Interesting. I'll see about trying this tomorrow.

For comparison, here's what I did:

Index: pci_550.c
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/alpha/pci/pci_550.c,v
retrieving revision 1.13
diff -c -r1.13 pci_550.c
*** pci_550.c	1999/02/12 06:25:13	1.13
--- pci_550.c	2000/03/16 18:39:01
***************
*** 114,119 ****
--- 114,127 ----
  #define	DEC_550_PCI_IRQ_BEGIN	8
  #define	DEC_550_MAX_IRQ		48
  
+ #define DEC_550_ISA_LINE_BEGIN 0xe0
+ #define DEC_550_ISA_LINE_END   0xef
+ #define DEC_550_ISA_LINE_MASK  0x0f
+ 
+ #define DEC_550_IS_ISA_LINE(line) ((line) >= DEC_550_ISA_LINE_BEGIN && \
+ 	(line) <= DEC_550_ISA_LINE_END)
+ #define DEC_550_ISA_LINE_IRQ(line) ((line) & DEC_550_ISA_LINE_MASK)
+ 
  /*
   * The Miata has a Pyxis, which seems to have problems with stray
   * interrupts.  Work around this by just ignoring strays.
***************
*** 203,208 ****
--- 211,218 ----
  	 * There will be no interrupt mapping for these devices, so just
  	 * bail out now.
  	 */
+ 	printf("dec_550_intr_map: about to map bus %d dev %d func %d pin %d line %d\n",
+ 		bus,device,function,buspin,line);
  	if (bus == 0) {
  		if ((hwrpb->rpb_variation & SV_ST_MASK) < SV_ST_MIATA_1_5) {
  			/* Miata 1 */
***************
*** 215,221 ****
  			if (device == 7) {
  				if (function == 0)
  					panic("dec_550_intr_map: SIO device");
! 				return (1);
  			}
  		}
  	}
--- 225,232 ----
  			if (device == 7) {
  				if (function == 0)
  					panic("dec_550_intr_map: SIO device");
! 				if ((function == 1) || (function == 2))
! 					return (1);
  			}
  		}
  	}
***************
*** 230,241 ****
  		return (1);
  	}
  
! 	/* Account for the PCI interrupt offset. */
! 	line += DEC_550_PCI_IRQ_BEGIN;
! 
! 	if (line >= DEC_550_MAX_IRQ)
! 		panic("dec_550_intr_map: dec 550 irq too large (%d)\n",
! 		    line);
  
  	*ihp = line;
  	return (0);
--- 241,265 ----
  		return (1);
  	}
  
! 	if (DEC_550_IS_ISA_LINE(line)) {
! 		/* This is not actually a PCI interrupt, but a PCI device
! 		 * whose interrupt is wired to an ISA interrupt line. Pass the
! 		 * value through unmolested so we can handle it in
! 		 * dec_550_intr_[dis]establish.  
! 		 */
! 		printf("Ah ha! ISA intr %d\n", DEC_550_ISA_LINE_IRQ(line));
! #if !NSIO
! 		panic("dec_550_intr_map: attempt to map ISA interrupt %d without " 
! 			"ISA ICU\n", DEC_550_IS_ISA_LINE(line));
! #endif
! 	} else {
! 		/* Account for the PCI interrupt offset. */
! 		line += DEC_550_PCI_IRQ_BEGIN;
! 
! 		if (line >= DEC_550_MAX_IRQ)
! 			panic("dec_550_intr_map: dec 550 irq too large (%d)\n",
! 			    line);
! 	}
  
  	*ihp = line;
  	return (0);
***************
*** 251,256 ****
--- 275,285 ----
  #endif
  	static char irqstr[16];		/* 12 + 2 + NULL + sanity */
  
+ #if NSIO
+ 	if (DEC_550_IS_ISA_LINE(ih))
+ 	  return sio_intr_string(NULL, DEC_550_ISA_LINE_IRQ(ih));
+ #endif
+ 	
  	if (ih >= DEC_550_MAX_IRQ)
  		panic("dec_550_intr_string: bogus 550 IRQ 0x%lx\n", ih);
  	sprintf(irqstr, "dec 550 irq %ld", ih);
***************
*** 269,274 ****
--- 298,309 ----
  #endif
  	void *cookie;
  
+ #if NSIO
+ 	if (DEC_550_IS_ISA_LINE(ih))
+ 		return sio_intr_establish(NULL, DEC_550_ISA_LINE_IRQ(ih),
+ 									IST_EDGE, level, func, arg);
+ #endif
+ 
  	if (ih >= DEC_550_MAX_IRQ)
  		panic("dec_550_intr_establish: bogus dec 550 IRQ 0x%lx\n", ih);
  
***************
*** 292,297 ****
--- 327,340 ----
  	int s;
   
  	s = splhigh();
+ 
+ #if 0 /* Need to figure out how to determine sio vs. 550 intr type from
+        * the cookie */
+ 	if (DEC_550_IS_ISA_LINE(ih)) {
+ 		sio_intr_disestablish(NULL, cookie);
+ 		return;
+ 	}
+ #endif
  
  	alpha_shared_intr_disestablish(dec_550_pci_intr, cookie,
  	    "dec 550 irq");