i bbought two 6800xt want to do a crossfire ,but i dont know how to do that
my motherboard is msi x570 tomahawk
Crossfire is a legacy feature which has been replaced by MGPU (Multi-GPU) instead.
Here is AMD Crossfire page on how to configure GPUs in Crossfire: https://www.amd.com/en/support/kb/faq/dh-018
The above aside, the most significant difference between CrossFire and SLI is that AMD phased out the brand name at the tail end of 2017 in favor of ‘’mGPU support’’ (multi-GPU), while Nvidia has stuck with the SLI acronym.
The logic behind AMD’s decision is that with the advent of DirectX 12, the way a game supports multiple GPUs has changed drastically.
With DirectX 11, the onus was on AMD to create profiles for games. In other words, drivers for their cards with settings geared towards allowing a specific game to make the most of CrossFire.
By comparison, DirectX 12 shifts responsibility to the developers, who must incorporate mGPU support into the game engine with clear instructions on how it must make use of the dual (or more) GPUs.
Although CrossFire has been officially and honorably discharged, AMD continues to provide support for games using DirectX 11 and create profiles.
The move towards DirectX 12 games and engines is a slow process, so AMD support looks to remain for the time being.