The current way of getting information about the memory
size fails if there are more than 64M RAM installed.
I've built a bootblock image which employs 2 more BIOS
functions (15e801 and 15e820) besides the old 1588 to get
the size of extended memory (look at R.Brown's interrupt
list for an explanation).
It's located at
zelof1.zel.kfa-juelich.de:~ftp/pub/NetBSD/bootcode/biosboot.sym

Please give it a try and report about problems!

best regards
Matthias