kernel hangs at serial port during boot (solved)

[walt]

walt wrote:

 > sio0: configured irq 4 not in bitmap of probed irqs 0
 > sio0: port may not be enabled
 > sio0 at port 0x3f8-0x3ff irq 4 flags 0x10 on isa0
 > sio0: type 16550A
 > <boot hangs forever at this point>

I changed the Plug-n-Play BIOS setting and now it works
normally again.

I also changed the ACPI-aware-OS BIOS setting to YES
and I notice that the acpi.ko module is now loading
at boot, which it didn't before.  I think this is an
old thing that I didn't notice until today, however.

What surprised me is that the machine has been working
for years with the 'wrong' BIOS setting and just now
stopped working.

Were there some kernel changes recently which would
account for this?