A tip for anyone who wants to run a UEFI operating system on a BIOS-only commuter

[Captain Flack]

Many readers of this forum will know:—

 

1. that Tianocore Duet allows UEFI operating systems to run on BIOS-only computers
2. that Clover and OpenCore both include their own variants of Duet
3. that all of the Duet variants are tricky to install and confusingly sensitive to the size and filesystem-type of the partition that they're installed on.

 

Less well known is that some of the difficulties of installing Duet can worked around by chainloading it from Grub4dos 0.4.6a (the most recent BIOS version of Grub4dos). The required Grub4dos menu.lst stanzas are curently published in two places oin the internet:

 

1. https://github.com/migle/BootDuet/issues/1#issuecomment-1461539918
2. https://winraid.level1techs.com/t/guide-nvme-boot-for-systems-with-legacy-bios-and-uefi-board-duet-refind/32251/977

 

I'm not the author of the above-linked comments, but I've tried the method (with Clover's "boot7" version of Duet and with OpenCore's bootx64 on a USB stick, and it works perfectly. Chainloading Duet in this way has some advantages:—

 

1. it doesn't matter whether the Duet file is in a FAT16 or a FAT32 filesystem (unlike the "orthodox" Duet booting scheme, where you need to choose carefully).
2. it doesn't matter whether the GPT partition type is "EFI System" or something else such as "Microsoft Basic Data".
3. it doesn't matter what name you choose to give to the Duet executable (but make sure you're using the 64-bit version on a 64-bit computer, and the 32-bit version on a 32-bit computer…).

 

In case the links above aren't working, here's a copy of the Grub4dos menu.lst stanzas:—

 

## This is a Grub4dos menu that chainloads various implementations of the Duet UEFI-from-BIOS
## software. Tested with Grub4dos 0.4.6a (BIOS version). Originally published in 2023 at
##
## https://winraid.level1techs.com/t/guide-nvme-boot-for-systems-with-legacy-bios-and-uefi-board-duet-refind/32251/977
##
## https://github.com/migle/BootDuet/issues/1#issuecomment-1461539918
##
## This may be helpful to anyone who wants to run a UEFI operating system on a BIOS-only computer. Unlike
## the orthodox method for booting Duet, it is tolerant of both FAT16 and FAT32 filesystems. It also works
## with various GPT partition types ("EFI System" and "Microsoft Basic Data" are both accepted).

title Load OpenCore EFI Emulator
## This assumes that the Duet file is "bootx64" at the root of some filesystems that is visible to
## Grub4dos. Adjust the name and path if you have named it differently.
find --set-root /bootx64
chainloader --force --load-segment=0x2000 --load-offset=0x200 --boot-cs=0x2000 --boot-ip=0x200 /bootx64


title Load Clover EFI Emulator
## This assumes that the Duet file is "boot7" at the root of some filesystems that is visible to
## Grub4dos. Adjust the name and path if you have named it differently.
find --set-root /boot7
chainloader --force --load-segment=0x2000 --load-offset=0x200 --boot-cs=0x2000 --boot-ip=0x200 /boot7


title Load DUET EFI Emulator
## This assumes that the Duet file is "efildr20" at the root of some filesystems that is visible to
## Grub4dos. Adjust the name and path if you have named it differently.
find --set-root /efildr20
chainloader --force --load-segment=0x2000 --load-offset=0x0 --boot-cs=0x2000 --boot-ip=0x200 /efildr20

 

For anyone who's not aware of Grub4dos: it's not just a DOS program. The download link for the pre-compiled BIOS version of it is https://github.com/chenall/grub4dos/releases/download/0.4.6a/grub4dos-0.4.6a-2024-02-26.7z

 

Don't be confused by the name: it's not just a DOS program. It come with DOS and Linux installers for installing it to the master boot record. Alternatively, it can be chainloaded from various other BIOS bootloaders such as [the BIOS version of] Grub2. The only minor glitch that I found when chainloading it was that it couldn't find its own menu (probably because it didn't set its own root variable) — but this was easily fixed by opening the executable (grub4dos.exe) in a text editor and adjusting the built-in menu).

Edited July 12, 2024 by Captain Flack
Added download link and hints for Grub4dos

[STLVNUB]

Good Info, Well Done