While we aim for a 100% free boot process, recent developments (and general unwillingness by some hardware companies to provide specifications) make it hard to achieve.
On Intel based chipsets (since Intel 5 Series) the following binary components persist:
- Panic level: 9000+ Management Engine firmware: The management engine is a separate CPU that does various management tasks and needs its own firmware. This firmware exists in a 1.5MB and a 5MB version, where the latter provides the "Intel AMT" functions (ie. remote access, "anti-theft", ...). Probably signed with an Intel key. It's unlikely that this is ever replaced by something open source. Firmware that runs on an ARC core inside the chipset. It runs entirely out-of-band with the main CPU. It has DMA access to the entire system memory and can access the networking adapters in a way transparent to the OS (separate MAC and IP).
- Panic level: medium VGABIOS: Runs on the CPU. Unless you can live with staying in the dark until Linux takes over, you'll need this. Luckily there's work in progress to replace it with open source code. Until then you can at least contain it by emulation (see YABEL).
- Panic level: small CPU microcode: Intel provides this as redistributable binary, the format is partially reverse engineered, it's covered by a 2048b RSA signature. Unlikely that it can be replaced. Depending on the CPU (incl. its stepping) it might be possible to get by without it. Check http://inertiawar.com/microcode/ for details.
- Panic level: medium-high Gigabit Ethernet Firmware: If your board uses the on-chipset GbE, it requires a small binary (8KB) with unknown content.
- Panic level: 8999 Memory Reference Code: (Sandybridge and newer). This is code that runs on the CPU and initializes RAM. Can be reverse engineered with enough persistence, but not done so far. The "Memory Reference Code", initializes memory and USB power states (other functions are yet unknown). It is provided by Google, and is wrapper for Intel PEI modules.
- IMC: An embedded controller of sorts in the southbridge. 8051-based, can probably be reimplemented (partially done, but unpublished) Check the AMD_IMC page. The controller is either enabled by hardware strap option. Or if you provide a firmware, the controller is enabled via soft strapping the chipset. It is 8051 controller.
- XHCI: Controller for USB 3.0 controllers. Analysis has shown that the firmware used in AMD system is most likely Renesas USB 3.0 IP Core. It seems it is V850 compatible controller If not present, USB3 (and related USB ports) won't work. Partial documentation is at AMD_XHCI.
- NIC Firmware: If your board uses the on-chip broadcom NIC, you need this firmware. Luckily few boards do (thanks to Broadcom seemingly having some "interesting" terms and conditions on its use) Check http://review.coreboot.org/#/c/2831/ for partial documentation
- CPU microcode: AMD provides this as redistributable binary, the format is partially reverse engineered, it's covered by a 2048b RSA signature. Unlikely that it can be replaced. Depending on the CPU (incl. its stepping) it might be possible to get by without it. Older microcode described http://www.securiteam.com/securityreviews/5FP0M1PDFO.html
- SMU: Another embedded controller, The SMU seems to be handling PCIe power management stuff in AMD northbridges (from RS880 onwards?) the firmware is loaded during system boot. It is unknown if the firmware has to be loaded. The SMU is most likely Altera LM32 CPU.
- VGA BIOS: The VGA BIOS contains bytecode which is motherboard/layout specific. Open source efforts here http://sourceforge.net/projects/openradeonbios/?source=dlp
In theory it might be possible to successfully boot an AMD board without all these binaries, with potentially reduced capabilities (no NIC, USB3, fan control)