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Aug 152014

This post is likely to change frequently after it first appears as experiments/research/etc. occurs to me.

To get to grips with it, let’s first define the software that is in control of your PC when it first starts as the system firmware rather than the BIOS, as system firmware is a generic term which can refer to BIOS, EFI, UEFI, OpenFirmware, or anything else that someone comes up with.

UEFI (and the older EFI) is a replacement for the legacy BIOS that we’ve been stuck with for decades. Despite advances in almost every aspect of the “PC standard”, the BIOS has hardly advanced at all. To be fair, the user interface has gotten a bit less 1980s, and it of course deals with hardware devices that weren’t even imagined decades ago. But there are still some rather nasty limitations, which UEFI is supposed to resolve.

Of course there are those who claim that UEFI is a complete mess with no redeeming qualities, but the truth is probably somewhere between it being that and the neatest bit of system firmware ever invented.

This posting are my working notes (with the addition of a bit of pointless waffling) on UEFI, given that I’m likely to be found staring at a UEFI shell on a broken server at some point in the future. All experiments (so far) have been carried out with a VirtualBox-powered virtual machine (with UEFI turned on) running Ubuntu server.


The boot process looks a little different … unsurprisingly. And there’s an error in relation to missing UEFI stuff from the hard disk. But once the CD is booted the process looks the same …

Partitioning: Initially “Guided – use entire disk and set up LVM”

Doesn’t give an opportunity to review the partitioning, but :-

  1. EFISystem (/boot/efi) is roughly 512Mbytes
  2. /boot is roughly 256Mbytes
  3. LVM Volume Group

The Ubuntu installation uses grub as the boot loader, although it’s hardly the only option and there are hints that grub has been known to have issues with EFI. Although this could be “early adopters pain”, and not applicable currently.

GPT Partitions

See the Wikipedia article, but basically GPT replaced the old Master Boot Record partitions. The main advantage is that there are fewer dumb limits with GPT – there is no limit of 4 primary partitions (and no hacks to support extended partitions), but instead a minimum maximum of 128 partitions, which basically translates as the required minimum size of a partition table allows for up to 128 partitions but the partition table can be bigger.  More than you’re likely to need anyway.

Cleverly (if you want to put it that way), GPT can live alongside the old MBR partitions as the GPT starts at block 1 rather than block 0. This has been used by Apple to keep two partition tables so that OSX can use GPT whilst Windows still uses MBR.

Keeping two partition tables in sync is perhaps not the best idea for stability and given the need for backwards compatibility has only a limited useful lifetime, I’d rather live without it. In fact it would be nice if I could fill up the old MBR with stuff that told all MBR tools not to mess around with the partition tables.

In theory, (U)EFI and GPT are independent of each other, but in practice, GPT implies booting from UEFI and MBR implies booting from BIOS (some UEFI implementation switch to a BIOS-compatibility mode when they see an MBR).

The EFI System Partition

(U)EFI requires that to boot, a disk must have an EFI System Partition formatted as FAT. This is used as effectively a replacement for the BIOS boot method of loading code from block 0. This file system is usually mounted under Linux as /boot/efi and contains various files that allow Linux to be booted (more details to be added).

It is perhaps a shame that the EFI standards people didn’t suggest making the EFI system partition part of the on-board firmware. It wouldn’t be impossible (or very expensive) to incorporate a small writeable FAT file system into the motherboard to avoid the need for the EFI partition on one of the storage disks. It is not as if the EFI system partition needs to be very big – Ubuntu configured one as 512Mbytes in size which is vastly larger than required for what is actually installed which adds up to less than 4Mbytes.


The tool efibootmgr is for interacting with the EFI boot manager and the EFI variables that control what gets booted in which order :-

# efibootmgr
BootCurrent: 0003
BootOrder: 0003,0000,0001,0002
Boot0001* EFI Hard Drive
Boot0002* EFI Internal Shell
Boot0003* ubuntu

The command has plenty of other options …

You can set the boot order with: efibootmgr -o 0002,0003,0001,0000. So I’ve set the preferred boot order to include the internal shell first … the purpose here is to look into EFI after all.

I also somehow managed to erase the “ubuntu” so re-created that with: efibootmgr -c -L UbuntuServer -l “\EFI\ubuntu\shimx64.efi” (yes unfortunately they use the wrong path separator).

EFI Shell

Is surprisingly limited. And rather too DOS-like for me.

Command Description
help Displays a list of the commands available … which very unhelpfully doesn’t pause at the end of the screen. Can also display additional details of a command if you try help command.
mode Displays a list of the available screen mode commands.
mode x y Sets the screen mode to the size specified (as listed with mode).
cls Clears the screen.
cls ${n} Clears the screen and sets the colours to a set specified by the number (0-7). Don’t bother; most of the choices are nasty.
map Displays teh mapping table showing the block devices (BLK${n}) and the recognised file systems (FS${n}). If you don’t have fs0 you’ve got problems!
fs0: Sets the specified file system as the current file system. This will change the prompt appropriately.
cd ${directory} Changes to the specified directory. Remember that the path separator is the DOS preferred character (\).
ls Lists files in the current directory.
edit ${filename} Edits the specified file (^S saves, ^Q quits), but don’t try editing files ending in .efi!
${filename}.efi Runs the EFI binary from the current directory, and yes that does mean you can boot Ubuntu Server by browsing to the \EFI\ubuntu directory and entering shimx64.efi (as I discovered after breaking the ubuntu boot option).
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