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Hyper-V generation 2 virtual machines – part 9

Part 1: Introduction to generation 2 virtual machines
Part 2: Networking and boot order
Part 3: Storage
Part 4: Keyboard for Windows 8 & Windows Server 2012
Part 5: Kernel debugging
Part 6: Secure Boot
Part 7: FAQ
Part 8: Manually migrating generation 1 virtual machines to generation 2
Part 9: Installing from ISO
Part 10: Utility for converting generation 1 virtual machines to generation 2 (Convert-VMGeneration)

This part of the series on generation 2 virtual machines in Hyper-V talks booting from CD/DVD. It’s not specific to Hyper-V, it’s more about PCAT and EFI, and the way in which Windows installation media is built.

Try the following – create a new generation 1 and generation 2 VMs with blank VHDXs, setting them both to boot from Windows 8 64-bit .ISO media, and start them.

Here’s what you’ll see in a generation 1 virtual machine (or PCAT physical system)


And here’s what you’ll see in a generation 2 virtual machine (or EFI physical system)


Before anyone comments, yes, we’re acutely aware that the generation 2 EFI boot loader messages persist on the screen and it can be a little confusing. Essentially what the generation 2 virtual machine did was:

  • Attempt to boot from the CD SCSI device containing the Windows 8 ISO. This is where the ‘Press any key…’ message came from. As no key was pressed, we went to the next boot entry. (And this is where we didn’t clear the message from the EFI CD boot loader)
  • I didn’t press F12 (in fact, this VM wasn’t connected to a network), so network boot failed
  • The SCSI VHDX is raw with no partitioning or file system, so this too failed

So why did the generation 1 virtual machine start setup from the Windows media? The answer is simply that it’s the way Windows media is built, and it’s inconsistent between the PCAT and EFI loaders.

However, it’s relatively simple to solve if you want to avoid the press any key message. In fact, we almost have all the pieces from previous parts. In particular, part 4 where we injected keyboard drivers into the Windows 8 media.

If you loopback mount Windows 8 or 8.1 RTM media, and navigate to the \efi\microsoft\boot directory, you will see there are two versions of cdboot.efi


The default version, cdboot.efi prompts for a keypress. The unused version, cdboot_noprompt.efi doesn’t prompt.

So it’s fairly simple to create modified media which uses the noprompt version. First, copy the contents of the ISO to a working directory, rename cdboot.efi to cdboot_prompt.efi, and rename cdboot_noprompt.efi to cdboot.efi.


Do the same with efisys.bin and efisys_prompt.bin


Then re-use our oscdimg command (in an elevated deployment and imaging tools environment) from part 4 to recreate the media.


Simply attach this modified media to the generation 2 VM and restart it (and don’t press any keys, not that you will be prompted…)


The surprise awaits! It’s worth it – stay tuned for part 10.