diff --git a/riscv-elf.adoc b/riscv-elf.adoc
index 10511e42..fd25fcc9 100644
--- a/riscv-elf.adoc
+++ b/riscv-elf.adoc
@@ -1209,6 +1209,9 @@ compatibility column in the following table.
 
 NOTE: Merging object files with the same ABI will result in the same ABI.
 
+NOTE: Program built without A-extension is classified into UNKNOWN_ATOMIC_ABI
+now, and we intend to define new value for those case in future.
+
 === Mapping Symbol
 
 The section can have a mixture of code and data or code with different ISAs.