1 files changed, 7 insertions, 0 deletions
diff --git a/MdePkg/Library/BaseLib/Ia32/ReadDr5.asm b/MdePkg/Library/BaseLib/Ia32/ReadDr5.asm
index bba8b602ba..091792dab9 100644
--- a/MdePkg/Library/BaseLib/Ia32/ReadDr5.asm
+++ b/MdePkg/Library/BaseLib/Ia32/ReadDr5.asm
@@ -33,6 +33,13 @@
 ;   );
 ;------------------------------------------------------------------------------
 AsmReadDr5  PROC
+    ;
+    ; DR5 is alias to DR7 only if DE (in CR4) is cleared. Otherwise, reading
+    ; this register will cause a #UD exception.
+    ;
+    ; MS assembler doesn't support this instruction since no one would use it
+    ; under normal circustances. Here opcode is used.
+    ;
     DB      0fh, 21h, 0e8h
     ret
 AsmReadDr5  ENDP