> The artifacts during the boot might be a regression in MAME.
Of course I am wrong about that. MAME is fine (that is, behavior has not changed between versions), the BIOS is doing something unexpected. I just checked with the real BIOS in 0.148 and the screen corruption is there. The custom BIOS from Calchase that I was using for testing does not have that problem, so I have some work to do to identify the difference.