Cannot assign more than 8GB RAM to VirtualBox Guest?

I’m using VirtualBox on a Windows Server 2008 R2 host system with 24GB RAM. I want to allocate 12GB of RAM to my 64-bit CentOS guest, but when I change the RAM slider to more than 8GB the VM will only see 8GB of RAM:

Reserving 129MB of memory at 48MB for crashkernel (System RAM: 8704MB)

and

# free -m
             total       used       free     shared    buffers     cached
Mem:          7870       5025       2845          0         13         90
-/+ buffers/cache:       4921       2949
Swap:         4099          0       4099

Is there a limitation on how much RAM a guest can have? I didn’t find anything on the net.

Ok it works now. I must shut down the VBoxVmService for the change to work. I had always shut down (powered off) the VM itself, but VBoxVmService leaves VBoxSVC running which seems to prevent any changes to the VM configuration. After I stopped the service, I could make the change and assign more RAM to the guest.