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ATI CrossFire: How It Works

We gave you a detailed insight into ATI CrossFire technology in our earlier reviews (Swords Crossed: ATI CrossFire Platform Review and ATI Crosses the Swords: Multi-GPU CrossFire Technology Previewed), so we are going to sum up just the basics here.

The main point of difference between ATI CrossFire and NVIDIA SLI is that the parts of the image rendered by the different GPUs are combined by the hardware, in a special compositing engine that consists of a TMDS receiver, a programmable Xilinx Spartan matrix, and the chips that send the final image onto the screen (a TMDS transmitter and a RAMDAC). The graphics cards in a CrossFire system are connected via a DVI interface with a special Y-shaped splitter cable.

A CrossFire graphics subsystem has four modes of operation – Scissor, Alternate Frame Rendering (AFR), SuperTiling, and Super AA – that differ in the way the total load is distributed between the Master and Slave graphics cards:

  • 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 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. This mode only works on graphics cards with 16 pixel processors and only in Direct3D.
  • 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.

The CrossFire rendering mode is selected automatically if the Catalyst A.I. feature is enabled. In this case the choice depends on the API employed by the particular application: SuperTiling for Direct3D and Scissor for OpenGL. 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.

That’s about enough of theory, so we can proceed now to the Master graphics card called ATI RADEON X1800 XT CrossFire Edition. It is the heart of ATI’s multi-GPU technology and we’ll use it to describe the features of the new-generation CrossFire.

 
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