diff --git a/sys/arch/alpha/pci/cia.c b/sys/arch/alpha/pci/cia.c
index c90859c4157b..64bf8b458f72 100644
--- a/sys/arch/alpha/pci/cia.c
+++ b/sys/arch/alpha/pci/cia.c
@@ -1,4 +1,41 @@
-/* $NetBSD: cia.c,v 1.41 1998/06/05 17:24:11 thorpej Exp $ */
+/* $NetBSD: cia.c,v 1.42 1998/06/05 19:25:19 thorpej Exp $ */
+
+/*-
+ * Copyright (c) 1998 The NetBSD Foundation, Inc.
+ * All rights reserved.
+ *
+ * This code is derived from software contributed to The NetBSD Foundation
+ * by Jason R. Thorpe of the Numerical Aerospace Simulation Facility,
+ * NASA Ames Research Center.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. All advertising materials mentioning features or use of this software
+ *    must display the following acknowledgement:
+ *	This product includes software developed by the NetBSD
+ *	Foundation, Inc. and its contributors.
+ * 4. Neither the name of The NetBSD Foundation nor the names of its
+ *    contributors may be used to endorse or promote products derived
+ *    from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
 
 /*
  * Copyright (c) 1995, 1996 Carnegie-Mellon University.
@@ -33,7 +70,7 @@
 
 #include <sys/cdefs.h>			/* RCS ID & Copyright macro defns */
 
-__KERNEL_RCSID(0, "$NetBSD: cia.c,v 1.41 1998/06/05 17:24:11 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: cia.c,v 1.42 1998/06/05 19:25:19 thorpej Exp $");
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -220,14 +257,23 @@ ciaattach(parent, self, aux)
 		    self->dv_xname);
 #endif
 
-	if ((ccp->cc_flags & CCF_ISPYXIS) != 0 && ccp->cc_rev < 1) {
+#ifdef DEC_550
+	if (hwrpb->rpb_type == ST_DEC_550 &&
+	    (hwrpb->rpb_variation & SV_ST_MASK) < SV_ST_MIATA_1_5) {
 		/*
-		 * Pass 1 Pyxis chips have a bug: DMA cannot cross
+		 * Miata 1 systems have a bug: DMA cannot cross
 		 * an 8k boundary!  Make sure PCI read prefetching
 		 * is disabled on these chips.  Note that secondary
 		 * PCI busses don't have this problem, because of
 		 * the way PPBs handle PCI read requests.
 		 *
+		 * In the 21174 Technical Reference Manual, this is
+		 * actually documented as "Pyxis Pass 1", but apparently
+		 * there are chips that report themselves as "Pass 1"
+		 * which do not have the bug!  Miatas with the Cypress
+		 * PCI-ISA bridge (i.e. Miata 1.5 and Miata 2) do not
+		 * have the bug, so we use this check.
+		 *
 		 * XXX We also need to deal with this boundary constraint
 		 * XXX in the PCI bus 0 (and ISA) DMA tags, but some
 		 * XXX drivers are going to need to be changed first.
@@ -244,6 +290,7 @@ ciaattach(parent, self, aux)
 		REGVAL(CIA_CSR_CTRL) = ctrl;
 		alpha_mb();
 	}
+#endif /* DEC_550 */
 
 	switch (hwrpb->rpb_type) {
 #ifdef DEC_KN20AA