diff --git a/src/core/main.c b/src/core/main.c
index da1c58a6..3cc777a8 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -144,7 +144,10 @@ static int initialized;
 
 
 /* Global instance of the current boot device */
-struct dev dev;
+DEV_BUS(struct {}, dev_bus);
+struct dev dev = {
+	.bus = &dev_bus,
+};
 
 /**************************************************************************
  * initialise() - perform any C-level initialisation
@@ -213,24 +216,6 @@ void exit(int status)
 }
 
 
-/*
- * Set PCI device to use.
- *
- * This routine can be called by e.g. the ROM prefix to specify that
- * the first device to be tried should be the device on which the ROM
- * was physically located.
- *
- * Note that this is deliberately in main.c rather than pci.c, because
- * this function should generalise to other bus types (e.g. ISAPnP),
- * and we don't want to end up dragging in pci.o unnecessarily.
- */
-void set_pci_device ( uint16_t busdevfn ) {
-	dev.devid.bus_type = PCI_BUS_TYPE;
-	dev.pci.busdevfn = busdevfn;
-	dev.pci.already_tried = 0;
-}
-
-
 #if 0
 
 static int main_loop(int state)
diff --git a/src/drivers/bus/eisa.c b/src/drivers/bus/eisa.c
index a294895d..c80974a8 100644
--- a/src/drivers/bus/eisa.c
+++ b/src/drivers/bus/eisa.c
@@ -13,6 +13,14 @@
 #define DBG(...)
 #endif
 
+/*
+ * Ensure that there is sufficient space in the shared dev_bus
+ * structure for a struct pci_device.
+ *
+ */
+DEV_BUS( struct eisa_device, eisa_dev );
+static char eisa_magic[0]; /* guaranteed unique symbol */
+
 /*
  * Fill in parameters for an EISA device based on slot number
  *
@@ -52,15 +60,14 @@ static int fill_eisa_device ( struct eisa_device *eisa ) {
  * Obtain a struct eisa * from a struct dev *
  *
  * If dev has not previously been used for an EISA device scan, blank
- * out dev.eisa
+ * out struct eisa
  */
 struct eisa_device * eisa_device ( struct dev *dev ) {
-	struct eisa_device *eisa = &dev->eisa;
+	struct eisa_device *eisa = dev->bus;;
 
-	if ( dev->devid.bus_type != EISA_BUS_TYPE ) {
+	if ( eisa->magic != eisa_magic ) {
 		memset ( eisa, 0, sizeof ( *eisa ) );
-		dev->devid.bus_type = EISA_BUS_TYPE;
-		eisa->slot = EISA_MIN_SLOT;
+		eisa->magic = eisa_magic;
 	}
 	eisa->dev = dev;
 	return eisa;
@@ -74,8 +81,13 @@ int find_eisa_device ( struct eisa_device *eisa, struct eisa_driver *driver ) {
 	unsigned int i;
 
 	/* Iterate through all possible EISA slots, starting where we
-	 * left off/
+	 * left off.  If eisa->slot is zero (which it will be if we
+	 * have a zeroed structure), start from slot EISA_MIN_SLOT,
+	 * since slot 0 doesn't exist.
 	 */
+	if ( ! eisa->slot ) {
+		eisa->slot = EISA_MIN_SLOT;
+	}
 	for ( ; eisa->slot <= EISA_MAX_SLOT ; eisa->slot++ ) {
 		/* If we've already used this device, skip it */
 		if ( eisa->already_tried ) {
@@ -101,6 +113,8 @@ int find_eisa_device ( struct eisa_device *eisa, struct eisa_driver *driver ) {
 						      eisa->prod_id ) );
 				if ( eisa->dev ) {
 					eisa->dev->name = driver->name;
+					eisa->dev->devid.bus_type
+						= ISA_BUS_TYPE;
 					eisa->dev->devid.vendor_id
 						= eisa->mfg_id;
 					eisa->dev->devid.device_id
diff --git a/src/drivers/bus/mca.c b/src/drivers/bus/mca.c
index 28b99f77..1715c588 100644
--- a/src/drivers/bus/mca.c
+++ b/src/drivers/bus/mca.c
@@ -19,6 +19,14 @@
 #define DBG(...)
 #endif
 
