Following up on the previous BIOS generation endeavour, as well as wanting to demonstrate to any naysayer that a universal recovery console (or panic room) in the BIOS bootblock of an x86 system is not something from the realm of fantasy, it is my pleasure to introduce UBRX, the Universal Bootblock Recovery console for X86 systems.
In its current version (0.1), it is mostly a Proof of Concept aimed at the coreboot team, since the crafting of a 'panic room' type bootblock has been on their mind for some time, and my understanding is that their approach would be to make such a bootblock platform specific (the motherboard would first need to be supported by coreboot), whereas I believe that there is a way to be a lot more generic to produce a 'panic room' feature that works on all platforms, even the ones that coreboot doesn't support yet. The current bootblock is less than 2 KB, which ought to leave plenty of room for a Y-modem, CAR, and a full console implementation.
If your aim is to develop a real BIOS from scratch, and you have a PC that is more recent than the year 2000 (most PCs from the late nineties should work too) UBRX might also be of interest to you, since it should provide you with a serial console. In short, UBRX can also be used as a base for BIOS development, regardless of the machine you have.
What UBRX implements then is a safe method for the detection of a PnP Super I/O chip, as well as the detection and initialisation of a 16550 compatible UART there, to then provide on-demand access to a serial console. The emphasis here has to be with the safety of the detection being performed, as it is intended to be executed at every boot, without resulting in advert consequences for non PnP Super I/O hardware residing in the same hardware space as the one we check, or any non UART function residing in the Super I/O. To find more about how we achieve detection safety, I suggest you check the "Detection Primer" section of the UBRX README.
Now, of course, as with "unlimited" broadband, the "universal" applicability of UBRX comes with some understanding that fairness needs to be applied to the claim. As such, non PnP Super I/Os, PnP Super I/Os that require uncommon initialization (I'm looking at you ITE), and platforms using CPUs other than Intel or AMD are not currently supported. Also, due to the lack of datasheets from nVidia, it is very likely that UBRX may not work with motherboards sporting an nVidia chipset. However, if you have an Intel motherboard with an ICH# chipset, or an AMD motherboard with an SB##0 SouthBridge (which probably covers more than 90% of PCs from the last few years), UBRX is expected to work.
Finally, as UBRX is only a Proof of Concept for now, the console is limited to a serial repeater, so there's not much you can do with it. Especially it it missing CAR (Cache As RAM) initialization and Y-modem functionality, to be able to transfer and run bare metal executables to do interesting things, such as flashing the remainder of the BIOS, initializing the full range of RAM according to the hardware, or loading a debugger. Therefore, if you choose to test and flash UBRX, remember that you must have means to reflash your BIOS using an external program, as your PC will become unusable.
Please head to the github project for more info. A source tar archive of the source can also be downloaded here.