DISCLAIMER:
I can’t really say whether for my question this is
- The proper sub forum
- The proper website in the first place.
I might be light years off.
Ok, here goes…:
Do you know someone, or have you done it yourself, writing your own minimum boot ROM code for any reasonably-recent machine?
I.e. >=Nehalem+Ibex Peak (that is, Core i7 CPU along with its particular chipset)
Your boot ROM should have at least shown one of the most basic signs of life, such as making the PC speaker or sound system play a tone, write “hello world!” text to the screen, or making a led blink at a USB/SD card reader output/Thunderbolt/whateves. No special/proprietary trace/debug solutions allowed!
Remember, I am NOT referring to writing your own boot loader, which I consider peanuts compared to writing such a BIOS “from scratch”, despite never having done a boot loader by myself.
Neither am I talking about removing all DXE modules and EFI apps from a particular BIOS, and let PEI load your own module you put there.
No, I refer to reflashing the SPI boot ROM with your own minimum silicon initialization code, and have your handmade code execute right from the reset vector at 0xfffffff0.
In assembly, or C (albeit after setting up Cache-As-RAM). No DRAM. Only the silicon YOU need for a sign of life.
Of course there is virtually 0 gain in doing such a pointless endeavor, however I at least consider pulling off such as something rather spectacular.
The closest I could come up with is UBRX, however this doesn’t seem to work if your machine doesn’t have a serial port, such as most newer notebooks/MacBooks.
Kind regards,
microwave89-hv
P.S. Of course reflashing would only work externally. After that your board is toast, unless you were to write/load a full fledged payload BIOS afterwards that has OS support 
.