3 Configuring virtual machines However, software support for I/O APICs has been unreliable with some operating sys- tems other than Windows. Also, the use of an I/O APIC slightly increases the overhead of virtualization and therefore slows down the guest OS a little. Warning: All Windows operating systems starting with Windows 2000 install different kernels depending on whether an I/O APIC is available. As with ACPI, the I/O APIC therefore must not be turned off after installation of a Windows guest OS. Turning it on after installation will have no effect however. Enable EFI This enables Extensible Firmware Interface (EFI), which replaces the legacy BIOS and may be useful for certain advanced use cases. Please refer to chapter 3.12, Alternative firmware (EFI), page 51 for details. Hardware clock in UTC time If checked, VirtualBox will report the system time in UTC format to the guest instead of local (host) time. This affects how the virtual real-time clock (RTC) operates and may be useful for Unix-like guest operating systems, which typically expect the hardware clock to be set to UTC. Enable absolute pointing device If enabled, VirtualBox reports to the virtual machine that a USB tablet device is present and communicates mouse events to the virtual machine through this device. If disabled, mouse events are communicated through a traditional PS/2 virtual mouse device. Using the virtual USB tablet has the advantage that movements are reported in absolute coordinates (instead of as relative position changes), which allows VirtualBox to translate mouse events over the VM window into tablet events without having to “capture” the mouse in the guest as described in chapter 1.8.2, Capturing and releasing keyboard and mouse, page 19. This makes using the VM less tedious even if Guest Additions are not installed.3 In addition, you can turn off the Advanced Configuration and Power Interface (ACPI) which VirtualBox presents to the guest operating system by default. ACPI is the current industry stan- dard to allow operating systems to recognize hardware, configure motherboards and other de- vices and manage power. As all modern PCs contain this feature and Windows and Linux have been supporting it for years, it is also enabled by default in VirtualBox. It can only be turned off on the command line see chapter 8.7, VBoxManage modifyvm, page 109. Warning: All Windows operating systems starting with Windows 2000 install different kernels depending on whether ACPI is available, so ACPI must not be turned off after installation of a Windows guest OS. Turning it on after installation will have no effect however. 3.4.2 “Processor” tab On the “Processor” tab, you can set how many virtual CPU cores the guest operating systems should see. Starting with version 3.0, VirtualBox supports symmetrical multiprocessing (SMP) and can present up to 32 virtual CPU cores to each virtual machine. You should not, however, configure virtual machines to use more CPU cores than you have available physically. 3The virtual USB tablet was added with VirtualBox 3.2. Depending on the guest operating system selected, this is now enabled by default for new virtual machines. 44