BIOS: ASUS EX-B560M-V5 (desired state)¶

The BIOS settings of this board are not declarable from NixOS: they live in the firmware NVRAM, and the kernel firmware-attributes interface (used by fwupdmgr get-bios-settings) only exists on Dell/Lenovo/HP business boards, not on consumer ASUS. fwupd here answers literally This system doesn't support firmware settings.

So this file is the closest thing to "declarative" available: the intent, versioned. It does not apply itself, but it survives a Clear CMOS and it is reproducible by hand in ~2 min.

Machine: Intel i5-11400 + Arc B580 (Battlemage). BIOS 2803 (2025/12/26).

Settings¶

Access: Del at POST, then F7 (Advanced Mode).

Setting	Value	Where	Status
Launch CSM	Disabled	Boot → CSM	confirmed (OS)
Above 4G Decoding	Enabled	Advanced → PCI Subsystem	confirmed (OS)
Re-Size BAR Support	Enabled	Advanced → PCI Subsystem	confirmed (OS)
Primary Display	PCIE	Advanced → System Agent / Graphics	recommended
Secure Boot → OS Type	Other OS	Boot → Secure Boot	recommended
Fast Boot	Disabled	Boot	recommended
Ai Overclock Tuner (XMP)	XMP I (3200 MT/s)	Ai Tweaker	confirmed (3200 MT/s)

Confirmed (OS): measured from Linux. CSM off (the Arc is UEFI-only and it booted), Above 4G + ReBAR active (a 16G VRAM BAR mapped above 4G, through lspci).
Recommended: good practice for the Arc; not verified, confirm on the next visit.
XMP: the TeamGroup UD4-3200 kit was running at 2400 MT/s (XMP off); with XMP on it is confirmed at 3200 MT/s (dmidecode -t 17).

Why CSM = Disabled is critical¶

Arc GPUs have no legacy VBIOS (they are UEFI-only). With CSM on, the motherboard tries to bring video up in legacy mode, finds no ROM on the card and hangs on the ASUS logo. That was the "CSM Forced Enablement Phenomenon" documented on the Intel forum, and it is not a defect of the board.

The root of the problem on this board: the EX-B560M-V5 had a default that re-enabled CSM automatically (CSM came back by itself after a reboot or a failed POST). That default was turned off, which is the real fix, at the source. With CSM off and that auto-enable off, you can enter the BIOS normally with the Arc installed (video comes up through the card's GOP, with no broken screen). The reboot=pci (OS side) stays as reinforcement.

TODO: write down here the exact name of the auto-CSM option that was turned off (its BIOS label), for precise reproduction after a Clear CMOS.

The OS side (this part IS declarative)¶

boot.kernelParams = [ "reboot=pci" ] in system/hardware/gpu.nix: it forces a full reset through 0xCF9 on reboot, the GPU reinitializes clean as in a cold boot and the POST does not fail, so CSM is never turned back on. It fixes the warm-reboot hang.
services.fwupd.enable = true in system/hardware/hardware.nix: it does NOT cover this board's BIOS (ASUS is outside LVFS); it serves SSD firmware and other components.

Backup and restore¶

Profile in the BIOS (native): Tool → ASUS User Profile (or Save & Exit → Save Profile), which saves into a slot (1-8) in NVRAM. After a Clear CMOS it is just Load Profile.
Recovering from a hang: hold the power button ~8s, and a cold boot always recovers the Arc. As a last resort, a Clear CMOS (the CLRTC jumper, or pulling the battery for ~1 min) goes back to defaults.

Do NOT do¶

Do NOT flash a modified BIOS (a mod for hidden menus): this board has no USB BIOS FlashBack, so a brick becomes a job for an external SPI programmer (CH341A + clip).
Do NOT edit the raw variables (AMITSESetup, CpuSetup and friends, through setup_var/efivarfs by offset): the offsets are undocumented and change per BIOS version, so the risk is corrupting or bricking it. The options that matter are already in F7 Advanced Mode.

Updating the BIOS (reference)¶

Download the version's .CAP from https://www.asus.com/motherboards-components/motherboards/expedition/ex-b560m-v5/helpdesk_bios/ and check the official SHA-256.
A FAT32 flash drive, with the .CAP at the root.
Del → F7 → Tool → ASUS EZ Flash 3 Utility → pick the file.
The flash resets the settings to defaults, so reapply the table above afterwards.