+/*
+ * Ensure that there is sufficient space in the shared dev_bus
+ * structure for a struct pci_device.
+ *
+ */
+DEV_BUS( struct mca_device, mca_dev );
+static char mca_magic[0]; /* guaranteed unique symbol */
+
 /*
  * Fill in parameters for an MCA device based on slot number
  *
@@ -53,14 +61,14 @@ static int fill_mca_device ( struct mca_device *mca ) {
  * Obtain a struct mca * from a struct dev *
  *
  * If dev has not previously been used for an MCA device scan, blank
- * out dev.mca
+ * out struct mca
  */
 struct mca_device * mca_device ( struct dev *dev ) {
-	struct mca_device *mca = &dev->mca;
+	struct mca_device *mca = dev->bus;
 
-	if ( dev->devid.bus_type != MCA_BUS_TYPE ) {
+	if ( mca->magic != mca_magic ) {
 		memset ( mca, 0, sizeof ( *mca ) );
-		dev->devid.bus_type = MCA_BUS_TYPE;
+		mca->magic = mca_magic;
 	}
 	mca->dev = dev;
 	return mca;
@@ -97,8 +105,10 @@ int find_mca_device ( struct mca_device *mca, struct mca_driver *driver ) {
 				      id->name, driver->name, id->id );
 				if ( mca->dev ) {
 					mca->dev->name = driver->name;
-					mca->dev->devid.vendor_id =
-						GENERIC_MCA_VENDOR;
+					mca->dev->devid.bus_type
+						= MCA_BUS_TYPE;
+					mca->dev->devid.vendor_id
+						= GENERIC_MCA_VENDOR;
 					mca->dev->devid.device_id = id->id;
 				}
 				mca->already_tried = 1;
diff --git a/src/drivers/bus/pci.c b/src/drivers/bus/pci.c
index 4b4e9a15..d23a647a 100644
--- a/src/drivers/bus/pci.c
+++ b/src/drivers/bus/pci.c
@@ -9,6 +9,14 @@
 #define DBG(...)
 #endif
 
+/*
+ * Ensure that there is sufficient space in the shared dev_bus
+ * structure for a struct pci_device.
+ *
+ */
+DEV_BUS( struct pci_device, pci_dev );
+static char pci_magic[0]; /* guaranteed unique symbol */
+
 /*
  * Fill in parameters (vendor & device ids, class, membase etc.) for a
  * PCI device based on bus & devfn.
@@ -112,19 +120,33 @@ void adjust_pci_device ( struct pci_device *pci ) {
  * Obtain a struct pci * from a struct dev *
  *
  * If dev has not previously been used for a PCI device scan, blank
- * out dev.pci
+ * out struct pci
  */
 struct pci_device * pci_device ( struct dev *dev ) {
-	struct pci_device *pci = &dev->pci;
+	struct pci_device *pci = dev->bus;
 
-	if ( dev->devid.bus_type != PCI_BUS_TYPE ) {
+	if ( pci->magic != pci_magic ) {
 		memset ( pci, 0, sizeof ( *pci ) );
-		dev->devid.bus_type = PCI_BUS_TYPE;
+		pci->magic = pci_magic;
 	}
 	pci->dev = dev;
 	return pci;
 }
 
