diff --git a/MdePkg/Library/BaseLib/Ia32/Thunk16.S b/MdePkg/Library/BaseLib/Ia32/Thunk16.S
index df205e9a19..d45a0d40b1 100644
--- a/MdePkg/Library/BaseLib/Ia32/Thunk16.S
+++ b/MdePkg/Library/BaseLib/Ia32/Thunk16.S
@@ -1,6 +1,6 @@
 #------------------------------------------------------------------------------
 #
-# Copyright (c) 2006 - 2010, Intel Corporation. All rights reserved.<BR>
+# Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>
 # This program and the accompanying materials
 # are licensed and made available under the terms and conditions of the BSD License
 # which accompanies this distribution.  The full text of the license may be found at
@@ -55,7 +55,8 @@ ASM_PFX(ThunkAttr): .space  4
     orb     $2, %al
     outb    %al, $0x92                  # deactivate A20M#
 2:
-    movw    %ss, %ax
+    xorw    %ax, %ax                    # xor eax, eax
+    movl    %ss, %eax                   # mov ax, ss
     .byte   0x67, 0x66, 0x8d, 0x6c, 0x24, 0x34, 0x66
     mov     %ebp, 0xffffffd8(%esi)
     mov     0xfffffff8(%esi), %ebx
diff --git a/MdePkg/Library/BaseLib/Ia32/Thunk16.asm b/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
index 31f9254ad5..6df0a4a46a 100644
--- a/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
+++ b/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
@@ -3,7 +3,7 @@
 
 ;------------------------------------------------------------------------------
 ;
-; Copyright (c) 2006 - 2010, Intel Corporation. All rights reserved.<BR>
+; Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>
 ; This program and the accompanying materials
 ; are licensed and made available under the terms and conditions of the BSD License
 ; which accompanies this distribution.  The full text of the license may be found at
@@ -109,7 +109,8 @@ _ThunkAttr  DD      ?
     or      al, 2
     out     92h, al                     ; deactivate A20M#
 @2:
-    mov     ax, ss
+    xor     ax, ax                      ; xor eax, eax
+    mov     eax, ss                     ; mov ax, ss
     DB      67h
     lea     bp, [esp + sizeof (IA32_REGS)]
     ;
diff --git a/MdePkg/Library/BaseLib/X64/Thunk16.S b/MdePkg/Library/BaseLib/X64/Thunk16.S
index 4c3c5605fa..d037a71043 100644
--- a/MdePkg/Library/BaseLib/X64/Thunk16.S
+++ b/MdePkg/Library/BaseLib/X64/Thunk16.S
@@ -1,6 +1,6 @@
 #------------------------------------------------------------------------------
 #
-# Copyright (c) 2006 - 2010, Intel Corporation. All rights reserved.<BR>
+# Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>
 # This program and the accompanying materials
 # are licensed and made available under the terms and conditions of the BSD License
 # which accompanies this distribution.  The full text of the license may be found at
@@ -109,7 +109,8 @@ L_1:
     orb     $2,%al
     outb    %al, $0x92                   # deactivate A20M#
 L_2: 
-    movw    %ss,%ax
+    xorw    %ax, %ax                     # xor eax, eax
+    movl    %ss, %eax                    # mov ax, ss
     lea     IA32_REGS_SIZE(%esp), %bp
     #
     # rsi in the following 2 instructions is indeed bp in 16-bit code
diff --git a/MdePkg/Library/BaseLib/X64/Thunk16.asm b/MdePkg/Library/BaseLib/X64/Thunk16.asm
index 829df1c2a0..8b208fdb58 100644
--- a/MdePkg/Library/BaseLib/X64/Thunk16.asm
+++ b/MdePkg/Library/BaseLib/X64/Thunk16.asm
@@ -3,7 +3,7 @@
 
 ;------------------------------------------------------------------------------
 ;
-; Copyright (c) 2006 - 2010, Intel Corporation. All rights reserved.<BR>
+; Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>
 ; This program and the accompanying materials
 ; are licensed and made available under the terms and conditions of the BSD License
 ; which accompanies this distribution.  The full text of the license may be found at
@@ -105,7 +105,8 @@ _ThunkAttr  DD      ?
     or      al, 2
     out     92h, al                     ; deactivate A20M#
 @2:
-    mov     ax, ss
+    xor     ax, ax                      ; xor eax, eax
+    mov     eax, ss                     ; mov ax, ss
     lea     bp, [esp + sizeof (IA32_REGS)]
     ;
     ; rsi in the following 2 instructions is indeed bp in 16-bit code