Graphics

On the graphics side there will be new "CrossFire Edition" graphics boards as well. There have been in the order of millions of X800, or above, class PCI Express graphics processors already sold, however unlike NVIDIA's SLI system they have no internal connector in order for them to be hooked up to a second board. In order to allow people with current PCI Express X800 and X850 boards to get the benefits of purchasing a second in order to increase the rendering performance ATI have come up with the concept of the master and slave boards, with the existing cards acting as the slave and the new CrossFire Editions acting as the master boards.

Click for a bigger version

Click for a bigger version

In order to support multi-board rendering the CrossFire editions have two primary elements: a "DMS" input / output port and its own dedicated compositing engine. The DMS I/O is the method by which the image data is transferred from the slave board to the master - with no internal connectors by which the boards can communicate, other than the PCI Express bus which may not be optimal performance, ATI are pushing out a digital image from the DVI output of the slave card and sending that to the DMS connector on the master board. The DMS connector is a high density connector that allows for both the input from the slave board and an output from the master, so this port can still be connected to an output device. One thing to note, though, is that it does appear that the CrossFire Edition board loses its TV output capability because of the size the DMS connector needs to be.

The compositing engine that is used on the master boards is a separate imaging chip that performs the joining of the images between the two boards, as well as blending of images, which is one element to an additional rendering mode presently unique to CrossFire boards. The compositing engine is a programmable device, which enables it to join, merge or blend digital images in numerous fashions, allowing ATI to support various different methods rendering across multiple graphics. The device does also have some limited buffering available to it, so the actual compositing task is done independently of the rest of the rendering process.

When the two graphics boards are placed in a supported platform the graphics drivers will split the memory a little differently than with a single card. In this instance each of the graphics boards will be assigned its own memory and Command Queue Buffers, enabling both boards to operate on independent instructions if needs be (which may be useful for doing things like distributing different Render To Texture operations across two boards), as well as having a shared pool of memory that both will access for synchronisation commands and other shared data such as textures. Once the two boards have finished rendering their sections of the screen they both output to the compositing engine on the master board (by virtue of the DMS connection on the slave board, and directly from the the graphics processor to the compositing engine on the master) such that the composting engine will perform the necessary function to get a single, correct image to send to the display. Note that as the frame data coming from either chip is effectively a digital representation of the display image, all the image resolving (such as FSAA down-sampling) has already occurred by this point, so the data being sent is only to the size of the display resolution.

With the master board requiring a new connector and the composite engine this means that there will need to be new board designs to facilitate the different components and that can also increase the price a little in relation to an equivalently performing standard board. Because of this, and the fact that the market for these boards are going to be fairly limited ATI have grouped up various current boards and are covering them with three different types of CrossFire Edition boards. These break down into the following groups:

*Note: we wanted to confirm the specification of these X800 boards with ATI and their reply was "Both X800 CrossFire cards are 16 pipe with XL clocks".

Seeing as there is a broader range of standard cards than CrossFire cards not all of them are going to be a like for like match, and as such some may need to be effected by the drivers dependant on the combination. For starters, only like RAM configurations will be used, so if you have a miss-matched card pairing in terms of the RAM quantities both support then the board with the least RAM will dictate the actual RAM use for the entire system - i.e. if you have a 256MB and 128MB board in the system the rendering will behave as though there is only 128MB. Also, some of the current chips have had pipelines disabled and in this instance, should you pair a board with 12 pipelines active and another with 16 the drivers will disable 4 of the pipelines on the 16 pipeline board.

In the cases of like for like in terms of configuration, but differing clockspeeds, both boards will operate at their native clocks, there will just be the instances where the faster board will end up waiting for the slower. However, the only X800 chips being produced now are R430 based chips (X800 XL) and so the master X800 boards will need to be based on that - the XL does actually have a 20% core clock deficit to the X800 XT and even greater to the X800 XT Platinum Edition, which can make for a reasonable difference in performance between them both. The X850 Crossfire board corresponds to X850 XT's clockspeeds. In the table above the like for like colourings in the respective rows indicate which board groupings are going to be the optimal in terms maximising the potential rendering capabilities of CrossFire.