diff --git a/cpu.cpp b/cpu.cpp
index 63289e8d..8d2ca24b 100644
--- a/cpu.cpp
+++ b/cpu.cpp
@@ -845,7 +845,8 @@ extern bool CPU_ProbeSHA256();
 extern bool CPU_ProbeSHA512();
 extern bool CPU_ProbeDARN();
 
-// Linux defines
+// Linux define values from 64-Bit ELF V2 ABI Specification.
+// http://openpowerfoundation.org/wp-content/uploads/resources/leabi/content/ch_preface.html
 #ifndef PPC_FEATURE_HAS_ALTIVEC
 # define PPC_FEATURE_HAS_ALTIVEC  0x10000000
 #endif