For more information, please refer to "Unmanaged RAM as Client Write Cache" in CCBoot v3.0 Wiki.

If the client PC has 4G physical RAM, the 32 bit Windows OS can manage at most less than 3.5G. We can use the rest physical RAM for client cache as bellow -

Add a DWORD key named "4G" in the registry "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CCache\Parameters", set its value as "1".
Now please click to select the "Enable Client Cache" check box and set the value as 512M for this client PC on the CCBoot server. It will make use of the RAM which has not been managed by Windows for client cache.