Re: EFI to grub to DOS?

[Chris Murphy]

On Jul 25, 2012, at 12:26 PM, Jake Thomas wrote:

> "I don't get the impression that the CSM provides any passthrough capability 
> to UEFI. With CSM the computer appears strictly as BIOS not UEFI. 
> 
> And not all UEFI firmware have a CSM."
> 
> Yeah, I was hoping that the memory which contains the setting that says 
> whether to boot into EFI mode or BIOS mode would be accessible both while in 
> BIOS mode and EFI mode.

I don't think this is a setting that's accessible except by the UEFI settings 
interface itself, and then only at the discretion of the IBV.

Since the CSM is not a real BIOS, there is no CMOS or NVRAM for its settings or 
even a settings menu. Any customization would be through the UEFI interface, 
and optional.

> Greg, I'm no expert when it comes manipulating system firmware settings from 
> the OS, but out of curiosity, when booted into Linux, do you have a 
> /dev/nvram when booted in BIOS mode? And what about when booted into EFI mode?

Apple hardware is Intel EFI 1.10 (not UEFI 2.x), but does have a CSM. 
Regardless of boot mode there is a /dev/nvram but the contents differ. Further, 
efibootmgr returns boot volume information when booted EFI mode, but when 
booted using the CSM, efibootmgr complains:
Fatal: Couldn't open either sysfs or procfs directories for accessing EFI 
variables.


Chris Murphy