1. Added comments to ASM files

[mirror_edk2.git] / MdePkg / Library / BaseLib / Ia32 / ReadDr4.asm

diff --git a/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm b/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm
index d1fffe4ec0792aa46e898e74fc66e41c023f60a1..606f26fe2abb59bb3e2e0c784ab92b15fb822ccb 100644 (file)
--- a/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm
+++ b/MdePkg/Library/BaseLib/Ia32/ReadDr4.asm
@@ -33,6 +33,13 @@
 ;   );\r
 ;------------------------------------------------------------------------------\r
 AsmReadDr4  PROC\r
+    ;\r
+    ; DR4 is alias to DR6 only if DE (in CR4) is cleared. Otherwise, reading\r
+    ; this register will cause a #UD exception.\r
+    ;\r
+    ; MS assembler doesn't support this instruction since no one would use it\r
+    ; under normal circustances. Here opcode is used.\r
+    ;\r
     DB      0fh, 21h, 0e0h\r
     ret\r
 AsmReadDr4  ENDP\r