diff --git a/src/drivers/bus/isapnp.c b/src/drivers/bus/isapnp.c
index 79268d44..b34108c7 100644
--- a/src/drivers/bus/isapnp.c
+++ b/src/drivers/bus/isapnp.c
@@ -529,7 +529,7 @@ static int isapnp_try_isolate ( void ) {
  *
  */
 static void isapnp_isolate ( void ) {
-	for ( isapnp_read_port = ISAPNP_READ_PORT_MIN ;
+	for ( isapnp_read_port = ISAPNP_READ_PORT_START ;
 	      isapnp_read_port <= ISAPNP_READ_PORT_MAX ;
 	      isapnp_read_port += ISAPNP_READ_PORT_STEP ) {
 		/* Avoid problematic locations such as the NE2000
diff --git a/src/include/gpxe/isapnp.h b/src/include/gpxe/isapnp.h
index 72ce0a6f..07797a99 100644
--- a/src/include/gpxe/isapnp.h
+++ b/src/include/gpxe/isapnp.h
@@ -49,7 +49,8 @@
 /* Port addresses */
 #define ISAPNP_ADDRESS		0x279
 #define ISAPNP_WRITE_DATA	0xa79
-#define ISAPNP_READ_PORT_MIN	0x213	/* ISAPnP spec says 0x203, but
+#define ISAPNP_READ_PORT_MIN	0x203
+#define ISAPNP_READ_PORT_START	0x213	/* ISAPnP spec says 0x203, but
 					 * Linux ISAPnP starts at
 					 * 0x213 with no explanatory
 					 * comment.  0x203 probably
@@ -63,6 +64,10 @@
 					 * any value less than 16.
 					 */
 
+/* Card select numbers */
+#define ISAPNP_CSN_MIN		0x01
+#define ISAPNP_CSN_MAX		0x0f
+
 /* Registers */
 #define ISAPNP_READPORT			0x00
 #define ISAPNP_SERIALISOLATION 		0x01
diff --git a/src/include/gpxe/netdevice.h b/src/include/gpxe/netdevice.h
index c0df7c96..0bc5311c 100644
--- a/src/include/gpxe/netdevice.h
+++ b/src/include/gpxe/netdevice.h
@@ -289,8 +289,9 @@ netdev_put ( struct net_device *netdev ) {
 }
 
 extern int netdev_tx ( struct net_device *netdev, struct io_buffer *iobuf );
-void netdev_tx_complete ( struct net_device *netdev, struct io_buffer *iobuf );
-void netdev_tx_complete_next ( struct net_device *netdev );
+extern void netdev_tx_complete ( struct net_device *netdev,
+				 struct io_buffer *iobuf );
+extern void netdev_tx_complete_next ( struct net_device *netdev );
 extern void netdev_rx ( struct net_device *netdev, struct io_buffer *iobuf );
 extern int netdev_poll ( struct net_device *netdev, unsigned int rx_quota );
 extern struct io_buffer * netdev_rx_dequeue ( struct net_device *netdev );
@@ -299,9 +300,9 @@ extern int register_netdev ( struct net_device *netdev );
 extern int netdev_open ( struct net_device *netdev );
 extern void netdev_close ( struct net_device *netdev );
 extern void unregister_netdev ( struct net_device *netdev );
-struct net_device * find_netdev ( const char *name );
-struct net_device * find_pci_netdev ( unsigned int busdevfn );
-
+extern struct net_device * find_netdev ( const char *name );
+extern struct net_device * find_netdev_by_location ( unsigned int bus_type,
+						     unsigned int location );
 extern int net_tx ( struct io_buffer *iobuf, struct net_device *netdev,
 		    struct net_protocol *net_protocol, const void *ll_dest );
 extern int net_rx ( struct io_buffer *iobuf, struct net_device *netdev,
diff --git a/src/interface/pxe/pxe_preboot.c b/src/interface/pxe/pxe_preboot.c
index 15752b23..a74e58af 100644
--- a/src/interface/pxe/pxe_preboot.c
+++ b/src/interface/pxe/pxe_preboot.c
@@ -28,6 +28,9 @@
 #include <stdlib.h>
 #include <gpxe/uaccess.h>
 #include <gpxe/dhcp.h>
+#include <gpxe/device.h>
+#include <gpxe/netdevice.h>
+#include <gpxe/isapnp.h>
 #include <basemem_packet.h>
 #include "pxe.h"
 #include "pxe_call.h"
@@ -196,41 +199,47 @@ PXENV_EXIT_t pxenv_restart_tftp ( struct s_PXENV_TFTP_READ_FILE
  * Status: working
  */
 PXENV_EXIT_t pxenv_start_undi ( struct s_PXENV_START_UNDI *start_undi ) {
+	unsigned int isapnp_read_port;
+	unsigned int isapnp_csn;
+	unsigned int pci_busdevfn;
+	unsigned int bus_type;
+	unsigned int location;
+	struct net_device *netdev;
 
