Microcode boot error

Microcode boot error

In rare cases there might be black screen after the GRUB menu. Also in that case the machine is completely frozen, doesnt react to keypresses, only a hard shutdown/reboot remain the only option for the user. This issue is documented in the Debian wiki too and the proposed workaround also works with Ubuntu.

Workaround

If the black screen appears during live USB booting or after a reboot, we have to disable the microcode loading temporarly:

1. At the GRUB menu, press e key to edit the GRUB bootmenu entry
2. Search for the line similar to linux /boot/vmlinuz-5.19... root=UUID=... ro splash and append dis_ucode_ldr at the end
3. Press F10 key to continue the boot with this modified bootmenu entry, and that way the microcode loading will be skipped

Disable the microcode loading permanently can be useful otherwise the previous process must be repeated on every boot. To disable to microcode loading do the following:

1. Edit the /etc/default/grub file
2. Search the line GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" (or similar) and append the dis_ucode_ldr to the end like this: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash dis_ucode_ldr"
3. Generate the new grub.cfg file with the sudo update-grub command
4. Reboot the machine, which shoudl boot in successfully