diff --git a/src/arch/i386/drivers/net/undinet.c b/src/arch/i386/drivers/net/undinet.c
index e479269a..835ec76d 100644
--- a/src/arch/i386/drivers/net/undinet.c
+++ b/src/arch/i386/drivers/net/undinet.c
@@ -605,15 +605,19 @@ int undinet_probe ( struct undi_device *undi ) {
 	DBGC ( undinic, "UNDINIC %p using UNDI %p\n", undinic, undi );
 
 	/* Hook in UNDI stack */
-	memset ( &start_undi, 0, sizeof ( start_undi ) );
-	start_undi.AX = undi->pci_busdevfn;
-	start_undi.BX = undi->isapnp_csn;
-	start_undi.DX = undi->isapnp_read_port;
-	start_undi.ES = BIOS_SEG;
-	start_undi.DI = find_pnp_bios();
-	if ( ( rc = undinet_call ( undinic, PXENV_START_UNDI, &start_undi,
-				   sizeof ( start_undi ) ) ) != 0 )
-		goto err_start_undi;
+	if ( ! ( undi->flags & UNDI_FL_STARTED ) ) {
+		memset ( &start_undi, 0, sizeof ( start_undi ) );
+		start_undi.AX = undi->pci_busdevfn;
+		start_undi.BX = undi->isapnp_csn;
+		start_undi.DX = undi->isapnp_read_port;
+		start_undi.ES = BIOS_SEG;
+		start_undi.DI = find_pnp_bios();
+		if ( ( rc = undinet_call ( undinic, PXENV_START_UNDI,
+					   &start_undi,
+					   sizeof ( start_undi ) ) ) != 0 )
+			goto err_start_undi;
+	}
+	undi->flags |= UNDI_FL_STARTED;
 
 	/* Bring up UNDI stack */
 	memset ( &undi_startup, 0, sizeof ( undi_startup ) );
@@ -703,6 +707,7 @@ void undinet_remove ( struct undi_device *undi ) {
 	memset ( &stop_undi, 0, sizeof ( stop_undi ) );
 	undinet_call ( undinic, PXENV_STOP_UNDI, &stop_undi,
 		       sizeof ( stop_undi ) );
+	undi->flags &= ~UNDI_FL_STARTED;
 
 	/* Free network device */
 	free_netdev ( netdev );
diff --git a/src/arch/i386/include/undi.h b/src/arch/i386/include/undi.h
index f6e22e70..f6fb85fe 100644
--- a/src/arch/i386/include/undi.h
+++ b/src/arch/i386/include/undi.h
@@ -7,6 +7,8 @@
  *
  */
 
+#ifndef ASSEMBLY
+
 #include <gpxe/device.h>
 #include <pxe_types.h>
 
@@ -42,8 +44,12 @@ struct undi_device {
 	 * Filled in only for the preloaded UNDI device by pxeprefix.S
 	 */
 	UINT16_t pci_device;
-	/** Padding */
-	UINT16_t pad;
+	/** Flags
+	 *
+	 * This is the bitwise OR of zero or more UNDI_FL_XXX
+	 * constants.
+	 */
+	UINT16_t flags;
 
 	/** Generic device */
 	struct device dev;
@@ -55,15 +61,6 @@ struct undi_device {
 	void *priv;
 } __attribute__ (( packed ));
 
-/** PCI bus:dev.fn field is invalid */
-#define UNDI_NO_PCI_BUSDEVFN 0xffff
-
-/** ISAPnP card select number field is invalid */
-#define UNDI_NO_ISAPNP_CSN 0xffff
-
-/** ISAPnP read port field is invalid */
-#define UNDI_NO_ISAPNP_READ_PORT 0xffff
-
 /**
  * Set UNDI driver-private data
  *
@@ -84,4 +81,18 @@ static inline void * undi_get_drvdata ( struct undi_device *undi ) {
 	return undi->priv;
 }
 
+#endif /* ASSEMBLY */
+
+/** PCI bus:dev.fn field is invalid */
+#define UNDI_NO_PCI_BUSDEVFN 0xffff
+
+/** ISAPnP card select number field is invalid */
+#define UNDI_NO_ISAPNP_CSN 0xffff
+
+/** ISAPnP read port field is invalid */
+#define UNDI_NO_ISAPNP_READ_PORT 0xffff
+
+/** UNDI flag: START_UNDI has been called */
+#define UNDI_FL_STARTED 0x0001
+
 #endif /* _UNDI_H */
diff --git a/src/arch/i386/prefix/pxeprefix.S b/src/arch/i386/prefix/pxeprefix.S
index 8c091df6..ca1a00c9 100644
--- a/src/arch/i386/prefix/pxeprefix.S
+++ b/src/arch/i386/prefix/pxeprefix.S
@@ -10,6 +10,8 @@
 	.section ".prefix.data", "aw", @progbits
 	.code16
 
+#include <undi.h>
+
 /*****************************************************************************
  * Entry point:	set operating context, print welcome message
  *****************************************************************************
@@ -278,6 +280,7 @@ unload_base_code:
  * Unload UNDI driver
  *****************************************************************************
  */
+#ifndef PXELOADER_KEEP_UNDI
 unload_undi:
 	/* Issue PXENV_STOP_UNDI */
 	movw	$PXENV_STOP_UNDI, %bx
@@ -286,12 +289,13 @@ unload_undi:
 	call	print_pxe_error
 	jmp	99f
 1:	/* Free base memory used by UNDI */
-#ifndef PXELOADER_KEEP_UNDI
 	movw	undi_fbms_start, %si
 	movw	undi_fbms_end, %di
 	call	free_basemem
-#endif /* PXELOADER_KEEP_UNDI */
+	/* Clear UNDI_FL_STARTED */
+	andw	$~UNDI_FL_STARTED, flags
 99:	
+#endif /* PXELOADER_KEEP_UNDI */
 
 /*****************************************************************************
  * Print remaining free base memory
@@ -678,12 +682,13 @@ entry_segment:		.word 0
 undi_fbms_start:	.word 0
 undi_fbms_end:		.word 0
 
-pci_busdevfn:		.word 0xffff
-isapnp_csn:		.word 0xffff
-isapnp_read_port:	.word 0xffff
+pci_busdevfn:		.word UNDI_NO_PCI_BUSDEVFN
+isapnp_csn:		.word UNDI_NO_ISAPNP_CSN
+isapnp_read_port:	.word UNDI_NO_ISAPNP_READ_PORT
 
 pci_vendor:		.word 0
 pci_device:		.word 0
+flags:			.word UNDI_FL_STARTED
 
 	.equ undi_device_size, ( . - undi_device )