diff --git a/src/arch/x86/prefix/mromprefix.S b/src/arch/x86/prefix/mromprefix.S
index b636b92a..568d1c00 100644
--- a/src/arch/x86/prefix/mromprefix.S
+++ b/src/arch/x86/prefix/mromprefix.S
@@ -456,6 +456,24 @@ pci_set_mem_access:
 	ret
 	.size	pci_set_mem_access, . - pci_set_mem_access
 
+/* Update image source address for UNDI loader
+ *
+ * Parameters:
+ *   %esi : Image source address
+ * Returns:
+ *   %esi : Image source address
+ */
+	.section ".prefix", "ax", @progbits
+	.globl	undiloader_source
+undiloader_source:
+	/* Always use expansion ROM BAR directly when installing via
+	 * the UNDI loader entry point, since the PMM-allocated block
+	 * may collide with whatever is calling the UNDI loader entry
+	 * point.
+	 */
+	xorl	%esi, %esi
+	ret
+
 /* Payload prefix
  *
  * We include a dummy ROM header to cover the "hidden" portion of the
diff --git a/src/arch/x86/prefix/undiloader.S b/src/arch/x86/prefix/undiloader.S
index 530b48e8..1d77110e 100644
--- a/src/arch/x86/prefix/undiloader.S
+++ b/src/arch/x86/prefix/undiloader.S
@@ -35,7 +35,8 @@ undiloader:
 	movw	%es:12(%di), %bx
 	movw	%es:14(%di), %ax
 	movl	image_source, %esi
-	movl	decompress_to, %edi
+	call	undiloader_source
+	xorl	%edi, %edi
 	orl	$0xffffffff, %ebp	/* Allow arbitrary relocation */
 	call	install_prealloc
 	popw	%di
@@ -57,3 +58,16 @@ undiloader:
 	popl	%edi
 	popl	%esi
 	lret
+
+/* Update image source address for UNDI loader
+ *
+ * Parameters:
+ *   %esi : Image source address
+ * Returns:
+ *   %esi : Image source address
+ */
+	.section ".prefix", "ax", @progbits
+	.globl	undiloader_source
+	.weak	undiloader_source
+undiloader_source:
+	ret