It will run whatever is in the flash chip. That one is generally written to outside the system's control. On modern PC mainboards with SPI flash (8 pins), ther

It will run whatever is in the flash chip. That one is generally written to outside the system's control. On modern PC mainboards with SPI flash (8 pins), there's often a set of pins (also 8) in close proximity to the flash chip - attach an external flasher to that, and you can write the flash while the PC is turned off. The alternative would be to write the flash before it's put on the board, but which method is chosen is an implementation detail.