-	DBG ( "PXENV_START_UNDI" );
+	DBG ( "PXENV_START_UNDI %04x:%04x:%04x",
+	      start_undi->AX, start_undi->BX, start_undi->DX );
 
-#if 0
-	/* Record PCI bus & devfn passed by caller, so we know which
-	 * NIC they want to use.
-	 *
-	 * If they don't match our already-existing NIC structure, set
-	 * values to ensure that the specified NIC is used at the next
-	 * call to pxe_intialise_nic().
-	 */
-	bus = ( start_undi->AX >> 8 ) & 0xff;
-	devfn = start_undi->AX & 0xff;
+	/* Determine bus type and location */
+	isapnp_read_port = start_undi->DX;
+	isapnp_csn = start_undi->BX;
+	pci_busdevfn = start_undi->AX;
 
-#warning "device probing mechanism has completely changed"
-#if 0
-	if ( ( pci->dev.driver == NULL ) ||
-	     ( pci->dev.bus != bus ) || ( pci->dev.devfn != devfn ) ) {
-		/* This is quite a bit of a hack and relies on
-		 * knowledge of the internal operation of Etherboot's
-		 * probe mechanism.
-		 */
-		DBG ( " set PCI %hhx:%hhx.%hhx",
-		      bus, PCI_SLOT(devfn), PCI_FUNC(devfn) );
-		dev->type = BOOT_NIC;
-		dev->to_probe = PROBE_PCI;
-		memset ( &dev->state, 0, sizeof(dev->state) );
-		pci->advance = 1;
-		pci->dev.use_specified = 1;
-		pci->dev.bus = bus;
-		pci->dev.devfn = devfn;
+	/* Use a heuristic to decide whether we are PCI or ISAPnP */
+	if ( ( isapnp_read_port >= ISAPNP_READ_PORT_MIN ) &&
+	     ( isapnp_read_port <= ISAPNP_READ_PORT_MAX ) &&
+	     ( isapnp_csn >= ISAPNP_CSN_MIN ) &&
+	     ( isapnp_csn <= ISAPNP_CSN_MAX ) ) {
+		bus_type = BUS_TYPE_ISAPNP;
+		location = isapnp_csn;
+	} else {
+		bus_type = BUS_TYPE_PCI;
+		location = pci_busdevfn;
 	}
-#endif
 
-#endif
+	/* Look for a matching net device */
+	netdev = find_netdev_by_location ( bus_type, location );
+	if ( ! netdev ) {
+		DBG ( " no net device found" );
+		start_undi->Status = PXENV_STATUS_UNDI_CANNOT_INITIALIZE_NIC;
+		return PXENV_EXIT_FAILURE;
+	}
+	DBG ( " using netdev %s", netdev->name );
+
+	/* Save as PXE net device */
+	pxe_set_netdev ( netdev );
+
+	/* Hook INT 1A */
+	pxe_hook_int1a();
 
 	start_undi->Status = PXENV_STATUS_SUCCESS;
 	return PXENV_EXIT_SUCCESS;
diff --git a/src/net/netdevice.c b/src/net/netdevice.c
index 8a099107..971830d9 100644
--- a/src/net/netdevice.c
+++ b/src/net/netdevice.c
@@ -356,15 +356,17 @@ struct net_device * find_netdev ( const char *name ) {
 /**
  * Get network device by PCI bus:dev.fn address
  *
- * @v busdevfn		PCI bus:dev.fn address
+ * @v bus_type		Bus type
+ * @v location		Bus location
  * @ret netdev		Network device, or NULL
  */
-struct net_device * find_pci_netdev ( unsigned int busdevfn ) {
+struct net_device * find_netdev_by_location ( unsigned int bus_type,
+					      unsigned int location ) {
 	struct net_device *netdev;
 
 	list_for_each_entry ( netdev, &net_devices, list ) {
-		if ( ( netdev->dev->desc.bus_type == BUS_TYPE_PCI ) &&
-		     ( netdev->dev->desc.location == busdevfn ) )
+		if ( ( netdev->dev->desc.bus_type == bus_type ) &&
+		     ( netdev->dev->desc.location == location ) )
 			return netdev;
 	}