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Windows Vista and EFI

Every now and then the question comes up why rEFIt doesn’t support booting Windows Vista through EFI. The short story is that neither Vista nor the Intel Mac firmware are up to it right now. Read on for the long story.

Microsoft’s Official Position

The official word from Microsoft (in their EFI and Windows Vista briefing) is that the final release of Windows Vista will not support EFI at all. EFI booting support will be added in a future service pack for Windows Vista, but will be limited to UEFI 2.0 on 64-bit platforms.

All IA64 (a.k.a. Itanium) versions of Windows support EFI as that is the standard firmware on that platform. But Apple doesn’t use IA64 CPUs and very likely never will, so that’s a moot point.

Microsoft’s Reality

An analysis of the publicly released Windows Vista RC1 DVD images yields the following results:

Since no current EFI firmware is capable of reading ISO9660 or UDF file systems, this means that even the x64 disk is not directly bootable on any EFI firmware.

Apple’s Reality

The current firmware on Intel Macs is based on EFI 1.10, and runs in 32-bit mode even on the 64-bit-capable Core 2 based machines (Mac Pro, late-2006 iMac).

This means the current Mac firmware is not capable of running the EFI binaries on the Windows Vista x64 disk, because those are compiled for a 64-bit EFI environment. (You just get an “Unsupported” error from EFI if you try anyway.) It will require a major firmware upgrade from Apple for this to change. And even then only the recent 64-bit Mac models would be able to boot Vista through EFI.

On a side note, the current state of El Torito support in Apple’s firmware is still in doubt. There is a possibility that it no longer detects file systems in secondary El Torito entries at all, but the details remain to be tested.

The State of rEFIt

Current versions of rEFIt are already coded to recognize the file names of the EFI boot loaders on the Windows Vista DVDs, and show an appropriate logo for them. However, the contents of the DVD are not accessible from within the EFI environment, so all that will show up is the BIOS-based boot code in the first El Torito entry.