Intel
82865G/82865GV GMCH Datasheet
159
Functional Description
5.3.1
GMCH AGP Support
5.3.2
Selecting between AGP 3.0 and AGP 2.0
The GMCH supports both AGP 3.0 and limited AGP 2.0, allowing a “Universal AGP 3.0
motherboard” implementation. Whether AGP 2.0 or AGP 3.0 mode is used is determined by the
graphics card installed. An AGP 2.0 card will put the system into AGP 2.0. An AGP 3.0 card will
put the system into AGP 3.0 mode. The mode is selected during RESET by a hardware mechanism
which is described in
Section 5.3.3.1
. The mode determines the electrical mode and can not be
dynamically changed once the system powers up.
5.3.3
AGP 3.0 Downshift (4X Data Rate) Mode
AGP 3.0 supports both an 8X data rate and a 4X data rate. The purpose of the 4X data rate is to
allow a fallback mode when board routing or other reasons make the 8X data rate marginal. Some
AGP4X graphics cards currently fall back to a 2X data rate when board layout or other issues arise.
This is referred as “downshift” mode. Since AGP 2X is not supported, any card falling back to 2X
will be running in a non supported mode. When in AGP 3.0 mode in the 4X data rate, all of the
AGP 3.0 protocols are used.
Table 27. AGP Support Matrix
Parameter
AGP 3.0
AGP 2.0
Comments
Data Rate
4X or 8X
4X, or 1X
GMCH does not Support AGP 2X
Electricals
0.8 V swing, parallel
terminated
1.5 V swing serial
terminated
Signal Polarity
Most control signals
active high
Most control signals
active low
This change was necessary to
eliminate current flow in the idle
state. Parallel termination has a
large current flow for a high level.
Hi / Low priority
commands
Only low priority
(renamed Async)
High and low priority
commands supported.
High priority does not have a good
usage model.
Strobe Protocol
Strobe First – Strobe
Second Protocol
Strobe – Strobe#
protocol.
Long transactions
Removed
Supported
PIPE# support
No
Yes
SBA required for AGP 3.0
Calibration Cycle
Required
No
New to AGP 3.0.
Dynamic Bus
Inversion
Yes
No
New to AGP 3.0
Coherency
Required for AGP
accesses outside of the
aperture, and for
FRAME-based accesses
Required only for
FRAME-based
accesses.