Technology
Fortunately, our technology preview is pretty much correct in how it explains Crossfire from a technical perspective. Mostly. Please go back and read it before returning here so we can explain the extras you'll need to know before we forge on with everything else.Compositing chip and data passing from slave to master
First off, the compositing chip. We said this in the technology previewLike NVIDIA's SLI, there's a master/slave(s) board setup with Crossfire. With SLI, the inter-GPU connector takes care of framebuffer compositing; frame data from the slave is passed over the connector to the master, for joining. With Crossfire, an external connector feeds the output data from the slave into the master board's compositing hardware. ATI are unspecific about where the compositing engine is on Crossfire boards (more on which later), be it on-GPU die silicon or something else (external chip?), but there's hardware that performs the compositing at full speed for both master and slave boards, before output. It's all done digitally, too.The compositing chip certainly does exist outside of the GPU die as a discrete board element that Crossfire Edition graphics boards have to implement for it all to work in this first generation of Crossfire Edition systems. Currently a programmable FPGA, the chip accepts frame data from the slave graphics board and combines it with the render output from the Crossfire Edition master board before outputting the data to your monitor.
That doesn't happen for all communication between master and slave, however. In Super AA mode, depth and colour samples from the SuperAA rendering modes are passed over the PCI Express bus rather than the cable. It makes sense given that the DVI output from the slave hardware can only transmit completely resolved and finished framebuffer contributions (what it has rendered as finished pixels) and not the encoded subpixel data that's needed for the depth or colour samples that Super AA needs to work with.
We'll come back to that later on when we examine the Super AA modes and their performance in more detail.
Display output limitations
This first generation of Crossfire works with all existing 12 and 16 pixel processor ATI Radeon X850 and X800 graphics boards on the PCI Express graphics interconnect. All consumer Radeon X850 and X800 graphics boards feature single-link DVI graphics outputs. By definition, a single-link DVI connection only has enough bandwidth at its maximum clock rate to carry a 1600x1200 image at up to 60Hz, or a 1900x1200 image displayed at 54Hz. Therefore in terms of what the slave can send the master board for output via the compositing chip, it's limited to those resolutions.And while the refresh rate essentially doesn't matter on a digital TFT LCD because of the way those screens work, for analogue outputs the digital signal is passed through a RAMDAC for display. With only 60Hz refresh at 1600x1200 - which also happens to be the maximum supported resolution for analogue outputs in Crossfire mode - coming out of the slave hardware via its single-link DVI transmitter, that's converted to a 60Hz maximum analogue transmission at that resolution for analogue displays.
That leaves analogue display owners with 1600x1200 at 60Hz in Crossfire mode as the maximum output combination. And while your eyes might be broken enough to suffer that, the majority of gamers with analogue displays looking for high resolution, flicker-free gaming with Crossfire are going to be disappointed. Digital fairs slightly better, but not by much.
In our recent Avivo technology discussion we confirm that dual-link DVI will help alleviate those limitations in future versions of Crossfire using upcoming next generation hardware but that certainly depends exactly how ATI configure the dual-link outputs on that hardware. We wait and see for that.
