Thanks to a kind tester, I can confirm that the latest sfb.c works
properly on a PMAGB-BB.  Kernels built from yesterday's -current are
in ftp.netbsd.org:/ftp/pub/NetBSD/arch/pmax/sfb-test-kernel/ for
anyone else who needs them. 

(I think there was someone in Oz with a PMAGB-BB?)