+/*
+ * Set PCI device to use.
+ *
+ * This routine can be called by e.g. the ROM prefix to specify that
+ * the first device to be tried should be the device on which the ROM
+ * was physically located.
+ *
+ */
+void set_pci_device ( uint16_t busdevfn ) {
+	pci_dev.magic = pci_magic;
+	pci_dev.busdevfn = busdevfn;
+	pci_dev.already_tried = 0;
+}
+
 /*
  * Find a PCI device matching the specified driver
  *
@@ -154,6 +176,7 @@ int find_pci_device ( struct pci_device *pci,
 		/* Fill in dev structure, if present */
 		if ( pci->dev ) {
 			pci->dev->name = driver->name;
+			pci->dev->devid.bus_type = PCI_BUS_TYPE;
 			pci->dev->devid.vendor_id = pci->vendor;
 			pci->dev->devid.device_id = pci->dev_id;
 		}
diff --git a/src/include/dev.h b/src/include/dev.h
index a9e2916b..b5a2edda 100644
--- a/src/include/dev.h
+++ b/src/include/dev.h
@@ -3,11 +3,6 @@
 
 #include "stdint.h"
 
-/* Bus types */
-#include "pci.h"
-#include "eisa.h"
-#include "mca.h"
-
 /* Device types */
 #include "nic.h"
 
@@ -18,8 +13,7 @@ struct dev_id {
 	uint8_t		bus_type;
 #define	PCI_BUS_TYPE	1
 #define	ISA_BUS_TYPE	2
-#define EISA_BUS_TYPE	3
-#define MCA_BUS_TYPE	4
+#define MCA_BUS_TYPE	3
 } __attribute__ ((packed));
 
 /* Dont use sizeof, that will include the padding */
@@ -29,18 +23,25 @@ struct dev {
 	struct dev_operations *dev_op;
 	const char *name;
 	struct dev_id	devid;	/* device ID string (sent to DHCP server) */
-	/* All possible bus types */
-	union {
-		struct pci_device	pci;
-		struct eisa_device	eisa;
-		struct mca_device	mca;
-	};
+	/* Pointer to bus information for device.  Whatever sets up
+	 * the struct dev must make sure that this points to a buffer
+	 * large enough for the required struct <bus>_device.
+	 */
+	void *bus;
 	/* All possible device types */
 	union {
 		struct nic	nic;
 	};
 };
 
+/*
+ * Macro to help create a common symbol with enough space for any
+ * struct <bus>_device.
+ *
+ * Use as e.g. DEV_BUS(struct pci_device);
+ */
+#define DEV_BUS(datatype,symbol) datatype symbol __asm__ ( "_dev_bus" );
+
 struct dev_operations {
 	void ( *disable ) ( struct dev * );
 	void ( *print_info ) ( struct dev * );
diff --git a/src/include/eisa.h b/src/include/eisa.h
index 4fe14fc6..ed0ccef3 100644
--- a/src/include/eisa.h
+++ b/src/include/eisa.h
@@ -27,6 +27,7 @@
  */
 struct dev;
 struct eisa_device {
+	char *magic; /* must be first */
 	struct dev *dev;
 	unsigned int slot;
 	uint16_t ioaddr;
diff --git a/src/include/mca.h b/src/include/mca.h
index e69b6560..c12b1cab 100644
--- a/src/include/mca.h
+++ b/src/include/mca.h
@@ -28,6 +28,7 @@
  */
 struct dev;
 struct mca_device {
+	char *magic; /* must be first */
 	struct dev *dev;
 	unsigned int slot;
 	unsigned char pos[8];
diff --git a/src/include/pci.h b/src/include/pci.h
index 36edc0f5..988259a3 100644
--- a/src/include/pci.h
+++ b/src/include/pci.h
@@ -232,13 +232,13 @@
 #define PCI_MSI_ADDRESS_HI	8	/* Upper 32 bits (if PCI_MSI_FLAGS_64BIT set) */
 #define PCI_MSI_DATA_32		8	/* 16 bits of data for 32-bit devices */
 #define PCI_MSI_DATA_64		12	/* 16 bits of data for 64-bit devices */
-
 /*
  * A physical PCI device
  *
  */
 struct dev;
 struct pci_device {
+	char *			magic; /* must be first */
 	struct dev *		dev;
 	uint32_t		membase;	/* BAR 1 */
 	uint32_t		ioaddr;		/* first IO BAR */