X-Git-Url: https://git.proxmox.com/?p=mirror_edk2.git;a=blobdiff_plain;f=MdePkg%2FLibrary%2FBaseLib%2FIa32%2FReadDr4.asm;h=606f26fe2abb59bb3e2e0c784ab92b15fb822ccb;hp=d1fffe4ec0792aa46e898e74fc66e41c023f60a1;hb=a9a812a0ed4ab3d83941fdc6da281aebdb9bb296;hpb=741fb364170375f89aa9c29488cc402db07a3776

diff --git a/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm b/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm
index d1fffe4ec0..606f26fe2a 100644
--- a/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm
+++ b/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm
@@ -33,6 +33,13 @@
 ;   );
 ;------------------------------------------------------------------------------
 AsmReadDr4  PROC
+    ;
+    ; DR4 is alias to DR6 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, 0e0h
     ret
 AsmReadDr4  ENDP