3 Configuring virtual machines In addition, the “Enable PAE/NX” setting determines whether the PAE and NX capabilities of the host CPU will be exposed to the virtual machine. PAE stands for “Physical Address Extension”. Normally, if enabled and supported by the operating system, then even a 32-bit x86 CPU can access more than 4 GB of RAM. This is made possible by adding another 4 bits to memory addresses, so that with 36 bits, up to 64 GB can be addressed. Some operating systems (such as Ubuntu Server) require PAE support from the CPU and cannot be run in a virtual machine without it. With virtual machines running modern server operating systems, VirtualBox also supports CPU hot-plugging. For details about this, please refer to chapter 9.5, CPU hot-plugging, page 140. 3.4.3 “Acceleration” tab On this page, you can determine whether and how VirtualBox should use hardware virtualization extensions that your host CPU may support. This is the case with most CPUs built after 2006. You can select for each virtual machine individually whether VirtualBox should use software or hardware virtualization.4 In most cases, the default settings will be fine VirtualBox will have picked sensible defaults depending on the operating system that you selected when you created the virtual machine. In certain situations, however, you may want to change these preconfigured defaults. Advanced users may be interested in technical details about software vs. hardware virtualiza- tion please see chapter 10.3, Hardware vs. software virtualization, page 157. If your host’s CPU supports the nested paging (AMD-V) or EPT (Intel VT-x) features, then you can expect a significant performance increase by enabling nested paging in addition to hardware virtualization. For technical details, see chapter 10.6, Nested paging and VPIDs, page 161. 3.5 Display settings Video memory size This sets the size of the memory provided by the virtual graphics card available to the guest, in MB. As with the main memory, the specified amount will be allocated from the host’s resident memory. Based on the amount of video memory, higher resolutions and color depths may be available. Monitor count With this setting VirtualBox can provide more than one virtual monitor to a virtual machine. If a guest operating system (such as Windows) supports multiple attached monitors, VirtualBox can pretend that multiple virtual monitors are present.5 Up to 8 such virtual monitors are supported. The output of the multiple monitors will be displayed on the host in multiple VM windows which are running side by side. However, in fullscreen and seamless mode, they will use the available physical monitors attached to the host. As a result, for fullscreen and seamless modes to work with multiple monitors, you will need at least as many physical monitors as you have virtual monitors configured, or VirtualBox will report an error. You can configure the relationship between guest and host monitors using the view menu by pressing Host key + Home when you are in fullscreen or seamless mode. Please see chapter 14, Known limitations, page 181 also. Enable 3D acceleration If a virtual machine has Guest Additions installed, you can select here whether the guest should support accelerated 3D graphics. Please refer to chapter 4.4.1, Hardware 3D acceleration (OpenGL and Direct3D 8/9), page 64 for details. 4Prior to VirtualBox version 2.2, software virtualization was the default starting with version 2.2, VirtualBox will enable hardware virtualization by default for new virtual machines that you create. (Existing virtual machines are not automatically changed for compatibility reasons, and the default can of course be changed for each virtual machine.) 5Multiple monitor support was added with VirtualBox 3.2. 45