How can I reduce the display resolution in Windows (Boot Camp) on a Retina 5k iMac? tl;dr How can I add additional valid display resolutions to the Retina 5k iMac under Windows 8.1?



So I just picked up a new Retina iMac, immediately threw Boot Camp and Windows 8.1 on it, and started pulling my Steam games over.

All is well except for the fact that the display refuses to switch from it's not-really-native-but-windows-thinks-it's-native resolution of 3840 x 2160. Apparently that's a theoretical max for Windows (or at least AMD drivers) at the moment.

However, since I want to play games in fullscreen (not windowed) modes, I really need to be able to decrease the resolution. How can I do that?
I was able to solve this myself by realizing: (a) Windows 8.1 upgrade had changed my display adapter to a Windows Basic Display Adapter, (b) AMD's web site doesn't apparently host installers that work for the Retina iMac (yet?), and (c) the Boot Camp package that Boot Camp Assistant adds to your Windows installer contains a version of AMD Catalyst that will work and will replace the errant driver:




The installer is in &lt;WININSTALL&gt;:\Bootcamp\Drivers\ATI\ATIGraphics\Setup.exe
Run install
Verify in Device Manager that the Display adapter is AMD... M29{0,5}X


Finally, I was able to update resolution: