How to debug coreboot

[Yajin]

Hi everyone, I have a question about coreboot debugging.

The BIOS in mainboard has two types(fix me if I am wrong): one is PLCC32
Slot and the other is SPI interface. The first type of BIOS can be fetched
from mainboard and the SPI type bios can not be fetched(it is welded on
mainboard).
So how do you debug coreboot? If the BIOS type is PLCC32, we can fetch it
from mainboard and program it using flash programmer and debug and debug.
But if the BIOS type is SPI interface, you can not fetch the BIOS from
mainboard. How do you put coreboot into BIOS? How do you reprogram the BIOS
if the coreboot you just put into BIOS is a bug program?
The BIOS part is the most mysterious part of computer. Please give me some
information. Thanks.