diff --git a/MdePkg/Library/BaseLib/BaseLib.msa b/MdePkg/Library/BaseLib/BaseLib.msa
index b208b2cbff..fd57084f63 100644
--- a/MdePkg/Library/BaseLib/BaseLib.msa
+++ b/MdePkg/Library/BaseLib/BaseLib.msa
@@ -175,7 +175,7 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
       <Filename>Ia32/CpuPause.asm</Filename>
       <Filename>Ia32/CpuBreakpoint.asm</Filename>
       <Filename>Ia32/CpuFlushTlb.asm</Filename>
-      <Filename FileType="x86_asm_autogen">Ia32/Thunk16.asm</Filename>
+      <Filename>Ia32/Thunk16.asm</Filename>
     </Arch>
     <Arch ArchType="X64">
       <Filename>X86LowLevel.c</Filename>
diff --git a/MdePkg/Library/BaseLib/Ia32/Thunk16.asm b/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
index 4d5b47017d..454b8905bc 100644
--- a/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
+++ b/MdePkg/Library/BaseLib/Ia32/Thunk16.asm
@@ -29,6 +29,10 @@ EXTERNDEF   C   m16Gdt:WORD
 EXTERNDEF   C   m16GdtrBase:WORD
 EXTERNDEF   C   mTransition:WORD
 
+THUNK_ATTRIBUTE_BIG_REAL_MODE               EQU 1
+THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15     EQU 2
+THUNK_ATTRIBUTE_DISABLE_A20_MASK_KBD_CTRL   EQU 4
+
 IA32_REGS   STRUC   4t
 _EDI        DD      ?
 _ESI        DD      ?