>Umm.. NBPG was constant but PAGE_SIZE was not?
>Or NBPG was a hardware value and PAGE_SIZE was for vm?

The latter.  This was all clearer in the 4.[123]BSD system before
the Mach VM went in:

	NBPG:	hardware page size in bytes
	CLSIZE: "cluster" size for making bigger VM pages out of
		small hardware pages
	CLBYTES: (CLSIZE*NBPG)

I forget whether PGOFSET was NBPG-1 or CLBYTES-1, though.  CLSIZE
was 2 (on the VAX) so that the OS could pretend the hardware had
1024-byte pages, and reduce the paging overhead.

On the Tahoe, 68020, and other "post-4.3BSD" non-VAX ports, CLSIZE
was effectively always 1.

Chris