tried to do so.

found that etherboot "recognizes" FreeBSD by checking if ELF entry address
is high (like >0xc0000000 in Net/FreeBSD).

so NetBSD GETS RECOGNIZED as FreeBSD, is successfully loaded and even
successfully executed, then first does

                printf("WARNING: CAN'T ALLOCATE EXTENDED MEMORY FROM IOMEM
EXTENT MAP!\n");

and then panics with uvm message that start >= end!

the problem is that etherboot passes params to kernel in FreeBSD's way:

                        (*entry)(freebsd_howto, NODEV, 0, 0, 0, &info.bsdinfo, 0, 0, 0);


while it should be:

(*entry)(0,NODEV,&info.bsdinfo,0,meminfo.basememsize,meminfo.memsize);


so:

how to detect if ELF image is NetBSD or FreeBSD? (what to check in ELF
header)

i'm not good in ELFs ;)