ATI CrossFire: The Third Incarnation

The third incarnation of the ATI CrossFire multi-GPU platform is very similar to the second one, based on the Radeon X1800. Customers have to use either Radeon Xpress CrossFire Edition (RD480)-based mainboard, or Intel 955/975-based chipset platforms that have two PCI Express x16 slots to build a CrossFire system. Additionally, in case you are building a Radeon X1800 or X1900 dual-GPU system, you will need one appropriate CrossFire Edition graphics card with compositing engine.

ATI’s CrossFire platform allows to use graphics cards clocked differently and with various memory configuration. This means that users will be able to use Radeon X1900 XTX, Radeon X1900 XT or even lower-clocked Radeon X1900-series graphics card along with the Radeon X1900 XT CrossFire Edition board. Theoretically, graphics cards will continue to operate at their default frequencies, unless the differences are significant. It should be noted that it is not possible to assemble a multi-GPU array consisting of one Radeon X1800 and one Radeon X1900.

There are no differences between the working principles of the Radeon X1900 CrossFire platform compared to earlier Radeon X1800 incarnation: those who are not familiar with the matter should refer to the previous article called Caught in the Cross-Fire: ATI RADEON X1800 XT CrossFire Edition Review .

The multi-GPU modes also remained the same:

• The Scissor mode works in the same way as SMR mode in Nvidia SLI: the frame is split in two parts each of which is rendered by a separate GPU. The driver may vary dynamically the size of these parts for better balance. The geometry information is doubled for each GPU. This mode supports Direct3D and OpenGL applications.
• In the alternate frame rendering (AFR) mode the frames are rendered and output on the screen alternately. This is the most efficient mode as each GPU processes the geometry for its own frame and there is no useless doubling of data. This mode supports Direct3D and OpenGL applications, but only if there is an appropriate application profile in the ATI Catalyst driver.
• The SuperTiling mode doesn’t have an analogy in Nvidia’s SLI. The frame is tessellated into tiles, 32x32 pixels each. Half the tiles are processed by the Master card and the other half by the Slave. This is an all-hardware mode as it does not depend on the driver. The geometry data are doubled for each GPU.
• The Super AA mode, unlike the rest of CrossFire operating modes, is not about higher performance but about higher image quality. It introduces new full-screen antialiasing modes: 8x, 10x, 12x and 14x. When using 8x or 12x FSAA, each of the CrossFire-linked cards works in 4x or 6x FSAA mode, respectively, but the samples are taken from different image parts. The frames are then united to yield full-screen antialiasing equivalent to 8x and 12x, respectively. The hybrid 10x and 14x FSAA modes combine traditional 4x/6x multi-sampling with additional antialiasing equivalent to 2x super- sampling. There are two times more texture samples in the Super AA mode and this allows using 32x anisotropic filtering. This CrossFire mode is available in both Direct3D and OpenGL applications.

As previously, the CrossFire rendering mode is activated automatically if the Catalyst A.I. feature is turned on. Generally, SuperTiling was designed for Direct3D, while Scissor was developed for OpenGL application programming interface (API). AFR is used only when there’s an appropriate application profile in Catalyst. If Catalyst A.I. is disabled, SuperTiling is used for Direct3D games and Scissor in all other cases. The Super AA mode is automatically activated as soon as 8x/10x/12x/14x FSAA is turned on.

Those who would like to have a closer look at the Radeon X1900 CrossFire Edition board can do it in our Radeon X1900 XTX review and also find some more details in the Radeon X1800 XT CrossFire-related article . Keep in mind that the CrossFire Edition boards do not have TV-outs, hence, it does not make sense to acquire them for multimedia-oriented systems if you want to use conventional cables. Nevertheless, if you have DVI or HDMI supporting TV, you will be able to output onto a big screen.