diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index cd43952f6f..c99f73930d 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -101,7 +101,7 @@ __start:
          */
         testb   $MBI_MEMLIMITS,(%ebx)
         jz      2f                  /* not available? BDA value will be fine */
-        mov     4(%ebx),%edx
+        mov     MB_mem_lower(%ebx),%edx
         cmp     $0x100,%edx         /* is the multiboot value too small? */
         jb      2f                  /* if so, do not use it */
         shl     $10-4,%edx
diff --git a/xen/arch/x86/x86_64/asm-offsets.c b/xen/arch/x86/x86_64/asm-offsets.c
index 3994f4df1f..447c650b1a 100644
--- a/xen/arch/x86/x86_64/asm-offsets.c
+++ b/xen/arch/x86/x86_64/asm-offsets.c
@@ -165,4 +165,5 @@ void __dummy__(void)
 
     OFFSET(MB_flags, multiboot_info_t, flags);
     OFFSET(MB_cmdline, multiboot_info_t, cmdline);
+    OFFSET(MB_mem_lower, multiboot_info_t, mem_lower);
 }