As an epilog to the previous discussion.  I'm using NetBSD 1.2.
The boot banner begins:
	>> NetBSD BOOT 639/23552 k [1.28]
So someone knows that there is not 640K of memory here.  (The
boot program does an "int 12" to get the conventional memory
size.)

When the kernel starts running it does a:
	 biosbasemem = (mc146818_read(NULL, NVRAM_BASEHI) << 8) |
            mc146818_read(NULL, NVRAM_BASELO);
and gets 640.

I've not an 1.3_alpha system to load onto the machine (hint ;-),
but reading the latest locore.s, it certainly seems that the 
biosbasemem and biosextmem are fed in from the boot program
which does the the right calculation. 

But sadly, it looks like the 1.2 boot loader also passes the 
correct mem info too, it's just not used.