Excerpts from netbsd: 5-Jan-98 Deng Hongwei: Re: NetBSD(i3.. "Perry E.
Metzger"@pierm (1896)

> I want to do some test about this( let BOOTSEC = 0x1000 in NetBSD ),
> but I don't know how many codes relative to it, how to make a boot 
> floppy, and where should I be in to build the install floppy(
> 'sys/arch/i386/boot/biosboot' or 'sys/arch/i386/stand')?

I just checked - there are no hidden dependencies on
BOOTSEC=0x100.
You can simply change it in i386/stand/lib/crt/bootsect/start_bootsect.S
and rebuild in i386/stand/biosboot.
Then put the new bootcode to the install floppy:
installboot -f -v biosboot.sym /dev/rfd0a

It it helps for your problem, please tell me.

best regards
Matthias