> Did MI SCSI support get pulled from -current?

Yes.  1.5 branch has it.

> I commented out the old-style SCSI, and included the MI SCSI stuff, but
> it won't link cleanly now (in ioconf.o there is an undefined reference
> to xasc_ioasic_ca).

You need to comment out the following (from GENERIC);
	asc*    at      ioasic? offset ?        # NCR53C94 SCSI with IOASIC
	asc*    at      tc? slot ? offset ?     # PMAZ-A single channel SCSI
	sii*    at      ibus0 addr ?            # DC7061 onboard SCSI
	oldscsibus*     at sii?
	oldscsibus*     at asc?
	include         "arch/pmax/conf/scsi.pmax"

and uncomment;
	#xasc*  at      ioasic? offset ?        #  [ MI SCSI version ]
	#include        "arch/pmax/conf/mi.scsi"

and make sure to comment out in mi.scsi; 

	scsibus*        at xsii?

(I don't like to scatter plain conf file into pieces....)

Device special files of /dev/sd* have;

	blk = 19, chr = 9

they are different from ULTRIX derived disk device tz.  MAKEDEV script
can be found at basesrc/etc/etc.pmax/ directory.  Stripped down version
is also available at;

	ftp://ftp.netbsd.org/pub/NetBSD/arch/pmax/MAKEDEV.MISCSI

Tohru Nishimura