diff --git a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBbs.c b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBbs.c
index c15e59972d..6ee43ad676 100644
--- a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBbs.c
+++ b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBbs.c
@@ -1,6 +1,6 @@
 /** @file
 
-Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2012, Intel Corporation. All rights reserved.<BR>
 
 This program and the accompanying materials
 are licensed and made available under the terms and conditions
@@ -352,8 +352,8 @@ LegacyBiosGetBbsInfo (
     // Pass in handoff data
     //
     TempData  = (UINTN) EfiToLegacy16BootTable;
-    Regs.X.ES = EFI_SEGMENT ((UINT32) TempData);
-    Regs.X.BX = EFI_OFFSET ((UINT32) TempData);
+    Regs.X.ES = NORMALIZE_EFI_SEGMENT ((UINT32) TempData);
+    Regs.X.BX = NORMALIZE_EFI_OFFSET ((UINT32) TempData);
 
     Private->LegacyBios.FarCall86 (
       This,
diff --git a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBiosInterface.h b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBiosInterface.h
index 71522a0ba4..a42e847250 100644
--- a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBiosInterface.h
+++ b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBiosInterface.h
@@ -1,6 +1,6 @@
 /** @file
 
-Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2012, Intel Corporation. All rights reserved.<BR>
 
 This program and the accompanying materials
 are licensed and made available under the terms and conditions
@@ -93,6 +93,14 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 #define PROTECTED_MODE_BASE_VECTOR_MASTER  0x68
 #define PROTECTED_MODE_BASE_VECTOR_SLAVE   0x70
 
+//
+// When we call CSM16 functions, some CSM16 use es:[offset + 0xabcd] to get data passed from CSM32, 
+// offset + 0xabcd could overflow which exceeds 0xFFFF which is invalid in real mode. 
+// So this will keep offset as small as possible to avoid offset overflow in real mode. 
+//
+#define NORMALIZE_EFI_SEGMENT(_Adr)      (UINT16) (((UINTN) (_Adr)) >> 4)
+#define NORMALIZE_EFI_OFFSET(_Adr)       (UINT16) (((UINT16) ((UINTN) (_Adr))) & 0xf)
+
 //
 // Trace defines
 //
diff --git a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
index 04013d9708..4e16880bd6 100644
--- a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
+++ b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
@@ -1,6 +1,6 @@
 /** @file
 
-Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2012, Intel Corporation. All rights reserved.<BR>
 
 This program and the accompanying materials
 are licensed and made available under the terms and conditions
@@ -1160,8 +1160,8 @@ GenericLegacyBoot (
   //
   // Pass in handoff data
   //
-  Regs.X.ES = EFI_SEGMENT ((UINTN)EfiToLegacy16BootTable);
-  Regs.X.BX = EFI_OFFSET ((UINTN)EfiToLegacy16BootTable);
+  Regs.X.ES = NORMALIZE_EFI_SEGMENT ((UINTN)EfiToLegacy16BootTable);
+  Regs.X.BX = NORMALIZE_EFI_OFFSET ((UINTN)EfiToLegacy16BootTable);
 
   Private->LegacyBios.FarCall86 (
     This,