Starting some time in December 1997, the FreeBSD project received reports of motherboards on which FreeBSD didn't recognize the UARTs (serial ports) when booting, notably motherboards from Iwill. In every case that we could verify, the boards in question uses UARTs from Acer, though we're not sure that they're the only culprits.
The problem was that the device probes didn't get an interrupt from the ports. The same motherboards worked normally with other operating systems and Microsoft products.
This problem was resolved in 3.0-CURRENT and 2.2-STABLE in mid-June 1998, and in FreeBSD-2.2.7-RELEASE in late July. If you're using older versions of FreeBSD, you shouldn't patch it, you should upgrade. The patch is no longer available.