So, faster FSB or AGP?

[keeper]

In all the OC talk, I don't think anyone's succinctly wrapped up the FSB vs. AGP bus speeds. In other words, sure, I step up the FSB but reduce the AGP. So, for the average gamer, what's the net-out? If I gain 12% FSB speed but lose %33 AGP, is it worth it?

[jaida]

why do you want to lower your agp if you up your fsb why dont you just leave the agp as the speed you get from upping your fsb and set your agp divider to 2/3

if your agp cannot handle the speed then i would just recommend leaving the fsb alone because its pointless lowering your agp speed

[This message has been edited by jaida (edited 03-14-2000).]

[krusty the klown]

Hopefully, this clears things up:

the mobo derives its AGP clock and PCI clock from the FSB.

Standard spec is for the AGP and PCI clocks to be 66 and 33MHz, respectively. Now, if I had a Celery designed to run on a 66MHz FSB, the AGP and PCI dividers would be set at /1 and /2 respectively to give the 'official' speeds.

If I overclocked the system by increasing the FSB to 75MHz, the AGP and PCI speeds would be 75 and 37.5MHz, so I would be overclocking my graphics bus and PCI bus and HDD controller bus. As most components will be OK at this speed, I would see a performance increase greater than that achieved by just increasing the CPU clock.

When Inhell produced the Celery and P2, later P2's were designed to run at 100MHz. This speed was an 'official' speed and the mobos automatically (or had manual adjustment) used a /1.5 and /3 divider to obtain 66 and 33MHz for the AGP and PCI clox. If this did not happen, you would be trying to run the AGP and PCI clocks way over spec and they wouldn't work.

The same thing happens with the 133MHz FSB speed for the Coppermine EB chips - a greater divider is used to keep the PCI and AGP clocks running at their official speeds.

The trouble starts when you want to overclock a 100MHz bus CPU to a very high FSB, like overclocking a P3 500E to a 133MHz (or greater) bus speed. This has only been an issue with the Coppermines because the overclock very well and they are multiplier is locked, so you can end up using very high FSB.

If you have an old BX board that was only designed for 100MHz CPUs, but runs up to a 133MHz bus, it probably only has the correct PCI and AGP dividers for 100MHz operation. If this is the case, you might run a 133MHz bus, but your AGP and PCI buses are 88.6 and 44.3MHz - way over spec. So, it might be that the CPU is OK, but the graphics card can't take 88MHz, so you have to reduce the FSB to get the system stable.

Newer BX boards and Appolo Pro 133 boards have the clock dividers to cope with Coppermine EB's, which are intended for a 133MHs FSB. In other words, you can run a 133MHz FSB and still have your PCI and AGP buses running at their official speeds. Also, notice that some BX/Appolo boards will run well over the 133MHz - what use would a 150MHz FSB be, if you only had /1.5 dividers: no AGP card would run at 100MHz (unlikely, anyway) and you would corrupt the hard disk at 50MHz.

So the good thing about these higher clock dividers is that it allows you to push the CPU to the limit and not be held back by the hard disk or graphics card.

I hope this has cleared things up 4U!!

Krusty.

[neo_otyugh]

"If I gain 12% FSB speed but lose %33 AGP, is it worth it?"

like for example on a gigabyte ga-5ax i had the fsb set at 140 which puts AGP at 47 for that board....how bad a hit in performance ar we going to see? or will we see it at all?

[krusty the klown]

neo_otyugh:

47MHz AGP and 140MHz FSB?

Mmmmm... don't know!

Read this though:
http://www.sysopt.com/forum/Forum3/HTML/005285.html

AGP at 47??? That is underclocking by a long way - I don't know at what point a 'slow' AGP bus starts to affect performance. Can you not run it at half FSB = 70MHz. Then it's only 4MHz over spec, which will be fine, rather than 19MHz unser spec??????????

Krusty

[alpha]

A lot of monitoring programs give incorrect results - sandra says I get 70mhz AGP when it's actually 63.