New Windows 7 WDDM 1.1 Drivers save you RAM
Windows 7 will now be more efficient in its use of RAM, which will have the positive effect of making everything run 
smoother – particularly on systems with shared graphics memory. In Windows Vista, the more application windows you had open, the more memory required.
The DWM (desktop window manager) is responsible for drawing the desktop with the GPU, which obviously requires the application window data in video memory. The same application window data is copied again in system RAM for the CPU to render independent of graphics hardware. This was ineffective obviously because of data redundancy.
Windows 7 altered this by getting rid of the system memory copy entirely, which effectively cuts the memory used in half. Unlike the changes to the GDI stack detailed earlier, this new memory-saving behavior requires new drivers.
The new drivers that support the memory-saving-change are designated WDDM 1.1. Older Windows Vista WDDM 1.0 drivers will still work fine with Windows 7.
This change positively impacts real-world usability, but benchmarks may show a degradation in performance since the CPU has to fetch data from video RAM.
“The elimination of the duplicate system memory copies which ‘speed up’ certain operations introduced slightly reduced performance as the CPU now has to read data back from the video memory. An analysis of real-world application statistics showed that these operations were rare,” Chitre said. “Our observation has been that these slow-downs do not impact the end-user functionality directly and that the memory savings directly result in Windows 7 being much responsive overall. The improvements overall are definitely noticeable on memory constrained PCs with shared memory graphics.”
