Back, when Intel
created the AGP 2.0 specification, its compatibility to AGP 1.0 was the most
important thing. Besides the lower signaling, AGP 2.0 is AGP 1.0 with some enhancements
(like SBA). By specifying the "Universal AGP Motherboard", Intel avoided
any trouble with older video cards
When Intel created
AGP 3.0, compatibility obviously wasn't an issue. Intel focused its attention
on professional users and workstations. First of all, Intel lowered the signaling
by 47% to 0.8V, while contemporaneously removing any support for AGP 3.3V. Still,
motherboard manufacturers can add support for 3.3V signaling, by implementing
the "Universal AGP 3.0 Motherboard" standard.
The complete table
of all AGP Motherboard standards is as follows:
Motherboard
Connector
Supported
signaling
Speed
AGP
3.3V Motherboard
3.3V
3.3V
1x,
2x
AGP
1.5V Motherboard
1.5V
1.5V
1x,
2x, 4x
Universal
AGP Motherboard
3.3V,
1.5V
3.3V,
1.5V
1x,
2x, 4x
AGP
3.0 Motherboard
1.5V
0.8V
4x,
8x
Universal
1.5V AGP 3.0 Motherboard
1.5V
1.5V,
0.8V
1x,
2x, 4x, 8x
Universal
AGP 3.0 Motherboard
3.3V,
1.5V
3.3V,
1.5V, 0.8V
1x,
2x, 4x, 8x
Explanation:
Motherboard:
This is the standard, the motherboard manufacturer has to implement, depending
on which AGP modes and video cards he wants to support
Connector:
This is the type of connector, the motherboard manufacturer has to implement,
depending on which AGP modes and video cards he wants to support
Supported signaling:
These are the various signaling types supported by the standard
AGP modes:
These are the AGP modes supported by the standard. For example: The „Universal
1.5V AGP 3.0 Motherboard“ supports AGP 1x mode (266 MB/s), but only
for video cards with 1.5V or 0.8V signaling (AGP 2.0 or 3.0), not for AGP
1.0 cards with 3.3V signaling.
Another change
causing incompatibility, makes perfect sense, from another point of view. The
new signaling scheme inverts the assertion levels of many signals, meaning that
if logic 1 (high voltage = 0.8V) deactivated something before, it's now done
by logic 0 (low power = 0V). One of these inverted signals is the SBA#, which
is deactivated on any nVidia card anyway. The primary motivation for doing so,
is to ensure that the unasserted logic state for any AGP 3.0 signal is „low
voltage“, which equates to 0V hence a lower power state. The software
requirements (required changes in software) are described as well. Surprisingly
enough, not only applications have to be rewritten:
„However,
optional new features introduced in AGP 3.0 may only be enabled with appropriate
changes to the OS and other software“
Required changes
to software and applications are nothing unusual, in order to ensure, that the
software uses new features provided by new hardware. One might wonder, what
exactly needs to be changed. Well, first of all, the OS has to support multiple
AGP-Ports. Further on, the new GART is now specified to be controlled and created
by the drivers. Support for existing GTLBs has to be enabled by the drivers,
as well as support for multiple GARTs for multiple AGP video cards.
As for today, it's not clear, whether MS Windows XP Home/Professional supports
these new AGP 3.0 features, however it's pretty implausible. If anything changes,
I will update the article.
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