Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <2jlur8F13c947U1@uni-berlin.de>, "M" <none@none.com> wrote:
> Yes it is. I have an 8x agp card in a 4x agp motherboard. Don't expect to
> get the full benefits of it though!
> "Steve Smith" <dudey1@earthlink.net> wrote in message
> news:2jlp73F12oehkU1@uni-berlin.de...
> > hey all,
> > i have a question for you pros. is it possible to use an 8x agp
> > card with 4x motherboard agp slot?
> > steve dudey1@earthlink.net
> >
> >
> >
AGP 2.0 spec (pg.249) and AGP 3.0 spec (pg.48)
ftp://download.intel.com/technology/agp/downloads/agp20.pdf
http://developer.intel.com/technology/agp/downloads/agp30_final_10.pdf
An AGP 2.0 generation device has status bits 3..0 equal to
reserved 4x_capable 2x_capable 1x_capable
and a AGP 3.0 generation device adds to that status bit 3..0 defined as:
AGP3.0_mode zero 8x_capable 4x_capable
The codes on AGP 2.0 look like
0 011 (2X and 1X capable card or host)
and a typical AGP 3.0 device might have
1 011 (8x and 4x capable card or host)
Now, a potential problem is with your BIOS. If the BIOS code reads
the host_bridge status and the video_card status, it should pick a
set of conditions that don't exceed the capabilities of the two. If
the BIOS looks at your new video card, and ignores the high order bit
AGP3.0_mode (i.e. the BIOS was written in the era of AGP2.0 and doesn't
know about AGP3.0), then a 011 code in the lower part, looks like
your new card supports 2X and 1X. Now, if the BIOS sets the interface
to run at 2X, instead of the actually advertised 8X or 4X, some cards don't
like that, and they won't run.
I only suggest this as a potential issue, of a BIOS that doesn't
understand the coding of a new card. Drivers like an ATI Catalyst,
which likes to ignore the info coming from the BIOS, would not have
a problem understanding the status register, so the hurdle you
face will likely be only at the BIOS level and getting the machine
to POST successfully.
A second issue with AGP cards, is the TYPEDET# pin A2. A video card
is supposed to ground this pin, indicating they want to use 1.5V
I/O voltage. This is important, to prevent damage to the Northbridge.
Most modern Asus motherboards, have a circuit labelled AGP_Warn, and
it checks this pin, to see if it is grounded. Now, some video cards
don't ground this pin well enough, to keep the Asus circuit happy.
If, when you plug in your new video card, you press the power button
on the front of the case, and the system refuses to power up, the
problem could be a disagreement between that circuit and the video
card, and the Asus circuit is erring on the safe side, preventing
what it thinks is a damaging combo.
If, on the other hand, you plug in the video card, and the screen
remains blank, yet the fans are spinning, my _theory_ is, that
the BIOS doesn't know how to handle the combo of an 8X video card
in a 4X system, and it is trying to use 2X mode on a card that
doesn't like it.
About all I would suggest at this point, is buy from a retailer with
a generous return policy, in case either of the above two problems
surface.
HTH,
Paul
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