On the first screen, you're asked pick a name for your system and to choose what type of operating system it will run. Pick whatever you want for a name, and choose Linux from the type dropdown menu and Debian (64 bit) from the version list.
On the next screen you'll choose how much of your system memory the virtual computer will use while running. Choose less than half of your total system memory — one or two gigabytes should be fine. Of course, most games call for at least 2-4GB of memory these days, and Valve recommends SteamOS systems have 4GB-plus of memory, but that's to play full-fledged modern games. VirtualBox isn't going to be the right kind of environment to run those. We recommend you pick a more hardware-forgiving game, such as Monaco or Superbrothers, and consider this virtual machine endeavor more of a test drive for the operating system itself.
Next you'll be asked about a hard drive. Choose to create one now, and then on the next screen, leave the first option (VDI) selected, and click Next. On the next screen, leave the Dynamically Allocated box checked and hit Next again. Finally, you'll be asked to pick a size for your virtual machine's hard drive. Note that you won't actually be giving up all this hard drive space right away — the dynamic allocation option you selected earlier means that virtual hard drive space is only created as you actually use it. The amount you specify here is just the maximum amount the virtual machine will ever be allowed to use. So go ahead and pick a fairly large value, such as 40 or 50GB.
Configure the SteamOS virtual PC
With that, your virtual SteamOS machine is created — you can see it in the column on the left side of VirtualBox. Before we "power on" the virtual machine for the first time, there are a few more settings we need to tweak.
With the SteamOS machine selected in the left column, click on Display in the main part of the VirtualBox window. In the display options window that opens, drag the Video Memory slider all the way to the right, and click on the box labelled "Enable 3D Acceleration." Next, click on the System options button and check the box labelled "Enable EFI."
Next, click on Network, and in the NAT dropdown box select "Bridged Adapter." If you receive an error warning, leave it as the default "NAT" option.
Lastly, click on the Storage options button. Here we'll load up the ISO we downloaded earlier, so our SteamOS virtual PC can boot from it. In the box marked Storage tree, click on the entry with a little CD icon that says "Empty." Then, click on the second CD icon at the very right of the screen, next to the CD/DVD drive dropdown menu. Select "Choose a virtual CD/DVD disk file" from the context menu that opens under the CD icon. Browse to the ISO file you downloaded in step one.
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