diff --git a/MdePkg/Library/BaseLib/BaseLib.msa b/MdePkg/Library/BaseLib/BaseLib.msa
index fd57084f63..daf4b86ddd 100644
--- a/MdePkg/Library/BaseLib/BaseLib.msa
+++ b/MdePkg/Library/BaseLib/BaseLib.msa
@@ -267,7 +267,7 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
       <Filename>X64/CpuPause.asm</Filename>
       <Filename>X64/CpuBreakpoint.asm</Filename>
       <Filename>X64/CpuFlushTlb.asm</Filename>
-      <Filename FileType="x86_asm_autogen">X64/Thunk16.asm</Filename>
+      <Filename>X64/Thunk16.asm</Filename>
     </Arch>
     <Arch ArchType="IPF">
       <Filename>Math64.c</Filename>
diff --git a/MdePkg/Library/BaseLib/X64/Thunk16.asm b/MdePkg/Library/BaseLib/X64/Thunk16.asm
index 7495ebe2c8..23450ed81b 100644
--- a/MdePkg/Library/BaseLib/X64/Thunk16.asm
+++ b/MdePkg/Library/BaseLib/X64/Thunk16.asm
@@ -26,6 +26,9 @@ EXTERNDEF   m16Gdt:WORD
 EXTERNDEF   m16GdtrBase:WORD
 EXTERNDEF   mTransition:WORD
 
+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      ?