfer rate not controlled by FSB. You will not find an AGP card anywhere that will run at 133mhz AGP speed!

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If you lived here, you'd be home by now...

[Mole]

I think you have misunderstood something major.

This is how the multipliers works.

If say you run 66 MHz bus, then the AGP would be set to 1/1 = 66 MHz.

AGP2X would then be 66 x 2 = 133 MHz
AGP4X would then be 66 x 4 = 266 MHz

Now, on 100 MHz bus, the AGP would be set to 2/3 = 66 MHz.

AGP2X and 4X would be same as above.

However, when the bus is OVER 100 MHz, the AGP speeds get divided by 2/3 as well.
In other words, at 133 MHz = (133/3) * 2 = 88.67 MHz.

AGP2X = 88.67 x 2 = 177.33 MHz
AGP4X = 88.67 x 4 = 354.67 MHz

Now, let's assume that setting the AGP to 1/1 would work on a 133 MHz bus, then:

AGP2X = 133.33 x 2 = 266.67 MHz
AGP4X = 133.33 x 4 = 533.33 MHz

Now, do you see that this is way way higher?

[Johnny GHz]

Wright ... If you run the ASUS P3B-F 440BX motherboard (which operates in AGP 2x mode)at 133 MHz FSB speed with an AGP multiplier of "1" you get:

AGP2X (at 133&1/3 AGP bus speed) = 133.33 x 2 = 266.67 MHz

Isn't that the same as AGP4X at 66&2/3 AGP bus speed

Explain to me what I'm missing here

Also, your calculation of (133/3) * 2 = 88.67 MHz for the AGP bus speed at 2/3 the 133&1/3 FSB speed is not quite correct.

Its not (133)*(2/3), its (133&1/3)*(2/3) = 88&8/9 or 88.889


[This message has been edited by Johnny GHz (edited 03-25-2000).]

[Tweety]

Then decimals aren't the important thing here...
The MAIN point is that all agp cards are supposed to run as close to 66Mhz as possible...ALL of them, both 4x and 2x...
The 4x/2x is an internal multiplier and doesn't concern the fsb or agp speed...
It defines weather the cards internal data speed is at 4*agpbus or 2*agpbus...
Running an 4x agp card at 133Mhz is like running a udma66 hdd at a pci speed of 66Mhz, just cuz it has the number 66 on it...

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I Thougth I Saw A Putty Cat...

[This message has been edited by Tweety (edited 03-26-2000).]

[Maxout]

BINGO!

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If you lived here, you'd be home by now...

[MaxCPUman]

What the hell r u guys talkin about??? I thought that ALL agp modes ran at 66mhz! And agp 1x 2x & 4x just meant twice and four times the bandwidth of agp 1x. And i thought that they were agp DIVIDERS and NOT MULTIPLIERS like you say. ie 100fsb=66agp via 2/3 divider. AND 66fsb=66agp via 1/1 divider.

[Johnny GHz]

If this is the case then if you are running your AGP bus above 66.667 MHz, you need to get into the graphics card bios to tweak it's internal multiplier somehow (This way you can maintain stability in 440BX boards with overclocked FSB speeds).

Thanks Tweety

P.S. Do you know of a good site that really explains AGP well