diff --git a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
index 5575d2ec8e..5848f9f164 100644
--- a/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
+++ b/IntelFrameworkModulePkg/Csm/LegacyBiosDxe/LegacyBootSupport.c
@@ -1615,7 +1615,7 @@ EfiMemoryTypeToE820Type (
   case EfiConventionalMemory:
   //
   // The memory of EfiRuntimeServicesCode and EfiRuntimeServicesData are
-  // usable memory for legacy OS.
+  // usable memory for legacy OS, because legacy OS is not aware of EFI runtime concept.
   // In ACPI specification, EfiRuntimeServiceCode and EfiRuntimeServiceData
   // should be mapped to AddressRangeReserved. This statement is for UEFI OS, not for legacy OS.
   //