> For the complete documentation index, see [llms.txt](https://open-rt.gitbook.io/open-surfacert/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://open-rt.gitbook.io/open-surfacert/common/boot-sequence/uefi/uefi-boot-sequence.md).

# UEFI Boot Sequence

According to [the UEFI spec](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_final.pdf) a device will initially start up and look for a particular file based on the architecture of the system:\
\
As we need to bypass the standard boot, we use the jailbreak to hack the process using a supplemental policy, which enables testsignmode.

The CPU will load the boot setup, and then load up UEFI "bios".\
\
UEFI will then load /efi/boot/bootarm.efi (which needs to be signed with a  secureboot-key) into ram and pass it control to then load other stuff in turn.\
\
We haven't been able to sign our own stuff yet ( if we could then we could just install uboot and bypass all the pain!).\
\
So.. we use Microsoft's, aka WindowsBootManager (/efi/boot/bootarm.efi) to eventually load our boot tool (i.e. grub2)<br>

Microsoft's bootarm.efi checks for&#x20;

efi/microsoft/boot/bcd (boot configuration data)

(which then loads our special "jailbreak" policy), then loads a test.efi shim, which finally loads a secondary EFI file - i.e. GRUB2 or uefi shell placed in the root.

/boot.efi \
\
Hardcoded bootloader boots "bootarm.efi"(1: windows bootmanager) Bootmanager loads BCD (2) \
Bootmanager loads SecureBootDebug.efi, and the SecureBootDebugPolicy.p7b\
Then loads  a test.efi (lolhax? shim) \
\[this can be changed to our own shim, eg <https://github.com/imbushuo/boot-shim/releases> ]\
Which finally loads our boot.efi\ <br>

```
+efi
+---microsoft
+------boot
+---------fonts (obvious)
+---------bcd(2)
+---------(automatically generated files)
+---boot
+------bootarm.efi(1)
+unicode.pf2
+test.efi(3)
+grub.cfg
+BOOT.efi
```

More detail here - \
[https://forum.xda-developers.com/windows-8-rt/rt-development/wip-secure-boot-linux-surface-rt-t3653848/page12](https://forum.xda-developers.com/showpost.php?p=82452439\&postcount=115)

## Further Reference

<https://wiki.debian.org/UEFI>\
<https://www.rodsbooks.com/efi-bootloaders/controlling-sb.html>\
\
<https://www.suse.com/media/article/UEFI_on_Top_of_U-Boot.pdf>\
(although we use GRUB2, the boot process is quite similar, and this is worth a read!)