diff --git a/src/drivers/net/rtl8139.c b/src/drivers/net/rtl8139.c
index 08130b23..81b6ccfb 100644
--- a/src/drivers/net/rtl8139.c
+++ b/src/drivers/net/rtl8139.c
@@ -367,10 +367,6 @@ static void rtl_close ( struct net_device *netdev ) {
  */
 static int rtl_transmit ( struct net_device *netdev, struct pk_buff *pkb ) {
 	struct rtl8139_nic *rtl = netdev->priv;
-	void *data;
-	size_t len;
-	size_t headroom;
-	signed int pad_len;
 
 	/* Check for space in TX ring */
 	if ( rtl->tx.pkb[rtl->tx.next] != NULL ) {
@@ -378,23 +374,8 @@ static int rtl_transmit ( struct net_device *netdev, struct pk_buff *pkb ) {
 		return -ENOBUFS;
 	}
 
-	/* Move packet data to start of packet buffer.  This will both
-	 * align the data (since packet buffers are aligned to
-	 * PKB_ALIGN) and give us sufficient space for the
-	 * zero-padding
-	 */
-	data = pkb->data;
-	len = pkb_len ( pkb );
-	headroom = pkb_headroom ( pkb );
-	pkb_push ( pkb, headroom );
-	memmove ( pkb->data, data, len );
-	pkb_unput ( pkb, headroom );
-	assert ( ( virt_to_bus ( pkb->data ) & 0x3 ) == 0 );
-
-	/* Pad to minimum packet length */
-	pad_len = ( ETH_ZLEN - pkb_len ( pkb ) );
-	if ( pad_len > 0 )
-		memset ( pkb_put ( pkb, pad_len ), 0, pad_len );
+	/* Pad and align packet */
+	pkb_pad ( pkb, ETH_ZLEN );
 
 	/* Add to TX ring */
 	DBG ( "TX id %d at %lx+%x\n", rtl->tx.next,
diff --git a/src/include/gpxe/pkbuff.h b/src/include/gpxe/pkbuff.h
index dc3d24c4..85bcc036 100644
--- a/src/include/gpxe/pkbuff.h
+++ b/src/include/gpxe/pkbuff.h
@@ -158,5 +158,6 @@ static inline size_t pkb_tailroom ( struct pk_buff *pkb ) {
 
 extern struct pk_buff * alloc_pkb ( size_t len );
 extern void free_pkb ( struct pk_buff *pkb );
+extern void pkb_pad ( struct pk_buff *pkb, size_t min_len );
 
 #endif /* _GPXE_PKBUFF_H */
diff --git a/src/net/pkbpad.c b/src/net/pkbpad.c
new file mode 100644
index 00000000..e12e46b9
--- /dev/null
+++ b/src/net/pkbpad.c
@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+/**
+ * @file
+ *
+ * Packet buffer padding
+ *
+ */
+
+#include <string.h>
+#include <gpxe/pkbuff.h>
+
+/**
+ * Pad packet buffer
+ *
+ * @v pkb		Packet buffer
+ * @v min_len		Minimum length
+ *
+ * This function pads and aligns packet buffers, for devices that
+ * aren't capable of padding in hardware, or that require specific
+ * alignment in TX buffers.  The packet data will end up aligned to a
+ * multiple of @c PKB_ALIGN.
+ *
+ * @c min_len must not exceed @v PKB_ZLEN.
+ */
+void pkb_pad ( struct pk_buff *pkb, size_t min_len ) {
+	void *data;
+	size_t len;
+	size_t headroom;
+	signed int pad_len;
+
+	assert ( min_len <= PKB_ZLEN );
+
+	/* Move packet data to start of packet buffer.  This will both
+	 * align the data (since packet buffers are aligned to
+	 * PKB_ALIGN) and give us sufficient space for the
+	 * zero-padding
+	 */
+	data = pkb->data;
+	len = pkb_len ( pkb );
+	headroom = pkb_headroom ( pkb );
+	pkb_push ( pkb, headroom );
+	memmove ( pkb->data, data, len );
+	pkb_unput ( pkb, headroom );
+
+	/* Pad to minimum packet length */
+	pad_len = ( min_len - pkb_len ( pkb ) );
+	if ( pad_len > 0 )
+		memset ( pkb_put ( pkb, pad_len ), 0, pad_len );
+}