A BIOS will search for bootable medium based on the configured boot order during system boot. A virtual BIOS for VirtualBox will do the same for a virtual machine. You can change the order of the boot device to either set it to boot from CD ROM to install an operating system or from the hard drive once the operating system is already installed. It can also be configured to boot from the network via PXE boot.
Boot device can temporarily be set during BIOS startup by pressing the F12 key. You can choose a boot device from the list, but the choice does not persist after the VM reboots.
You can make the boot device selection permanent by configuring the boot order in the VM's settings page.
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