PCI latency & fast writes on AGP bus

Stephen M. Smit · Oct 3, 2005, 11:41 PM

Can someone give me a simple explanation of how PCI latency affects AGP cards?

Also, the ATI help on SmartGart page says "fast writes enable a direct connection b/t card's frame buffer and cpu". Does this mean that enabling fast writes bypasses the PCI bus?

cswiger · Oct 4, 2005, 01:56 AM

The simple explanation is that PCI latency settings control how long devices like your video card are allowed to monopolize the PCI bus, before having to make time available for other devices to run.

"Fast Writes" mean that the CPU is allowed to send data directly to the video card, instead of having to send the data to main memory, and then have the data DMA'ed from RAM to the video card. The data still passes through the Northbridge chip via the AGP port to the video card; whether you want to consider that as passing over the "PCI bus" specificly is more a matter of perspective.

H3X4D3C1M4L · Oct 5, 2005, 02:45 AM

THe only reason to fiddle with PCI latency is if you hear popping or sound distortion in games, otherwise it doesn't do much. It fixed 1 - 2 games for me other than thatr it didnt make them perform too too much better if any at all

cswiger · Oct 8, 2005, 06:21 PM

Oh, agreed, tweaking PCI latency is going to make around a 1% difference, give or take, at best. Which is why I think it's silly for any device to set their latency value to max the way many video card vendors do...

This type of problem seems to affect cards with little or no on-board memory (think the Creative Audigy). Even the old SB AWE32/64 cards had 8MB or so, which was enough to cache a reasonable amount of sound and avoid the stutter. In particular, I kinda wonder if HL2 does any better on an older SoundBlaster then on an Audigy...?

Stephen M. Smit · Oct 9, 2005, 05:41 AM

Yeah, that's what prompted me to start this thread... I can't find any impact at all after changing my X850XTPE (AGP) from 255 to 64 latency. So I'm wondering what's the point of ATI hardcoding the 255 latency in the hardware? All I can find is that this causes sound stuttering for some people. I don't have any stuttering but I set it to 64 to see if maybe I'd get smoother response from my USB mouse. Can't really tell if it helps or not.

Are there any types of activities an AGP card does that need more than 64 clock ticks on the PCI bus?

H3X4D3C1M4L · Oct 9, 2005, 04:44 PM

Yeah, games

If it ain't broke don't fix it, best way to describe this kinda thing

Oct 3, 2005, 11:41 PM	#1
Stephen M. Smit DriverHeaven Junior Member Join Date: Sep 2004 Posts: 79 Rep Power: 0	PCI latency & fast writes on AGP bus Can someone give me a simple explanation of how PCI latency affects AGP cards? Also, the ATI help on SmartGart page says "fast writes enable a direct connection b/t card's frame buffer and cpu". Does this mean that enabling fast writes bypasses the PCI bus?

Oct 4, 2005, 01:56 AM	#2
cswiger DriverHeaven Addict Join Date: Nov 2004 Location: NYC Posts: 233 Rep Power: 0	The simple explanation is that PCI latency settings control how long devices like your video card are allowed to monopolize the PCI bus, before having to make time available for other devices to run. "Fast Writes" mean that the CPU is allowed to send data directly to the video card, instead of having to send the data to main memory, and then have the data DMA'ed from RAM to the video card. The data still passes through the Northbridge chip via the AGP port to the video card; whether you want to consider that as passing over the "PCI bus" specificly is more a matter of perspective. __________________ Do you remember Shalebridge Cradle? Beware! The Cradle doesn't forget.... AMD Opteron 185 @ 2.6GHz/200HTT \| 2GB G.Skill DDR3200 @ 2-3-2-5/1T \| 512MB eVGA 8800GT @ 600/1800 \| SB Audigy 2ZS eX, Outlaw 1050 6.1ch amp, Paradigm speakers \| MSI K8N Diamond Plus mainboard \| black Lian-Li USB68 case + Foxconn FX600-GLN 600W PSU \| 2xWDC3200AAKS (RAID1) \| Sony GDM-F500R CRT, BENQ G2400W LCD

Oct 8, 2005, 06:21 PM	#4
cswiger DriverHeaven Addict Join Date: Nov 2004 Location: NYC Posts: 233 Rep Power: 0	Oh, agreed, tweaking PCI latency is going to make around a 1% difference, give or take, at best. Which is why I think it's silly for any device to set their latency value to max the way many video card vendors do... This type of problem seems to affect cards with little or no on-board memory (think the Creative Audigy). Even the old SB AWE32/64 cards had 8MB or so, which was enough to cache a reasonable amount of sound and avoid the stutter. In particular, I kinda wonder if HL2 does any better on an older SoundBlaster then on an Audigy...? __________________ Do you remember Shalebridge Cradle? Beware! The Cradle doesn't forget.... AMD Opteron 185 @ 2.6GHz/200HTT \| 2GB G.Skill DDR3200 @ 2-3-2-5/1T \| 512MB eVGA 8800GT @ 600/1800 \| SB Audigy 2ZS eX, Outlaw 1050 6.1ch amp, Paradigm speakers \| MSI K8N Diamond Plus mainboard \| black Lian-Li USB68 case + Foxconn FX600-GLN 600W PSU \| 2xWDC3200AAKS (RAID1) \| Sony GDM-F500R CRT, BENQ G2400W LCD

Oct 5, 2005, 02:45 AM	#3
H3X4D3C1M4L DriverHeaven Extreme Member Join Date: May 2005 Posts: 6,794 Rep Power: 0	THe only reason to fiddle with PCI latency is if you hear popping or sound distortion in games, otherwise it doesn't do much. It fixed 1 - 2 games for me other than thatr it didnt make them perform too too much better if any at all

Oct 9, 2005, 05:41 AM	Thread Starter #5
Stephen M. Smit DriverHeaven Junior Member Join Date: Sep 2004 Posts: 79 Rep Power: 0	Yeah, that's what prompted me to start this thread... I can't find any impact at all after changing my X850XTPE (AGP) from 255 to 64 latency. So I'm wondering what's the point of ATI hardcoding the 255 latency in the hardware? All I can find is that this causes sound stuttering for some people. I don't have any stuttering but I set it to 64 to see if maybe I'd get smoother response from my USB mouse. Can't really tell if it helps or not. Are there any types of activities an AGP card does that need more than 64 clock ticks on the PCI bus?

Oct 9, 2005, 04:44 PM	#6
H3X4D3C1M4L DriverHeaven Extreme Member Join Date: May 2005 Posts: 6,794 Rep Power: 0	Yeah, games If it ain't broke don't fix it, best way to describe this kinda thing