Just like on all other similar products, AMD's two Cayman variants, called Cayman Pro and Cayman XT, are based on the exact same GPU silicon. The model variant a GPU chip becomes is decided after the die is produced, at some point before it is put on the card. Creating new SKUs from the same silicon by locking features has been common practice in the industry since at least the Radeon 9500 in 2002.
Apparently currently shipping Radeon HD 6950 cards from all manufacturers — which actually are all the same card with different sticker — have their shaders locked via the BIOS method, so we can exploit it easily.
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