Searching for AMD driver for iMac 2011, W8.1

thank's to Mr. Backlin in the Apple Community for his helpfull tips, installing W8.1 in EFI-Mode on my iMac 2011. Couldn't install the update from W8 to W8.1 before without the known graphic driver issues at the end of the setup.

So I followed his tips:
https://discussions.apple.com/message/23604163#23604163#23604163

now I have successfully installed W.8.1 on my iMac, but couldn't get the right AMD driver. Tried latest Catalyst driver, Beta driver from AMD's website, tried the driver-check-program. All failed after extracting at the beginning of the installation.

has anyone found the right driver for my Radeon 6770? I think, it's a mobility graphic card, so has anyone a link to a working driver? Thank's!

found a workaround

no AMD driver from their support site worked for me, neither the catalyst suite, nor the beta drivers, found no special mobility drivers for W8.1.

so I decided to do do a clean install of my W8 Pro Upgrade DVD in EFI-mode with the new Bootcamp driver on a USB stick while the standard install is going on. At the end I was asked to install the Bootcamp drivers. Updated Windows.

Decided to try a full version of a OEM-W8.1 as an Upgrade install to W8.1, all at default settings. After the reboots, no bluescreen, like it is the way on upgrading from Windows store ;-). But first desktop screen hat a low resolution, but waited for around 15 minutes and suddenly the display driver was updated and recognized the iMac display at full resolution due to the automatic driver and Windows settings at the installation! Did the following updates of W8.1 and later changed the generic key of my W8.1 install to my W8 key ;-). Maybe that helps someone!